Correspondence
Base Deficit as a Diagnostic Test for Abdominal Injury
Ken Zafren, MD Tom Purcell, MD/ James W Davis, MD, FACS Troy L Holbrook, PhD [V Adenosine in the Management of PSVT
Charles V Pollack, Jr, MA, MD Traumatic Uvulitis
Robert A Partridge, MD Robert M McNamara, MD
Base Deficit as a Diagnostic Test for Abdominal Injury To the Editor. How useful is"Base Deficit as an Indicator of Significant Abdominal Injury" [August 1991;20:842-844]? Davis et al show that for base deficit less than or equal to -6, the odds ratio for abdominal injury is 6.2. The odds ratio, an approximation of the relative risk {or risk ratio), is applied when data cannot be gathered from a complete cohort. However, in this study, virtually the entire cohort was included in the data analysis, and the preferred measure--relative risk--might have been used: Relative risk = (77/272)/(163/2,739) = 4.72 The use of the odds ratio or risk ratio alone as a guide to making clinical decisions has a serious limitation. Neither reflects the influence of the prevalence of a disease an the interpretation of test results. The traditional measures of a diagnostic test give a more complete picture of the implications of a base deficit of -6 or less in a blunt trauma patient. Of these measures, sensitivity and specificity are independent of prevalence. The following two-way table is based on values given in the article: Base Deficit
-6 or tess More than -6
Abdominal Injury
No Abdominal Injury
77
195
163
2,576
This yields the values of sensitivity, ;7/(77 + 163) = 0.32; specificity, 2,576/(2,576 + 195) = 0.93; positive-
NOVEMBER1992
21:11
ANNALS OF EMERGENCY MEBICINE
predictive value, 77/(77 + 195) = 0.28; negative-predictive value, 2,576/(2,576 + 163) = 0.94; and accuracy, (77 + 2,576)/3,011 = 0.88. Although the negative-predictive value is high, the authors mention that "4% of patients with a normal base deficit had an abdominal injury requiring surgical repair in this study." Unless these patients were identified by other tests, the use of base deficit criteria for diagnostic peritoneal lavage would have allowed these 84 injuries to be missed. With a sensitivity of .32, the test will miss 68% of those at risk for abdominal injury regardless of the prevalence of abdominal injury in a given population. The ability of a diagnostic test to separate normals from "non-normals" is best measured by the receiver operating characteristic curve. Based on the data given in the article, we calculated the area under the receiver operating characteristic curve to be 0.69, which places it in the category of "rather low" discriminatory power (1 .O0 being best and 0.50 being worst). 1 The use of the odds ratio to evaluate a diagnostic test adds dimension to our understanding, but taken alone it provides an incomplete picture of the virtues and limitations of the test.
Ken Zafren, MD TomPurcell, MD Departmentof EmergencyMedicine Kern Medical Center/UCLA Bakersfield, Cafifornia 1. Swets JA: Measuring the accuracy of diagnostic systems. Science 1988;240:1285-1293.
In Reply: We appreciate the interest shown by Drs Zafren and Purcell in the use of base deficit as a significant indicator of abdominal injury. Their comments and analysis are interesting. We agree that the odds ratio is used when data cannot be gathered from a complete cohort. Our design was a retrospective case control analysis in which cases of significant abdominal injury first were selected and then controls were drawn with frequency matching for Injury Severity Score from the remaining blunt trauma patient admissions. Given this design, the odds ratio will approximate the relative risk only when the disease of interest is rare. Abdominal injury is not a rare condition among blunt trauma patients in our institution; thus, as shown by Zafren and Purcell, the relative risk estimate does not closely approximate the corresponding odds ratio, which is to be expected. We also chose to use the odds ratio because we wanted to examine the independent contribution of base deficit as an indicator of abdominal injury using multivariate logistic regression. This statistical methodology yields adjusted estimates of the odds ratio (adjusted for other covariares), not the relative risk. When the disease of interest is not rare, logistic regression analysis will not approximate the relative risk and the use of the odds ratio is more appropriate. We have fundamental disagreement with some inferences made by Zafren and Purcell. The base deficit was
1406/153
Base Deficit as a Diagnostic Test for Abdominal Injury
Ken Zafren, MD Tom Purcell, MD/ James W Davis, MD, FACS Troy L Holbrook, PhD [V Adenosine in the Management of PSVT
Charles V Pollack, Jr, MA, MD Traumatic Uvulitis
Robert A Partridge, MD Robert M McNamara, MD
Base Deficit as a Diagnostic Test for Abdominal Injury To the Editor. How useful is"Base Deficit as an Indicator of Significant Abdominal Injury" [August 1991;20:842-844]? Davis et al show that for base deficit less than or equal to -6, the odds ratio for abdominal injury is 6.2. The odds ratio, an approximation of the relative risk {or risk ratio), is applied when data cannot be gathered from a complete cohort. However, in this study, virtually the entire cohort was included in the data analysis, and the preferred measure--relative risk--might have been used: Relative risk = (77/272)/(163/2,739) = 4.72 The use of the odds ratio or risk ratio alone as a guide to making clinical decisions has a serious limitation. Neither reflects the influence of the prevalence of a disease an the interpretation of test results. The traditional measures of a diagnostic test give a more complete picture of the implications of a base deficit of -6 or less in a blunt trauma patient. Of these measures, sensitivity and specificity are independent of prevalence. The following two-way table is based on values given in the article: Base Deficit
-6 or tess More than -6
Abdominal Injury
No Abdominal Injury
77
195
163
2,576
This yields the values of sensitivity, ;7/(77 + 163) = 0.32; specificity, 2,576/(2,576 + 195) = 0.93; positive-
NOVEMBER1992
21:11
ANNALS OF EMERGENCY MEBICINE
predictive value, 77/(77 + 195) = 0.28; negative-predictive value, 2,576/(2,576 + 163) = 0.94; and accuracy, (77 + 2,576)/3,011 = 0.88. Although the negative-predictive value is high, the authors mention that "4% of patients with a normal base deficit had an abdominal injury requiring surgical repair in this study." Unless these patients were identified by other tests, the use of base deficit criteria for diagnostic peritoneal lavage would have allowed these 84 injuries to be missed. With a sensitivity of .32, the test will miss 68% of those at risk for abdominal injury regardless of the prevalence of abdominal injury in a given population. The ability of a diagnostic test to separate normals from "non-normals" is best measured by the receiver operating characteristic curve. Based on the data given in the article, we calculated the area under the receiver operating characteristic curve to be 0.69, which places it in the category of "rather low" discriminatory power (1 .O0 being best and 0.50 being worst). 1 The use of the odds ratio to evaluate a diagnostic test adds dimension to our understanding, but taken alone it provides an incomplete picture of the virtues and limitations of the test.
Ken Zafren, MD TomPurcell, MD Departmentof EmergencyMedicine Kern Medical Center/UCLA Bakersfield, Cafifornia 1. Swets JA: Measuring the accuracy of diagnostic systems. Science 1988;240:1285-1293.
In Reply: We appreciate the interest shown by Drs Zafren and Purcell in the use of base deficit as a significant indicator of abdominal injury. Their comments and analysis are interesting. We agree that the odds ratio is used when data cannot be gathered from a complete cohort. Our design was a retrospective case control analysis in which cases of significant abdominal injury first were selected and then controls were drawn with frequency matching for Injury Severity Score from the remaining blunt trauma patient admissions. Given this design, the odds ratio will approximate the relative risk only when the disease of interest is rare. Abdominal injury is not a rare condition among blunt trauma patients in our institution; thus, as shown by Zafren and Purcell, the relative risk estimate does not closely approximate the corresponding odds ratio, which is to be expected. We also chose to use the odds ratio because we wanted to examine the independent contribution of base deficit as an indicator of abdominal injury using multivariate logistic regression. This statistical methodology yields adjusted estimates of the odds ratio (adjusted for other covariares), not the relative risk. When the disease of interest is not rare, logistic regression analysis will not approximate the relative risk and the use of the odds ratio is more appropriate. We have fundamental disagreement with some inferences made by Zafren and Purcell. The base deficit was
1406/153
