Checking IOP: Comparing iCare and GAT Results

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This month, News in Review highlights selected papers from the original papers sessions at AAO 2022. Each was chosen by the session chairs prior to the meeting because it presents important news or illustrates a trend in the field. Only three subspecialties are included here; papers sessions also were held in six other fields.

THE COVID-19 pandemic sparked a re-evaluation of the iCare tonometer, a handheld portable device that mea­sures IOP without topical anesthetic and minimizes eye surface contact. In light of the possibility that iCare might also minimize viral transmis­sion, researchers at Wills Eye Hospital in Philadelphia evaluated the tonometer’s accuracy by comparing it to the gold standard, Goldmann applanation tonometry (GAT).¹

The researchers “did this study to investigate if iCare is an accurate method to measure IOP in the midst of COVID-19—and if it truly could be a substitute for GAT,” said Shreya Swamina­than, BS, at Sidney Kimmel Medical College. Both devices touch the surface of the eye, although iCare’s diameter is smaller.

IN PURSUIT OF ACCURACY. How should IOP be measured? Although GAT (shown here) is considered the gold standard, the iCare tonometer may also be used.

A retrospective assessment. For this retrospective cohort study, the research­ers considered clinical data from 350 patients (700 eyes) seen between March 2020 and March 2021. They chose 350 right eyes for final analysis, repeating the analysis in the left eye for verifica­tion.

Initially, IOP was measured by oph­thalmic technicians using iCare. Repeat measurements were performed within 90 minutes by a glaucoma specialist using GAT. Primary outcome measures included the correlation between iCare and GAT IOP readings and factors influencing the differences in those readings.

IOP correlation. IOP readings from both tonometers showed high levels of correlation, with an intraclass correlation coefficient (ICC) of .899. Bland-Altman plots indicated a 95% limit of agreement between –5.8 to 5.5 mm Hg, suggesting the devices are not interchangeable for measuring IOP. This trend was consistent in eyes with IOPs both above and below 21 mm Hg.

Additional results. Other findings included the following:

• Greater central corneal thickness (CCT) was significantly associated with increased divergence in IOP measure­ments obtained with the two approaches.
• For each decade of increase in age, the IOP reading was .4 mm Hg lower with iCare than with GAT.
• Greater variation between the two tonometers was noted at higher IOP ranges. Specifically, there were signifi­cantly more patients with an IOP dif­ference greater than 3 mm Hg among those with IOP ≥21 mm Hg than among those with IOP ≤21 mm Hg.
• Factors not associated with differ­ences in IOP measurements achieved with the two approaches included prior glaucoma surgery, mean retinal nerve fiber layer thickness, axial length, and factors involving the technicians (i.e., years of practice and level of certifica­tion).

A caution: not interchangeable. Despite the high intraclass correlation between the two devices, they cannot be used interchangeably, Ms. Swami­nathan said. “The fact that more than 20% of patients had a difference of IOP readings greater than 3 mm Hg is strik­ing and indicates that although iCare is close most of the time, it may not be an appropriate substitute for all patients,” she commented.

What’s next? Ms. Swaminathan stressed that further research needs to be done to find a method of measuring IOP that is both accurate and carries a low risk for viral transmission. “While iCare may not be completely accurate when it comes to IOP measurement, it may provide a clinical picture in an en­vironment in which virus transmission is a large concern.”

—Miriam Karmel

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1 Swaminathan S et al. A comparison of iCare and Goldmann applanation tonometry mea­surements during the COVID-19 pandemic: a retrospective study. Paper presented at: AAO 2022; Oct. 3, 2022; Chicago.

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Relevant financial disclosures—Ms. Swamina­than: None.

For full disclosures and the disclosure key, see below.

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