International Ophthalmology 16: 287-290, 1992. 9 1992 Kluwer Academic Publishers. Printed in the Netherlands.

X-PERT NCT advanced logic tonometer valuation Gilles Kretz & Philippe Demailly

Glaucoma Institute, Saint-Joseph Hospital Foundation, 7, rue Pierre-Larousse, 75014 Paris, France

Key words: intraocular pressure, Goldmann tonometer, air-pulsed tonometer, glaucoma Abstract

The air pulsed automatic tonometer X-PERT NCT has been tested in hospitals on glaucomatous patients. This new technique avoids direct contact with the patient's eyes and therefore avoids any risk of contamination. The population studied consisted of 118 eyes of 60 patients whose intraocular pressure (I.O.P.) is measured by applanation with the Goldmann tonometer and then by the air pulsed tonometer. The average I.O.P. obtained by Goldmann tonometer is 17.5 _+ 5mmHg while by X-PERT tonometer, the average I.O.P. is 17.4 __+1.5 mmHg. Statistical analysis of these data showed a very good correlation (r = 0.969) in the two methods of measurement.

Introduction

Since applanation tonometer was first described in 1885 by Maklakoff, many improvements have been made on intraocular pressure measurement techniques by automatisation and indirect corneal contact. In 1974, during International Glaucoma Symposium in Albi, Moses [1] described different developments in tonometry. He presented a non-contact air-pulsed tonometer. The principle is based on air-pulsed corneal applanation. When air-pulsed force on cornea is higher than intraocular pressure, cornea gets out of shape. This deformation is aimed by optical system and conversed in pressure. Each measure is recorded and mean of 3 measures is done. Forbes [2] indicated that correlation with Goldmann tonometer was good with standard deviation of 2.86 mmHg. More recently, Fischer [3] related his experiment of 'Pulsair' tonometer founded on the same principle. The correlation was good with Goldmann tonometer. Correlation coefficients vary from 0.88 to

0.95 and standard deviations from 1.56 to 2.66 mmHg. These results are similar to those of Vernon [4] on 48 eyes in post operative stage. Correlation coefficient was 0.92.

Materials and methods

X-PERT NCT tonometer has been tested in hospitals on patients during general consultation with routine intraocular pressure measurements. The population studied consisted of 118 eyes of 60 patients in whom intraocular pressure is measured by applanation with the Goldmann tonometer (GAT) and then by the air pulsed tonometer X-PERT five minutes later. All measurements were carried by the same operator. Data were analysed and presented as mean and standard deviations, regression coefficients and coefficients of correlations, and matched pair difference distribution.

288

G. K r e t z & P. D e m a i l l y

Table 1. Clinical population characteristics.

Population (N) Mean (m) Std. dev (S)

GAT IOP

XPERT IOP

Differences (XPERT-GAT)

Age

118 17.5 5.0

118 17.4 5.7

118 - 0.1 mmHg 1.5

60 59.6 18.7

Table 2. XPERT's clinical performance.

Slope (M) Y intercept (B)

XPERT vs GAT

Diff vs GAT

GAT vs age

1.110 - 2.1

0.110 - 2.1

0.040 15.1

Sdiff

1.5

1.5

--

Coef: corr. (r)

0.969

0.360

0.150

Results

In the present study, 4 variables were used i.e. G A T IOPs, X P E R T lOPs, matched pair differences ( X P E R T minus G A T ) and age of patients. The m e a n and standard deviation of each of these variables is shown in Table 1. In the clinical population ( I O P ranges from 4 to 30 m m H g ) , the mean and SD of I O P as determined by X P E R T and G A T are as follows: Mxpert = 1 7 . 4 m m H g , Sxpert = 5 . 7 m m H g , M g a t = 17.5; S g a t = 5.1. The lesser magnitude of Sgat may be attributed to the competence of the G A T operator. The X P E R T calibration is defined by the X P E R T vs. G A T regression coefficients. Table 2

Table 3. Clinical calibration.

GAT

XPERT

10mmHg 15 20 25 30 35 40 45 50

9.1 mmHg 14.6 20.1 25.7 31.2 36.8 42.3 47.9 53.4

shows these coefficients and coefficients of correlation for X P E R T N C T vs. G A T , matched pair differences vs. G A T IOPs, and I O P vs. age. Results in Table 2 shows that slope m = 0.110 and at Y intercept, b = - 2 . 1 m m H g . X P E R T ' s clinical performance, as predicted by the regression equation: X P E R T = 1.110; G A T = 2.1. While the predicted matched pair disparities are clinically insignificant, results in Table 3 indicate that the X P E R T slightly overestimates I O P at 30 m m H g and above.

Discussion

By convention, the G A T reference instrument is considered error-free, and, in the calculation of the linear regression, all matched pair differences are attributed as errors in the Test instrument, X P E R T NCT. The Test instrument's single most relevant performance index is the standard deviation of differences (Sdiff) of matched pairs. Realistically~ Sdiff is a measure of the study's aggregate variability. It includes variability due to - physiologically related factors, reference instrument and its operator(s), test instrument and its operator(s), I O P time dependence, and - the effect of one m e a s u r e m e n t on another.

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Fig. 1. XPERT NCT vs GAT: 3 scatter plots showing close agreement (as indicated by the regression line) between XPERT and GAT. (A) XPERT vs GAT IOPs with line of regression. (B) XPERT vs. GAT IOPs with _+1 SD of difference limits (Sai.). (C) XPERT vs. GAT IOPs with + 2 SD of difference limits (Sdi~).

T h e finding of a r e m a r k a b l y small X P E R T vs G A T Sa~f~ ( 1 . 5 m m H g ) , c o u p l e d with a high positivive correlation coefficient r (0.969), confirms the X P E R T ' S outstanding clinical p e r f o r m a n c e . T h e

G A T vs G A T reliability literature, cited b e l o w , provides a frame of reference for judging the excellence of these findings. Thorburn [5] reported a Sai, = 0.9 m m H g for a o n e operator, o n e G A T instrument, and no interval b e t w e e n two successive sets of three m e a s u r e m e n t s , each of which in a g r e e m e n t within a range of 1 m m H g . M o s e s [6] reported a Sai, = 1.4 m m H g and r = 0.94, for a o n e operator, o n e G A T , and a short w a l k b e t w e e n t w o successive sets of m e a s u r e s ,

G. Kretz & P. Demailly

290 50

lOP A S A F T N OF AGE

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