0021-972x/92/7506-1413$03.00/0 Journal of Clinical Endocrinology and Metabolism Copynght 0 1992 by The Endocrine Society
Vol. 75, No. 6 Printed in U.S.A.
Combining Insulin-Like Growth Factor-I and Mean Spontaneous Nighttime Growth Hormone Levels for the Diagnosis of Growth Hormone Deficiency KAREN JAMES
E. OERTER*, D. MALLEY,
ARON M. SOBEL, SUSAN R. ROSE, AUDREY CRISTIANO, GORDON B. CUTLER, JR.*, AND JEFFREY BARON*
Developmental Endocrinology Branch, National Institute of Child Health and Human Development, and the Laboratory of Statistical and Mathematical Methodology (J.D.M.), Division of Computer Research and Technology, National Institutes of Health, Bethesda, Maryland 20892 ABSTRACT
deficiency. A child failed this bivariate test if his S score was less than -2 SD. We applied this model to 47 normal children and 48 short or slowly growing children (all prepubertal). We measured spontaneous nighttime GH levels and IGF-I levels in all children. In addition, the short children underwent 3 GH stimulation tests. Forty-six of the 47 normal children passed the bivariate test for GH sufficiency. Twenty-three of the 48 short or slowly growing children failed the bivariate test, whereas only 11 children had an abnormally low mean spontaneous nighttime GH measurement alone. Sixteen of 23 children who were GH deficient by the bivariate test were also GH deficient by the stimulation tests. In summary, the bivariate test for GH deficiency appears 1) to be independent of body mass, unlike either IGF-I or GH individually; 2) to identify more children than the mean spontaneous nighttime GH level alone; and 3) to be highly specific in the normal population, unlike stimulation tests. (J Clin Endocrinol Metab 75: 1413-1420, 1992)
There is no gold standard for the diagnosis of GH deficiency. Recent data show that spontaneous GH levels may lack sensitivity, and that GH stimulation tests lack specificity as currently performed. Serum insulin-like growth factor-I (IGF-I) measurements lack both sensitivity and specificity. Some of these problems may be explained by nutritional effects. In children, overnutrition decreases GH and increases IGF-I, while undernutrition decreases IGF-I and increases GH. To overcome these difficulties and improve diagnostic accuracy, we combined mean spontaneous nighttime GH levels with IGF-I levels in a statistically based bivariate model. On a two-dimensional plot of mean soontaneous nighttime GH level (in SD units) us. IGF-I level (in SD units), we defined a new variable, S (sum) score, where S = (l/A) X (nighttime mean GH SD + IGF-I SD). While IGF-I (SD) and the mean spontaneous nighttime GH (SD) showed a significant correlation with body mass index, the S score was independent of body mass. We, therefore, used the S score to define a new test for GH
I
T HAS been suggestedthat both GH-deficient and nonGH-deficient children respond to GH therapy, and therefore, that the diagnosis of GH deficiency is unnecessary (1, 2). However, we believe that an accurate diagnosisis important for several reasons: 1) patients with GH deficiency are at higher risk for other pituitary deficiencies and for central nervous system abnormalities, including tumors, and, therefore, may require further endocrine and neurological evaluation; 2) patients without GH deficiency may have another occult cause for growth failure and, therefore, may require further gastrointestinal, psychosocial, or other evaluation; and 3) neither the long term safety nor the efficacy of GH therapy for GH-sufficient patients has been established (35). Investigators, clinicians, patients, and parents need to know whether they are replacing a missing hormone, as clinicians have done for almost 3 decades (6), or giving supplemental hormone, a treatment for which long term data are lacking. Unfortunately, there is no gold standard laboratory test for making the diagnosisof GH deficiency. Becausedaytime Received January 8, 1992. Address all correspondence and requests Oerter, Building 10, Room lON262, National thesda, Maryland 20892. * Commissioned Officer in the USPHS.
for reprints Institutes
to: Dr. Karen of Health, Be-
GH levels are generally undetectable, a number of GH stimulation tests have been introduced, including arginine, insulin, L-dopa, clonidine, and exercise tests. No single stimulation test has been found to provide adequate specificity. Therefore, at least two stimulation tests are usually performed to diagnose GH deficiency. However, the criteria for passingthe combined tests have not been rigorously defined. There is wide variation among clinicians and investigators in the specific tests performed and their interpretation. Recent results show that a large percentage of normal children fail two or more stimulation tests(7), and that the reproducibility of stimulation tests is poor (8). The value of measuring spontaneous GH levels for the diagnosisof GH deficiency remains controversial. Spiliotis et al. (9) advocated measurementsof spontaneousGH levels as a more sensitive measure of abnormal GH secretion. However, Roseet al. (10) concluded that the spontaneousGH test is lesssensitive than GH stimulation tests. Insulin-like growth factor-I (IGF-I) levels are often used to screenpatients for GH deficiency. While IGF-I levels may be low in GH deficiency, they are often in the normal range (11). Likewise, IGF-I levels are frequently low in short children who have no other evidence of GH deficiency (11, 12). The strong influence of nutrition may explain some of the failures of GH or IGF-I levels in the diagnosis of GH defi-
1413
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OERTER
1414 TABLE
1. Clinical
characteristics
JCE & M. 1992 Vol75.No6
ET AL.
of the subjects
category
NO.
Sex (M/F)
Normal Short
47 48
31/16 33/15
Age
Bone age
(yr)
(Yd
8.2 t 1.8 a.2 k 2.7
7.1 + 1.9 6.1 f 2.8
Body mass index (SD units)”
Height (SD units) -0.02 -3.0
+ 1.0 + 1.0
-0.3 -0.3
f 0.6 + 0.9
Midparental heighth (SD units)” -0.002 -1.1
+ 1.9 + 0.8
Mean + SD. ’ Standard deviation units are relative to values in North American children of the same age and sex (20, 27). * Adjusted for the sex of the child according to the following formula: father’s height (-13 cm if a girl) plus mother’s divided by 2 and expressed in standard deviations from the mean normal adult height at the age of 18 yr. Measure levels for
patient’s GH every 20 min. 12 h (bp - 8a)
Measure
patient’s I
Calculate mean niahttime GH -(MNGH)
t
height
0.4 + 1.1 -1.6 + 1.8
(+13
cm if a boy)
IGF-I levels in a statistically based bivariate model. applied this model to 47 children of normal height growth velocity and 48 children who were evaluated short stature or decreased growth velocity.
IGF-I
i
Calculate logarithm
Growth rate (SD units)”
Materials
We and for
and Methods
Subjects the base IO of this value
Converl to SD units [MNGH (SD units)] (eq.
Convert [IGF-I m
1)
\
to SD units units11 (eq. 4)
/
[S = (I /?%MNGH
Calculate S score (SD units) + IGF- I (SD units))] (es 5)
The study included 47 prepubertal children of normal height for age and 48 prepubertal children with a height (n = 44) or growth velocity (n = 18) more than 2 SD below the mean for age (Table 1). The short children were referred to the Pediatric Endocrinology Clinic of the NICHHD for evaluation of their short stature. The normal children were recruited through the Normal Volunteer Office at the NIH. For 38 of the 48 short or slowly growing children, no underlying cause for the growth abnormality was found by history, physical examination, or laboratory determination. Ten of the short or slowly growing children had a specific diagnosis, including idiopathic panhypopituitarism (n = 2), chiasmatic glioma treated with irradiation (n = 2), septo-optic dysplasia (n = l), birth trauma with subdural hematoma (n = I), neurofibromatosis (n = l), and familial autosomal dominant isolated GH deficiency (n = 1). Previously reported normal and short children (10) were included in this study if they met the above criteria.
Protocol Abnormal bivariate test result
FIG. 1. Flow diagram equations in Materials TABLE different
2. Mean ages
Normal brvartate result
of the bivariate and Methods). +
SD
IGF-1
levels
test
Age (yd
*
SD
0.41 0.46 0.83 1.03 1.13
+ + + + +
0.2 0.3 0.2 0.3 0.3
to the
girls
indicated
and boys at
(U/mL)
Boys
2-3 4-5 6-7 8-9 10-11 Mean
(refer
in normal
IGF-1
test
Girls (7) (11) (20) (17) (8)
0.66 1.01 1.04 0.99
+& + If:
0.1 0.3 0.4 0.4
(10) (6) (20) (12)
(n).
ciency. Nutritional status alters GH and IGF-I in opposite directions. Acute fasting or chronic malnutrition raises GH (13-15) and lowers IGF-I (16, 17); obesity lowers GH and raises IGF-I (18, 19). In contrast, true GH deficiency causes a decrease in both GH and IGF-I. Thus, we hypothesized that combining IGF-I and GH measurements might improve the detection of GH deficiency while reducing the confounding effects of nutritional state. We combined mean spontaneous nighttime GH levels with
The study was approved by the institutional review board of the NICHHD. Informed consent was obtained from a parent, and assent was obtained from all children of an appropriate age. Height was measured 10 times with the use of a stadiometer.
Stimulation
tests
All of the short or slowly growing children underwent 3 GH stimulation tests. Arginine (0.5 g/kg) was given iv over 30 min. Insulin (0.1 U/kg) was given iv 30 min after the arginine infusion was completed. Levodopa was given orally (125 mg for body weight 30 kg). GH levels were measured in plasma samples obtained every 15 min for 2 h during the arginine-insulin test and every 30 min for 2 h during the levodopa test. Children with a bone age greater than 8 yr were treated with sex steroids before the stimulation tests [ethinyl estradiol (40 pg/ m*.day orally, for 2 days) or testosterone enanthate (200 mg by im injection) 5-10 days before testing]. A GH level greater than 7 pg/L in response to arginine, insulin, or levodopa stimulation was considered normal, based on historical criteria.
Spontaneous
hormone
levels
All children had blood obtained for GH measurement every 20 min from 2000-0800 h. GH was measured by RIA at Hazleton Biotechnologies (Vienna, VA). The detection limit averaged 0.5 @g/L. Levels below the detection limit were set equal to the detection limit for purposes of analysis. The mean spontaneous nighttime GH level was the average of the values obtained for a given patient. IGF-I levels were measured by
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l
c 3 =i’ c
4.!A
Mean Spontaneous Nighttime Growth Hormone (SD units)
N-
Mean Spontaneous Nighttime Growth Hormone (SD units) The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 23 June 2016. at 17:42 For personal use only. No other uses without permission. . All rights reserved.
OERTER
--ET -_-. AT,.
JCE8zM.1992 Vol75.No6
the height in meters (20, 21). The significance levels for the correlations between body mass index and measures of GH and IGF-I were evaluated after conversion to SD units by a one-tailed test, since the direction of change had been established in other studies (13-19, 22). Conversion
of data to
SD
units
The bivariate test combined mean spontaneous nighttime GH and IGF-I levels after they were converted to SD units (Fig. 1). The logarithmic transformation of the mean spontaneous nighttime GH level has been shown to provide the closest approximation to a Gaussian distribution (10) and, therefore, was used to convert the mean spontaneous nighttime GH level (MNGH) to SD units. MNGH
(SD
units) =
patient’s value - mean SD
-4 -5 -6-
., -6
., -5
.,
.,
-4
-3
-2
IGF-1
, -,
.,
., 0
(SD
.,., ,
., ;
j
, 4
r
;
units)
FIG. 5. A graph of mean spontaneous nighttime GH (SD) us. IGF-I (SD) in 47 normal children (0) and 48 short or slowly growing children (0, passed GH stimulation tests; +, failed GH stimulation tests). The line S = -2 is shown. Patients whose value fell below this line failed the bivariate test.
where patient’s value is the lo&MNGH), mean is the mean of [loglo(MNGH)] for the normal population (0.47 in this study), and SD is the SD of [loglo(MNGH)] for the normal population (0.24 in this study). IGF-I levels change significantly with age and sex (11, 2324). Using IGF-I measurements from normal children, including values from prepubertal children reported by Endocrine Sciences and values from the 47 normal children in this report, the mean and SD were established for each age and sex category. IGF-I was found to increase linearly with age in each sex (r = 0.33 for girls; r = 0.93 for boys; P < 0.02 for girls and boys; Table 2). This linear relationship allowed us to determine the mean IGFI for each sex over the age ranges of 2-11 yr in boys and 2-9 yr in girls by using the following equations: for boys: mean IGF-I = 0.101 X (age in yr) + 0.146 for girls: mean IGF-I = 0.063 X (age in yr) + 0.57 These values were used to convert each individual to SD units by the equation: IGF-I (SD units) =
Mean
Spontaneous
q
Bivariate
I3
Stimulation
0
IGF-1
Nighttime
Growth
Hormone
where patient’s value is the from Eq II or III, and SD is the in individual IGF-I values regression lines for IGF-I us.
(III)
child’s value
patient’s value - mean SD
(11)
(IV
measured IGF-I, mean is the value root mean square of the differences of the normal subjects from the age (0.27 in boys and 0.36 in girls).
test tests
test
FIG. 6. Venn diagram comparing the number of children who failed each of four tests. Of the 48 short or slowly growing children, 29 failed at least 1 of the following tests: mean spontaneous nighttime GH test, GH stimulation tests, IGF-I test, and bivariate test. The numbers on the diagram indicate how many children fall into each subset.. For example, 8 children failed all 4 tests, whereas 2 failed only the IGF-I test. Endocrine Sciences (Tarzana, CA), using an acid-ethanol extraction. Each reported level represents the average of a 2000 h value and an 0800 h value. Body mass index Nutritional state was assessed by determining the body mass index, which is defined as the weight in kilograms divided by the square of
Results Bivariate
analysis
A two-dimensional graph of mean spontaneous nighttime GH level in SD units zts. IGF-I level in SD units for the normal prepubertal children showed that the two variables were not correlated and, therefore, could be treated mathematically as independent variables (Fig. 2). The GH and IGF-I axes divide this two-dimensional graph into four quadrants (Fig. 3). Because patients with GH deficiency tend to have low levels of both GH and IGF-I, their values on such a two-dimensional graph would be hypothesized to fall in quadrant III. Conversely, patients with acromegaly, who tend to have high levels of both GH and IGF-I, would be expected to fall in quadrant I. Children with overnutrition tend to have high IGF-I and low GH values and would be hypothesized to fall
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DIAGNOSIS
r = 0.26
p
0.05
l
.
. .
l
. 9
0
Body
Mass
-_ ‘I
1
Index
0.9
l
c
-1
.
l
.
*
.
. .
l
” .
.
l
l
.
.
.
-34
.
-1
Ii
Body FIG. 7. The relationship correlation coefficients
Mass
21
1-
Index
(SD units)
between body and corresponding
mass index probabilities
-2
score = $
X
(
[IGF-I
(SD
units)
+ mean
units)
- mean
nighttime
GH
(SD units)]
(V)
units)]
(VI)
1
and D (difference) 5
score = X
[IGF-I
(SD
nighttime
GH
7
-1
Body and IGF-I (A), mean spontaneous are shown in the upper lefthand
in quadrant IV. Patients with undernutrition have the reverse pattern and would be expected to fall in quadrant II. On this graph (Fig. 3), we defined a new set of axeswhere:
(
l
. .
.
. .
.
-2
S (sum)
D .
0
.
M .
l .
0
.
-1.
;
l
(SD units)
!I Q
.
.
.o.**
.
-I
Index
3
p < 0.01
l.
0’
-2.
Mass
2
.
.
0
r = 0.34
I
.
0.
- 1
Body
(SD units)
r < 0.01
p
1417
A
.
.
.
OF GH DEFICIENCY
(SD
1
As shown in Fig. 3, patients with GH deficiency would be hypothesized to have low S scores, patients with acromegaly high S scores, patients with overnutrition high D scores, and patients with undernutrition low D scores.
nighttime GH (B), corner of each panel.
0
Mass
Index
S score
(C),
2
1
(SD unlts) and D score
(D).
The
Thus, we hypothesized that the D score would reflect nutritional state, while the S score would reflect somatotroph state. We, therefore, used the S score to define a new test for GH deficiency, which we hypothesized would be independent of nutritional state. On this graph of two normally distributed independent variables approximately 2.3% of the normal population falls outside any line drawn tangent to the circle whose origin is (O,O), and whose radius is 2 SD (Fig. 4). Therefore, 2.3% of the normal population should fall below the line S = -2 SD. We used this line as the criterion for passing the bivariate test. Patients with an S score greater than or equal to -2 SD passed the test; patients with an S score less than -2 SD failed the test. Forty-six of the 47 normal children had S scoresgreater
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OERTER
I418
TABLE
3. Comparison
of three
tests
for GH
deficiency.
Bivariate
a.
Stimulation
test
Pass
Fail
Total
21 4 25
I 16 23
28 20 48
25 0 25
12 11 23
37 11 48
23 2 25
19
21
23
48
tests
Pass
Fail Total
b. Spontaneous test Pass Fail Total
c. ZGF-1 level Pass Fail Total
4
27
than -2 SD and, thus, passed the bivariate test. In contrast, 23 of the 48 short or slowly growing children (48%) failed the bivariate test (Figs. 5 and 6). Influence
of body mass
To test the hypothesis that the bivariate test decreasedthe influence of body mass in the diagnosis of GH deficiency, we examined the relationship between body massindex and the measuresof GH sufficiency in our population of normal children. We found that, as expected, IGF-I correlated positively with body mass index (SD units; r = 0.26; P < 0.05; Fig. 7A), while mean spontaneous nighttime GH correlated negatively with body massindex (r = -0.27; P < 0.05; Fig. 7B). Also, in agreement with our hypothesis, the S score appeared independent of body mass index (r = 0.01; P > 0.9; Fig. 7C), while the D score showed the strongest correlation with body massindex of any variable examined (r = 0.34; P < 0.01; Fig. 7D). Factors other than body massindex must account for much of the variance. Comparison
ET AL.
JCE & M. 1992 Vol75.No6
children (42%) evaluated for short stature or slow growth rate failed all three stimulation tests (Table 3A and Fig. 6). Of the 23 children who failed the bivariate test, 16 also failed three GH stimulation tests (Table 3A and Fig. 6). Similarly, of the 25 short or slowly growing children who passedthe bivariate test, 21 passed the stimulation tests (Fig. 5 and Table 3A). The normal range of mean spontaneous nighttime GH levels based on the 47 normal children was 1.0-7.8 pg/L. Only 11 of the 48 short or slowly growing children (23%) had mean spontaneous nighttime GH levels below this normal range (Fig. 8). All of the 11 children who had mean spontaneous nighttime GH levels more than 2 SD below the mean also failed the bivariate test. However, 12 of the 23 (52%) children who failed the bivariate test had mean spontaneous nighttime GH levels within the normal range (Table 3B). Twenty-one of the 48 short or slowly growing children (44%) had IGF-I levels more than 2 SD below the mean for age and sex. Of the 23 children who failed the bivariate test, 19 also had IGF-I levels more than 2 SD below the mean for age and sex (Fig. 6 and Table 3c. Similarly, of the 25 children who passedthe bivariate test, 23 had IGF-I levels within the normal range (Table 3C). Ten of the 11 children who failed mean spontaneous nighttime GH tests failed the GH stimulation tests (Figs. 5, 6, and 8). However, 10 of the 20 (50%) children who failed stimulation testshad mean spontaneousnighttime GH levels within the normal range. Only 13 of the 21 children who had IGF-I levels more than 2 SD below the mean (62%) failed stimulation tests (Figs. 5, 6, and 8). Nine of the 21 (43%) had mean spontaneous nighttime GH levels more than 2 SD below the mean. The 10 children who had a specific diagnosis failed both the bivariate test and the stimulation tests. Discussion
of tests
There was strong, but not absolute, agreement between the bivariate test and the stimulation tests. Twenty of the 48
Children with GH deficiency tend to have both low nighttime GH levels and low IGF-I levels. Unfortunately, neither
B
0
.E g =c AZ 8. A, Mean spontaneous nighttime GH levels (in SD units) in normal and short or slowly growing children. B, IGFI levels (in SD units) in normal and short or slowly growing children. +, Children who failed GH stimulation tests; 0, children who passed GH stimulation tests.
FIG.
2% u) 22 g 0 -mEL :I” 85
2 --------*----. 1.. I ;I;
---a-------e 0---I x
o
00 0 Fjl
‘$” -2
________
1% _______
J+ _______ +$ T Ot+ ++
I
000 t +
+ Normal
Short
Normal
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Short
DIAGNOSIS
OF GH DEFICIENCY
measure alone has proven satisfactory for the diagnosis of GH deficiency. Some of these problems may be explained by nutritional effects. In children, overnutrition decreases GH levels and increases IGF-I levels, while undernutrition decreases IGF-I levels and increases GH levels. To overcome these difficulties and improve diagnostic accuracy, we combined mean spontaneous nighttime GH levels with IGF-I levels in a statistically based bivariate model. The bivariate model combined mean spontaneous nighttime GH levels (in SD units) with IGF-I levels (in SD units) in a two-dimensional plot. On this two-dimensional plot we defined a new set of axes, the S (sum) score, which is proportional to the sum of IGF-I (SD units) and nighttime mean GH (SD units), and the D (difference) score, which is proportional to the difference between these values. The S score was independent of body mass as we had hypothesized, unlike either of its components (mean spontaneous nighttime GH or IGF-I). Thus, we used the S score to define a new test for GH deficiency, the bivariate test. The theoretically derived criterion for passing the test was evaluated with data from a population of 47 normal children. Forty-six of the 47 normal children (97.9%) passed the bivariate test, which agrees well with the prediction that 97.7% of a normal population should have an S score greater than -2 SD. Thus, the bivariate test was highly specific among normal children. The specificity among the short children cannot be definitively determined in the absence of a gold standard test for GH deficiency. There was reasonably close, but not absolute, agreement between the bivariate test and GH stimulation tests. The 2 methods were discrepant in 11 of 48 short or slowly growing children. Although GH stimulation tests are currently the most commonly used tests, the interpretation of these tests remains unclear. Recent data indicate a high incidence of false positive results among normal children (7). Zadik et al. (8) recently demonstrated the poor reproducibility of the stimulation tests. Furthermore, certain patients, especially those who have received cranial irradiation, may respond normally to pharmacological stimulation but not secrete adequate GH spontaneously (25, 26). Thus, the 4 patients who failed stimulation tests but passed the bivariate test may be false positives of the stimulation tests, similar to the false positives seen when normal children are tested (7). The 7 patients who passed stimulation tests but failed the bivariate test could be children who do not secrete adequate GH under physiological circumstances yet can respond to pharmacological stimuli. Three of these 7 patients were in the lower 14% of the normal range for body mass index, which may have accounted for the relatively greater responses to stimulation tests. More short or slowly growing children fail the bivariate test (23 of 48) than fail the mean spontaneous nighttime GH test alone (11 of 48). Therefore, the bivariate test identifies an abnormality in more short children than does the mean spontaneous nighttime GH test. However, the actual sensitivity of the bivariate test cannot be definitively determined in the absence of a gold standard test for GH deficiency. Thus, the bivariate test appears to have advantages over
existing tests for GH deficiency in terms of sensitivity, specificity, and independence of body mass. The principle disadvantages of the bivariate test are that it cannot be performed as an outpatient, and it requires a greater blood volume and is more expensive than stimulation tests. For these reasons, we do not propose the use of the bivariate test in the routine evaluation of short stature. However, the bivariate test may prove useful when the initial studies of a short child are equivocal or inconsistent. It may also prove helpful in patients approaching the extremes of body mass, in whom measures of either GH or IGF-I alone are most likely to be misleading. The bivariate test requires normative data from a control group that is appropriately matched for age, sex, and pubertal status. Different normative data for IGF-I levels and mean spontaneous nighttime GH would need to be applied to use the bivariate test in pubertal children. Furthermore, normative data should be obtained from the same laboratory as the patient data. In this analysis, IGF-I levels were combined with mean spontaneous nighttime GH levels. A bivariate test could also be developed, combining IGF-I levels with stimulated rather than spontaneous GH levels, thus permitting outpatient testing. To develop such a test, the distribution of stimulated GH levels in normal children would first need to be established. In conclusion, IGF-I and mean spontaneous GH levels are dependent on nutritional state. On the other hand, the bivariate test, which combines IGF-I and mean spontaneous GH levels, appears to be independent of nutritional state, as assessed by the body mass index. We found the bivariate test for GH deficiency to be 1) independent of body mass; 2) highly specific in the normal population, unlike stimulation tests; and 3) apparently more sensitive than the mean spontaneous nighttime GH level alone.
References 1. Allen DB, Fost NC. 1990 Growth hormone therapy for short stature: panacea or Pandora’s box? J Pediatr. 117:16-21. 2. Brook CGD, Hindmarsh PC, Smith PJ. 1987 Is growth hormone deficiency a useful diagnosis? Acta Pediatr Stand. 331(suppl):7075. 3. Underwood LE, Rieser PA. 1989 Is it ethical to treat healthy short children with growth hormone? Acta Pediatr Stand. 362(suppl):lS23. 4. Albertsson-Wikland K, Bischofberger E, Brook CGD, et al. 1989 Growth hormone treatment of short stature. Acta Pediatr Stand. 362(Suppl):9-13. 5. Lippe B, Frasier SD. 1989 How should we test for growth hormone deficiencv, and whom should we treat? T Pediatr. 115:585-587. 6. Raiti S, yolman RA. 1986 Human growth hormone. New York: Plenum Press. 7. Marin G, Barnes K, Rose S, Cutler Jr GB, Cassorla F. 1990 Responses to 4 growth hormone provocative tests in normal children and adolescents. Pediatr Res. 27:468A. 8. Zadik Z, Chalew SA, Gilula Z, Kowarski AA. 1990 Reproducibility of growth hormone testing procedures: a comparison between 24. hour integrated concentration and pharmacological stimulation. J Clin Endocrin Metab. 71:1127-1130. 9. Spiliotis BE, August GP, Hung W, Sonis W, Mendelson W, Bercu BB. 1984 Growth hormone neurosecretory dysfunction: a treatable cause of short stature. JAMA. 251:2223-2301.
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1420
OERTER
10. Rose SR, Ross JL, Uriarte M, Barnes KM, Cassorla FG, Cutler Jr GB. 1988 The advantage of measuring stimulated as compared with spontaneous growth hormone levels in the diagnosis of growth hormone deficiency. N Engl J Med. 319:201-207. 11. Rosenfeld RG, Wilson DM, Lee PDK, Hintz RL. 1986 Insulin-like growth factors I and II in evaluation of growth retardation. J Pediatr. 109:428-433. 12. Rudman D, Kutner M, Chawla RK. 1985 The short child with subnormal plasma somatomedin-C. Pediatr Res. 19:975-980. 13. Hartman ML, Thorner MO. Fasting-induced enhancement of pulsatile growth hormone (GH) secretion is rapidly abolished by refeeding. Proc of the 72nd Annual Meeting of The Endocrine Sot. p 55. 14. Pimstone BL, Wittmann W, Hansen JDL, Murray P. 1966 Growth hormone and kwashiorkor. Lancet. 2:779-780. 15. Marks V, Howorth N. 1965 Plasma growth hormone levels in chronic starvation in man. Nature. 208:686-687. 16. Isley WL, Underwood LE, Clemmons DR. 1983 Dietary components that regulate serum somatomedin-C concentrations in humans. J Clin Invest. 71:175-182. 17. Clemmons DR, Klibanski A, Underwood LE, et al. 1981 Reduction of plasma immunoreactive somatomedin C during fasting in humans. J Clin Endocrin Metab. 53:1247-1250. 18. Cacciari E, Cicognani A, Pirazzoli I’, et al. 1985 Differences in somatomedin-C between short-normal subjects and those of normal height. J Pediatr. 106:891-894. 19. Loche S, Cappa M, Borrelli P, et al. 1987 Reduced growth hormone response to growth hormone-releasing hormone in children with simple obesity: evidence for somatomedin-C mediated inhibition. Clin Endocrinol (Oxf). 27:145-153.
ET AL.
JCE & M - 1992 Vol75.No6
20. Najjar MF, Rowland M. 1987 National Center of Health Statistics: anthropometric reference data and prevalence of overweight, United States, 1976-80. Vital and Health Statistics, ser 11, no. 238 p. l-73. DHHS publication (PHS) 87-1688. Washington DC: USPHS. 21. Rolland-Cachera MF, Semp& M, Guilloud-Bataille M, Patois E, Pe’quignot-Guggenbuhl F, Fautrad V. 1982 Adiposity indices in children. Am J Clin Nutr. 36:178-184. 22. Kamp GA, Manasco PK, Barnes KM, et al. 1991 Low growth hormone levels are related to increased body mass index and do not reflect impaired growth in luteinizing hormone-releasing hormone agonist-treated children with precocious puberty. J Clin Endocrin Metab. 72:301-307. 23. Rose SR, Municchi G, Barnes KM, et al. 1991 Spontaneous growth hormone secretion increases during puberty in normal girls and boys. J Clin Endocrin Metab. 73:428-435. 24. Bala RM, Lopatka J, Leung A, McCoy E, McArthur RG. 1981 Serum immunoreactive somatomedin levels in normal adults, pregnant women at term, children at various ages, and children with constitutionally delayed growth. J Clin Endocrin Metab. 52:508512. 25. Romsche CA, Zipf WB, Miser A, Miser J, Sotos JF, Newton WA. 1984 Evaluation of growth hormone release and human growth hormone treatment in children with cranial irradiation-associated short stature. J Pediatr. 105:177-181. 26. Blatt J, Bercu BB, Gillin JC, Mendelson WB, Poplack DG. 1984 Reduced pulsatile growth hormone secretion in children after therapy for acute lymphoblastic leukemia. J Pediatr. 104:182-186. 27. Tanner JM, Davis PSW. 1985 Clinical longitudinal standards for height and weight for North American children. J Pediatr. 107:317329.
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Vol. 75, No. 6 Printed in U.S.A.
Combining Insulin-Like Growth Factor-I and Mean Spontaneous Nighttime Growth Hormone Levels for the Diagnosis of Growth Hormone Deficiency KAREN JAMES
E. OERTER*, D. MALLEY,
ARON M. SOBEL, SUSAN R. ROSE, AUDREY CRISTIANO, GORDON B. CUTLER, JR.*, AND JEFFREY BARON*
Developmental Endocrinology Branch, National Institute of Child Health and Human Development, and the Laboratory of Statistical and Mathematical Methodology (J.D.M.), Division of Computer Research and Technology, National Institutes of Health, Bethesda, Maryland 20892 ABSTRACT
deficiency. A child failed this bivariate test if his S score was less than -2 SD. We applied this model to 47 normal children and 48 short or slowly growing children (all prepubertal). We measured spontaneous nighttime GH levels and IGF-I levels in all children. In addition, the short children underwent 3 GH stimulation tests. Forty-six of the 47 normal children passed the bivariate test for GH sufficiency. Twenty-three of the 48 short or slowly growing children failed the bivariate test, whereas only 11 children had an abnormally low mean spontaneous nighttime GH measurement alone. Sixteen of 23 children who were GH deficient by the bivariate test were also GH deficient by the stimulation tests. In summary, the bivariate test for GH deficiency appears 1) to be independent of body mass, unlike either IGF-I or GH individually; 2) to identify more children than the mean spontaneous nighttime GH level alone; and 3) to be highly specific in the normal population, unlike stimulation tests. (J Clin Endocrinol Metab 75: 1413-1420, 1992)
There is no gold standard for the diagnosis of GH deficiency. Recent data show that spontaneous GH levels may lack sensitivity, and that GH stimulation tests lack specificity as currently performed. Serum insulin-like growth factor-I (IGF-I) measurements lack both sensitivity and specificity. Some of these problems may be explained by nutritional effects. In children, overnutrition decreases GH and increases IGF-I, while undernutrition decreases IGF-I and increases GH. To overcome these difficulties and improve diagnostic accuracy, we combined mean spontaneous nighttime GH levels with IGF-I levels in a statistically based bivariate model. On a two-dimensional plot of mean soontaneous nighttime GH level (in SD units) us. IGF-I level (in SD units), we defined a new variable, S (sum) score, where S = (l/A) X (nighttime mean GH SD + IGF-I SD). While IGF-I (SD) and the mean spontaneous nighttime GH (SD) showed a significant correlation with body mass index, the S score was independent of body mass. We, therefore, used the S score to define a new test for GH
I
T HAS been suggestedthat both GH-deficient and nonGH-deficient children respond to GH therapy, and therefore, that the diagnosis of GH deficiency is unnecessary (1, 2). However, we believe that an accurate diagnosisis important for several reasons: 1) patients with GH deficiency are at higher risk for other pituitary deficiencies and for central nervous system abnormalities, including tumors, and, therefore, may require further endocrine and neurological evaluation; 2) patients without GH deficiency may have another occult cause for growth failure and, therefore, may require further gastrointestinal, psychosocial, or other evaluation; and 3) neither the long term safety nor the efficacy of GH therapy for GH-sufficient patients has been established (35). Investigators, clinicians, patients, and parents need to know whether they are replacing a missing hormone, as clinicians have done for almost 3 decades (6), or giving supplemental hormone, a treatment for which long term data are lacking. Unfortunately, there is no gold standard laboratory test for making the diagnosisof GH deficiency. Becausedaytime Received January 8, 1992. Address all correspondence and requests Oerter, Building 10, Room lON262, National thesda, Maryland 20892. * Commissioned Officer in the USPHS.
for reprints Institutes
to: Dr. Karen of Health, Be-
GH levels are generally undetectable, a number of GH stimulation tests have been introduced, including arginine, insulin, L-dopa, clonidine, and exercise tests. No single stimulation test has been found to provide adequate specificity. Therefore, at least two stimulation tests are usually performed to diagnose GH deficiency. However, the criteria for passingthe combined tests have not been rigorously defined. There is wide variation among clinicians and investigators in the specific tests performed and their interpretation. Recent results show that a large percentage of normal children fail two or more stimulation tests(7), and that the reproducibility of stimulation tests is poor (8). The value of measuring spontaneous GH levels for the diagnosisof GH deficiency remains controversial. Spiliotis et al. (9) advocated measurementsof spontaneousGH levels as a more sensitive measure of abnormal GH secretion. However, Roseet al. (10) concluded that the spontaneousGH test is lesssensitive than GH stimulation tests. Insulin-like growth factor-I (IGF-I) levels are often used to screenpatients for GH deficiency. While IGF-I levels may be low in GH deficiency, they are often in the normal range (11). Likewise, IGF-I levels are frequently low in short children who have no other evidence of GH deficiency (11, 12). The strong influence of nutrition may explain some of the failures of GH or IGF-I levels in the diagnosis of GH defi-
1413
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OERTER
1414 TABLE
1. Clinical
characteristics
JCE & M. 1992 Vol75.No6
ET AL.
of the subjects
category
NO.
Sex (M/F)
Normal Short
47 48
31/16 33/15
Age
Bone age
(yr)
(Yd
8.2 t 1.8 a.2 k 2.7
7.1 + 1.9 6.1 f 2.8
Body mass index (SD units)”
Height (SD units) -0.02 -3.0
+ 1.0 + 1.0
-0.3 -0.3
f 0.6 + 0.9
Midparental heighth (SD units)” -0.002 -1.1
+ 1.9 + 0.8
Mean + SD. ’ Standard deviation units are relative to values in North American children of the same age and sex (20, 27). * Adjusted for the sex of the child according to the following formula: father’s height (-13 cm if a girl) plus mother’s divided by 2 and expressed in standard deviations from the mean normal adult height at the age of 18 yr. Measure levels for
patient’s GH every 20 min. 12 h (bp - 8a)
Measure
patient’s I
Calculate mean niahttime GH -(MNGH)
t
height
0.4 + 1.1 -1.6 + 1.8
(+13
cm if a boy)
IGF-I levels in a statistically based bivariate model. applied this model to 47 children of normal height growth velocity and 48 children who were evaluated short stature or decreased growth velocity.
IGF-I
i
Calculate logarithm
Growth rate (SD units)”
Materials
We and for
and Methods
Subjects the base IO of this value
Converl to SD units [MNGH (SD units)] (eq.
Convert [IGF-I m
1)
\
to SD units units11 (eq. 4)
/
[S = (I /?%MNGH
Calculate S score (SD units) + IGF- I (SD units))] (es 5)
The study included 47 prepubertal children of normal height for age and 48 prepubertal children with a height (n = 44) or growth velocity (n = 18) more than 2 SD below the mean for age (Table 1). The short children were referred to the Pediatric Endocrinology Clinic of the NICHHD for evaluation of their short stature. The normal children were recruited through the Normal Volunteer Office at the NIH. For 38 of the 48 short or slowly growing children, no underlying cause for the growth abnormality was found by history, physical examination, or laboratory determination. Ten of the short or slowly growing children had a specific diagnosis, including idiopathic panhypopituitarism (n = 2), chiasmatic glioma treated with irradiation (n = 2), septo-optic dysplasia (n = l), birth trauma with subdural hematoma (n = I), neurofibromatosis (n = l), and familial autosomal dominant isolated GH deficiency (n = 1). Previously reported normal and short children (10) were included in this study if they met the above criteria.
Protocol Abnormal bivariate test result
FIG. 1. Flow diagram equations in Materials TABLE different
2. Mean ages
Normal brvartate result
of the bivariate and Methods). +
SD
IGF-1
levels
test
Age (yd
*
SD
0.41 0.46 0.83 1.03 1.13
+ + + + +
0.2 0.3 0.2 0.3 0.3
to the
girls
indicated
and boys at
(U/mL)
Boys
2-3 4-5 6-7 8-9 10-11 Mean
(refer
in normal
IGF-1
test
Girls (7) (11) (20) (17) (8)
0.66 1.01 1.04 0.99
+& + If:
0.1 0.3 0.4 0.4
(10) (6) (20) (12)
(n).
ciency. Nutritional status alters GH and IGF-I in opposite directions. Acute fasting or chronic malnutrition raises GH (13-15) and lowers IGF-I (16, 17); obesity lowers GH and raises IGF-I (18, 19). In contrast, true GH deficiency causes a decrease in both GH and IGF-I. Thus, we hypothesized that combining IGF-I and GH measurements might improve the detection of GH deficiency while reducing the confounding effects of nutritional state. We combined mean spontaneous nighttime GH levels with
The study was approved by the institutional review board of the NICHHD. Informed consent was obtained from a parent, and assent was obtained from all children of an appropriate age. Height was measured 10 times with the use of a stadiometer.
Stimulation
tests
All of the short or slowly growing children underwent 3 GH stimulation tests. Arginine (0.5 g/kg) was given iv over 30 min. Insulin (0.1 U/kg) was given iv 30 min after the arginine infusion was completed. Levodopa was given orally (125 mg for body weight 30 kg). GH levels were measured in plasma samples obtained every 15 min for 2 h during the arginine-insulin test and every 30 min for 2 h during the levodopa test. Children with a bone age greater than 8 yr were treated with sex steroids before the stimulation tests [ethinyl estradiol (40 pg/ m*.day orally, for 2 days) or testosterone enanthate (200 mg by im injection) 5-10 days before testing]. A GH level greater than 7 pg/L in response to arginine, insulin, or levodopa stimulation was considered normal, based on historical criteria.
Spontaneous
hormone
levels
All children had blood obtained for GH measurement every 20 min from 2000-0800 h. GH was measured by RIA at Hazleton Biotechnologies (Vienna, VA). The detection limit averaged 0.5 @g/L. Levels below the detection limit were set equal to the detection limit for purposes of analysis. The mean spontaneous nighttime GH level was the average of the values obtained for a given patient. IGF-I levels were measured by
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l
c 3 =i’ c
4.!A
Mean Spontaneous Nighttime Growth Hormone (SD units)
N-
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OERTER
--ET -_-. AT,.
JCE8zM.1992 Vol75.No6
the height in meters (20, 21). The significance levels for the correlations between body mass index and measures of GH and IGF-I were evaluated after conversion to SD units by a one-tailed test, since the direction of change had been established in other studies (13-19, 22). Conversion
of data to
SD
units
The bivariate test combined mean spontaneous nighttime GH and IGF-I levels after they were converted to SD units (Fig. 1). The logarithmic transformation of the mean spontaneous nighttime GH level has been shown to provide the closest approximation to a Gaussian distribution (10) and, therefore, was used to convert the mean spontaneous nighttime GH level (MNGH) to SD units. MNGH
(SD
units) =
patient’s value - mean SD
-4 -5 -6-
., -6
., -5
.,
.,
-4
-3
-2
IGF-1
, -,
.,
., 0
(SD
.,., ,
., ;
j
, 4
r
;
units)
FIG. 5. A graph of mean spontaneous nighttime GH (SD) us. IGF-I (SD) in 47 normal children (0) and 48 short or slowly growing children (0, passed GH stimulation tests; +, failed GH stimulation tests). The line S = -2 is shown. Patients whose value fell below this line failed the bivariate test.
where patient’s value is the lo&MNGH), mean is the mean of [loglo(MNGH)] for the normal population (0.47 in this study), and SD is the SD of [loglo(MNGH)] for the normal population (0.24 in this study). IGF-I levels change significantly with age and sex (11, 2324). Using IGF-I measurements from normal children, including values from prepubertal children reported by Endocrine Sciences and values from the 47 normal children in this report, the mean and SD were established for each age and sex category. IGF-I was found to increase linearly with age in each sex (r = 0.33 for girls; r = 0.93 for boys; P < 0.02 for girls and boys; Table 2). This linear relationship allowed us to determine the mean IGFI for each sex over the age ranges of 2-11 yr in boys and 2-9 yr in girls by using the following equations: for boys: mean IGF-I = 0.101 X (age in yr) + 0.146 for girls: mean IGF-I = 0.063 X (age in yr) + 0.57 These values were used to convert each individual to SD units by the equation: IGF-I (SD units) =
Mean
Spontaneous
q
Bivariate
I3
Stimulation
0
IGF-1
Nighttime
Growth
Hormone
where patient’s value is the from Eq II or III, and SD is the in individual IGF-I values regression lines for IGF-I us.
(III)
child’s value
patient’s value - mean SD
(11)
(IV
measured IGF-I, mean is the value root mean square of the differences of the normal subjects from the age (0.27 in boys and 0.36 in girls).
test tests
test
FIG. 6. Venn diagram comparing the number of children who failed each of four tests. Of the 48 short or slowly growing children, 29 failed at least 1 of the following tests: mean spontaneous nighttime GH test, GH stimulation tests, IGF-I test, and bivariate test. The numbers on the diagram indicate how many children fall into each subset.. For example, 8 children failed all 4 tests, whereas 2 failed only the IGF-I test. Endocrine Sciences (Tarzana, CA), using an acid-ethanol extraction. Each reported level represents the average of a 2000 h value and an 0800 h value. Body mass index Nutritional state was assessed by determining the body mass index, which is defined as the weight in kilograms divided by the square of
Results Bivariate
analysis
A two-dimensional graph of mean spontaneous nighttime GH level in SD units zts. IGF-I level in SD units for the normal prepubertal children showed that the two variables were not correlated and, therefore, could be treated mathematically as independent variables (Fig. 2). The GH and IGF-I axes divide this two-dimensional graph into four quadrants (Fig. 3). Because patients with GH deficiency tend to have low levels of both GH and IGF-I, their values on such a two-dimensional graph would be hypothesized to fall in quadrant III. Conversely, patients with acromegaly, who tend to have high levels of both GH and IGF-I, would be expected to fall in quadrant I. Children with overnutrition tend to have high IGF-I and low GH values and would be hypothesized to fall
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DIAGNOSIS
r = 0.26
p
0.05
l
.
. .
l
. 9
0
Body
Mass
-_ ‘I
1
Index
0.9
l
c
-1
.
l
.
*
.
. .
l
” .
.
l
l
.
.
.
-34
.
-1
Ii
Body FIG. 7. The relationship correlation coefficients
Mass
21
1-
Index
(SD units)
between body and corresponding
mass index probabilities
-2
score = $
X
(
[IGF-I
(SD
units)
+ mean
units)
- mean
nighttime
GH
(SD units)]
(V)
units)]
(VI)
1
and D (difference) 5
score = X
[IGF-I
(SD
nighttime
GH
7
-1
Body and IGF-I (A), mean spontaneous are shown in the upper lefthand
in quadrant IV. Patients with undernutrition have the reverse pattern and would be expected to fall in quadrant II. On this graph (Fig. 3), we defined a new set of axeswhere:
(
l
. .
.
. .
.
-2
S (sum)
D .
0
.
M .
l .
0
.
-1.
;
l
(SD units)
!I Q
.
.
.o.**
.
-I
Index
3
p < 0.01
l.
0’
-2.
Mass
2
.
.
0
r = 0.34
I
.
0.
- 1
Body
(SD units)
r < 0.01
p
1417
A
.
.
.
OF GH DEFICIENCY
(SD
1
As shown in Fig. 3, patients with GH deficiency would be hypothesized to have low S scores, patients with acromegaly high S scores, patients with overnutrition high D scores, and patients with undernutrition low D scores.
nighttime GH (B), corner of each panel.
0
Mass
Index
S score
(C),
2
1
(SD unlts) and D score
(D).
The
Thus, we hypothesized that the D score would reflect nutritional state, while the S score would reflect somatotroph state. We, therefore, used the S score to define a new test for GH deficiency, which we hypothesized would be independent of nutritional state. On this graph of two normally distributed independent variables approximately 2.3% of the normal population falls outside any line drawn tangent to the circle whose origin is (O,O), and whose radius is 2 SD (Fig. 4). Therefore, 2.3% of the normal population should fall below the line S = -2 SD. We used this line as the criterion for passing the bivariate test. Patients with an S score greater than or equal to -2 SD passed the test; patients with an S score less than -2 SD failed the test. Forty-six of the 47 normal children had S scoresgreater
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OERTER
I418
TABLE
3. Comparison
of three
tests
for GH
deficiency.
Bivariate
a.
Stimulation
test
Pass
Fail
Total
21 4 25
I 16 23
28 20 48
25 0 25
12 11 23
37 11 48
23 2 25
19
21
23
48
tests
Pass
Fail Total
b. Spontaneous test Pass Fail Total
c. ZGF-1 level Pass Fail Total
4
27
than -2 SD and, thus, passed the bivariate test. In contrast, 23 of the 48 short or slowly growing children (48%) failed the bivariate test (Figs. 5 and 6). Influence
of body mass
To test the hypothesis that the bivariate test decreasedthe influence of body mass in the diagnosis of GH deficiency, we examined the relationship between body massindex and the measuresof GH sufficiency in our population of normal children. We found that, as expected, IGF-I correlated positively with body mass index (SD units; r = 0.26; P < 0.05; Fig. 7A), while mean spontaneous nighttime GH correlated negatively with body massindex (r = -0.27; P < 0.05; Fig. 7B). Also, in agreement with our hypothesis, the S score appeared independent of body mass index (r = 0.01; P > 0.9; Fig. 7C), while the D score showed the strongest correlation with body massindex of any variable examined (r = 0.34; P < 0.01; Fig. 7D). Factors other than body massindex must account for much of the variance. Comparison
ET AL.
JCE & M. 1992 Vol75.No6
children (42%) evaluated for short stature or slow growth rate failed all three stimulation tests (Table 3A and Fig. 6). Of the 23 children who failed the bivariate test, 16 also failed three GH stimulation tests (Table 3A and Fig. 6). Similarly, of the 25 short or slowly growing children who passedthe bivariate test, 21 passed the stimulation tests (Fig. 5 and Table 3A). The normal range of mean spontaneous nighttime GH levels based on the 47 normal children was 1.0-7.8 pg/L. Only 11 of the 48 short or slowly growing children (23%) had mean spontaneous nighttime GH levels below this normal range (Fig. 8). All of the 11 children who had mean spontaneous nighttime GH levels more than 2 SD below the mean also failed the bivariate test. However, 12 of the 23 (52%) children who failed the bivariate test had mean spontaneous nighttime GH levels within the normal range (Table 3B). Twenty-one of the 48 short or slowly growing children (44%) had IGF-I levels more than 2 SD below the mean for age and sex. Of the 23 children who failed the bivariate test, 19 also had IGF-I levels more than 2 SD below the mean for age and sex (Fig. 6 and Table 3c. Similarly, of the 25 children who passedthe bivariate test, 23 had IGF-I levels within the normal range (Table 3C). Ten of the 11 children who failed mean spontaneous nighttime GH tests failed the GH stimulation tests (Figs. 5, 6, and 8). However, 10 of the 20 (50%) children who failed stimulation testshad mean spontaneousnighttime GH levels within the normal range. Only 13 of the 21 children who had IGF-I levels more than 2 SD below the mean (62%) failed stimulation tests (Figs. 5, 6, and 8). Nine of the 21 (43%) had mean spontaneous nighttime GH levels more than 2 SD below the mean. The 10 children who had a specific diagnosis failed both the bivariate test and the stimulation tests. Discussion
of tests
There was strong, but not absolute, agreement between the bivariate test and the stimulation tests. Twenty of the 48
Children with GH deficiency tend to have both low nighttime GH levels and low IGF-I levels. Unfortunately, neither
B
0
.E g =c AZ 8. A, Mean spontaneous nighttime GH levels (in SD units) in normal and short or slowly growing children. B, IGFI levels (in SD units) in normal and short or slowly growing children. +, Children who failed GH stimulation tests; 0, children who passed GH stimulation tests.
FIG.
2% u) 22 g 0 -mEL :I” 85
2 --------*----. 1.. I ;I;
---a-------e 0---I x
o
00 0 Fjl
‘$” -2
________
1% _______
J+ _______ +$ T Ot+ ++
I
000 t +
+ Normal
Short
Normal
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Short
DIAGNOSIS
OF GH DEFICIENCY
measure alone has proven satisfactory for the diagnosis of GH deficiency. Some of these problems may be explained by nutritional effects. In children, overnutrition decreases GH levels and increases IGF-I levels, while undernutrition decreases IGF-I levels and increases GH levels. To overcome these difficulties and improve diagnostic accuracy, we combined mean spontaneous nighttime GH levels with IGF-I levels in a statistically based bivariate model. The bivariate model combined mean spontaneous nighttime GH levels (in SD units) with IGF-I levels (in SD units) in a two-dimensional plot. On this two-dimensional plot we defined a new set of axes, the S (sum) score, which is proportional to the sum of IGF-I (SD units) and nighttime mean GH (SD units), and the D (difference) score, which is proportional to the difference between these values. The S score was independent of body mass as we had hypothesized, unlike either of its components (mean spontaneous nighttime GH or IGF-I). Thus, we used the S score to define a new test for GH deficiency, the bivariate test. The theoretically derived criterion for passing the test was evaluated with data from a population of 47 normal children. Forty-six of the 47 normal children (97.9%) passed the bivariate test, which agrees well with the prediction that 97.7% of a normal population should have an S score greater than -2 SD. Thus, the bivariate test was highly specific among normal children. The specificity among the short children cannot be definitively determined in the absence of a gold standard test for GH deficiency. There was reasonably close, but not absolute, agreement between the bivariate test and GH stimulation tests. The 2 methods were discrepant in 11 of 48 short or slowly growing children. Although GH stimulation tests are currently the most commonly used tests, the interpretation of these tests remains unclear. Recent data indicate a high incidence of false positive results among normal children (7). Zadik et al. (8) recently demonstrated the poor reproducibility of the stimulation tests. Furthermore, certain patients, especially those who have received cranial irradiation, may respond normally to pharmacological stimulation but not secrete adequate GH spontaneously (25, 26). Thus, the 4 patients who failed stimulation tests but passed the bivariate test may be false positives of the stimulation tests, similar to the false positives seen when normal children are tested (7). The 7 patients who passed stimulation tests but failed the bivariate test could be children who do not secrete adequate GH under physiological circumstances yet can respond to pharmacological stimuli. Three of these 7 patients were in the lower 14% of the normal range for body mass index, which may have accounted for the relatively greater responses to stimulation tests. More short or slowly growing children fail the bivariate test (23 of 48) than fail the mean spontaneous nighttime GH test alone (11 of 48). Therefore, the bivariate test identifies an abnormality in more short children than does the mean spontaneous nighttime GH test. However, the actual sensitivity of the bivariate test cannot be definitively determined in the absence of a gold standard test for GH deficiency. Thus, the bivariate test appears to have advantages over
existing tests for GH deficiency in terms of sensitivity, specificity, and independence of body mass. The principle disadvantages of the bivariate test are that it cannot be performed as an outpatient, and it requires a greater blood volume and is more expensive than stimulation tests. For these reasons, we do not propose the use of the bivariate test in the routine evaluation of short stature. However, the bivariate test may prove useful when the initial studies of a short child are equivocal or inconsistent. It may also prove helpful in patients approaching the extremes of body mass, in whom measures of either GH or IGF-I alone are most likely to be misleading. The bivariate test requires normative data from a control group that is appropriately matched for age, sex, and pubertal status. Different normative data for IGF-I levels and mean spontaneous nighttime GH would need to be applied to use the bivariate test in pubertal children. Furthermore, normative data should be obtained from the same laboratory as the patient data. In this analysis, IGF-I levels were combined with mean spontaneous nighttime GH levels. A bivariate test could also be developed, combining IGF-I levels with stimulated rather than spontaneous GH levels, thus permitting outpatient testing. To develop such a test, the distribution of stimulated GH levels in normal children would first need to be established. In conclusion, IGF-I and mean spontaneous GH levels are dependent on nutritional state. On the other hand, the bivariate test, which combines IGF-I and mean spontaneous GH levels, appears to be independent of nutritional state, as assessed by the body mass index. We found the bivariate test for GH deficiency to be 1) independent of body mass; 2) highly specific in the normal population, unlike stimulation tests; and 3) apparently more sensitive than the mean spontaneous nighttime GH level alone.
References 1. Allen DB, Fost NC. 1990 Growth hormone therapy for short stature: panacea or Pandora’s box? J Pediatr. 117:16-21. 2. Brook CGD, Hindmarsh PC, Smith PJ. 1987 Is growth hormone deficiency a useful diagnosis? Acta Pediatr Stand. 331(suppl):7075. 3. Underwood LE, Rieser PA. 1989 Is it ethical to treat healthy short children with growth hormone? Acta Pediatr Stand. 362(suppl):lS23. 4. Albertsson-Wikland K, Bischofberger E, Brook CGD, et al. 1989 Growth hormone treatment of short stature. Acta Pediatr Stand. 362(Suppl):9-13. 5. Lippe B, Frasier SD. 1989 How should we test for growth hormone deficiencv, and whom should we treat? T Pediatr. 115:585-587. 6. Raiti S, yolman RA. 1986 Human growth hormone. New York: Plenum Press. 7. Marin G, Barnes K, Rose S, Cutler Jr GB, Cassorla F. 1990 Responses to 4 growth hormone provocative tests in normal children and adolescents. Pediatr Res. 27:468A. 8. Zadik Z, Chalew SA, Gilula Z, Kowarski AA. 1990 Reproducibility of growth hormone testing procedures: a comparison between 24. hour integrated concentration and pharmacological stimulation. J Clin Endocrin Metab. 71:1127-1130. 9. Spiliotis BE, August GP, Hung W, Sonis W, Mendelson W, Bercu BB. 1984 Growth hormone neurosecretory dysfunction: a treatable cause of short stature. JAMA. 251:2223-2301.
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