Repeatability of central corneal thickness measurements measured with the Topcon SP2000P specular microscope
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Graefes Arch Clin Exp Ophthalmol(2005) 243: 798802
Kelechi C. OgbuehiTurki M. Almubrad
Received: 11 November 2004Revised: 7 January 2005Accepted: 14 January 2005Published online: 15 March 2005# Springer-Verlag 2005
Repeatability of central corneal thicknessmeasurements measured with the TopconSP2000P specular microscope
Abstract Background: The non-contact specular microscope has be-come the method of choice for aquick, accurate and non-invasive as-sessment of central corneal thickness(CCT), which is an important variableto monitor before and after refractivesurgery. The consistency of the resultsproduced by such widely used meth-ods/equipment must be assessed todetermine their reliability. The pur-pose of this study was to assesswithin- and between-observer repeat-ability of, and to determine if asystematic bias exists in the measure-ments made by, the Topcon SP2000Pspecular microscope. Methods: TheCCT of the right eyes of 70 adultsubjects, divided equally between menand women, was assessed on twoseparate occasions (47 days apart) byeach of two examiners using the low-intensity auto mode of the SP2000Pspecular microscope. Results: Theaverage CCT values for men andwomen, measured by one observer,were 0.520.03 mm (mean SD) and0.520.04 mm, respectively. Theaverage for the entire sample was 0.520.04 mm. Within- and between-observer repeatability were assessedby plotting the mean difference (foreach subject) between two readingsmade by the same observer or oneeach by both observers against thecombined average CCT reading ofboth sessions; the mean difference
between two sets of observations wasnot significantly different from zero(P
Evaluation of central corneal thickness (CCT) -and cornealthickness profiles- is an important indicator of the patencyof the corneal endothelial pump , and it can be used as asensitive indicator for a wide range of disorders that affectthe anterior segment of the eye (and, as a consequence, thecornea), including glaucoma , corneal ectasias such askeratoconus and keratoglobus , and dry eye [5, 7, 11].The assessment of corneal central thickness and profile hasbecome even more important with the advent of modernrefractive surgical techniques, which have made cornealsurgery more widespread. Repeated measures of cornealcentral thickness and profile have thus become even morerelevant. Avariety of modern equipment is available for theassessment of corneal thickness, the most versatile andpopular of which is the non-contact specular microscope,which has rapidly become amethod of choice for pachometryassessments. In addition to its non-invasive method, it com-bines an assessment of the mid-peripheral thickness of thecornea in four locations with a differential focusing of cor-neal epithelial and endothelial surfaces, which consequentlyallows for the simultaneous assessment of corneal endothe-lial cell density. The first description of the corneal endo-thelium in vivo viewed by specular reflection was made byVogt . Since then, advances in computer technologyhave simplified the tedious nature of corneal endothelialmorphometric analysis, making non-contact specular mi-croscopy a convenient and popular method for assessingcorneal endothelial morphology while simultaneously ana-lysing corneal thickness profiles.To the knowledge of the authors, only one previous study
has assessed the intra-observer and inter-observer reproduc-ibility of repeated corneal thickness assessments with thenon-contact specular microscope SP2000P . The presentstudy was undertaken to confirm the findings of Cho andCheung , and we go further to suggest criteria for val-idating repeat measures of the same quantity made by amethod.
Subjects and methods
The subjects were drawn from patients scheduled forroutine refraction at the King Saud University Optometryclinic. All subjects were pre-presbyopic patients presentingfor a routine eye exam. Those subjects with a positive his-tory for contact lens wear, anterior segment disease or sur-gery, trauma or amblyopia were excluded. Seventy subjects,divided equally between men and women, participated inthis study. Only the right eye was assessed for each subject.The subjects age range was 1835 (24.04.3; mean SD)years. Informed consent was obtained from each subjectbefore themeasurements were carried out, and the studywasconducted in conformance with the ethical considerationslaid out in the 1964 Declaration of Helsinki. To avoid theeffects of diurnal corneal thickness variations, all measure-ments were carried out in the afternoon . Triplicate CCTassessments using the specularmicroscope (TopconSP2000P;Abdulrehman Al-Gosaibi GTB, Riyadh, Saudi Arabia) wereobtained. Corneal images were captured with the subjectfixating on the central target, the subjects chin on the chinrest and his/her head on the forehead rest (in a position iden-tical to that for a slitlamp examination). The automatic imagecapture, low-intensity mode of the specular microscope wasemployed in this study.
The average corneal thickness for each subject formed thedata point for that subject. The data points for all the subjectswere collected and analyzed. The level of significance for allcomparisons was set at 5%.
Assessment of intra- and inter-observer reproducibility
To determine the reliability of amethod, repeated readings bythe same observer and by different observers were compared.
Table 2 Mean differenceswithin- and between-observers
Observer 1(1st2nd readings)
Observer 2(1st2nd readings)
Observer 1observer 2
Mean difference (mm) 0.001 0.0001 0.000Standard deviation 0.008 0.009 0.00895% CI (1.96 SD) 0.015 to 0.017 0.018 to 0.018 0.016 to 0.016
Table 1 Central corneal thickness (CCT) readings determinedby both observers
Observer 1 (1st reading +2nd reading)/2
Observer 2 (1st reading +2nd reading)/2
(Observer 1 +observer 2)/2
Mean CCT (mm) 0.522 0.524 0.522Standard deviation 0.036 0.036 0.03695% CI (1.96 SD) 0.4510.593 0.4530.595 0.4510.593
The Bland and Altman  statistical analysis method wasemployed to assess the limits of agreement (LoA) betweentwo measurements of CCT (either by the same observer orby two different observers) using the same technique. Thedifference between the means of two readings taken on twoseparate days (separated by a time frame of 1 week or less)was plotted against the average readings of the two ses-sions to determine the limits of agreement. Pearsons orSpearman rank correlation coefficient analyses were car-ried out on the scatter plot (differences between meansagainst the combined mean CCT) to exclude a systemat-ic bias of the differences between means with the mag-nitude of the measured CCT.
There was no statistically significant difference (P>0.05)between the CCT values for men and women. Therefore,only the values for one set of measurements (by one ob-server) are quoted for male and female CCT values, and theresults of the entire sample are pooled in the statisticalanalyses of each set of measurements by each observer. Theaverage CCT values for men and women, measured by thefirst observer, were 0.520.03 mm (mean SD) and 0.520.04 mm, respectively. The average for the entire samplewas 0.520.04 mm. The average readings determined byboth observers are summarised in Table 1.The 95% LoA for two sets of CCT measurements made
by each observer and for one measurement each made byboth observers is summarised in Table 2. The mean dif-
Fig. 1 Graph plot of the meandifference, first reading minussecond reading, (ordinate)against the mean corneal thick-ness readings (abscissa) of twoseparate sets of measurementstaken by the first observer. APearson correlation coefficientanalysis (P>0.05) excluded asystemic bias of the meandifference with the magnitudeof the measured central cornealtickness (CCT).
Fig. 2 Mean difference plot forthe second observer. A Spear-man rank correlation coefficientanalysis (P>0.05) excluded asystemic bias of the meandifference with the magnitudeof the measured central cornealthickness (CCT).
ference of the two readings made by the first observer was0.0010.008 mm (mean SD). For the second observer, themean difference was 0.00010.009 mm. The between-observer mean difference was 0.0000.008 mm.The 95% LoA for the two readings made by the first
observer ranged from 0.015 to 0.017 mm. For the secondobserver, the 95% LoA ranged from 0.018 to 0.018 mm.The between-observer 95% LoA ranged from 0.016 to0.016 mm. As a percentage of the mean CCT of two as-sessments, the 95% LoA of the mean difference rangedfrom 2.9 to 3.3% for the first observer, 3.4 to 3.4% for thesecond observer and 3.1 to 3.1% for between observers.Figures 1, 2 and 3 are the mean difference plots for two setsof CCT values measured by the first , second and both ob-servers, respectively.The 95% LoA for the mean differences for each observer
and between observers were all within the range of 1 SD ofthe combined mean of the two observations. For the firstobserver, for example, the 95% confidence interval for themean difference between two assessments ranged from0.015 to 0.017 mm (range=0.032 mm). The average CCTof two sets of readings made by the first observer was 0.522mm (0.036 SD). Therefore, 95% of the differences be-tween both readings would be expected to fall within therange of 1 SD of the mean CCT measured by observer 1.
The mean difference method described by Bland andAltman  is perhaps the most appropriate method forassessment of repeatability and agreement. Thismethodwasemployed in this study to assess the repeatability of CCTmeasures made with the SP 2000 P non-contact specularmicroscope. The measu