Single vs multiple intraocular pressure measurements in glaucoma surgical trials.

Importance: Little is known about the necessity of multiple same-day intraocular pressure (IOP) measurements in describing the effect of IOP-lowering surgical procedures, and such evidence could affect surgical trial recruitment and retention of participants.
Objective: To determine whether a single IOP measurement might adequately approximate the mean of several measurements in glaucoma surgical trials.
Design, Setting, and Participants: A prospective, multicenter, interventional cohort from the prerandomization phase of a randomized clinical trial evaluating use of a supraciliary implant for treatment of IOP was conducted at multiple ophthalmology clinics. A total of 609 patients (609 eyes) with primary open-angle glaucoma and cataract were included.
Interventions: One IOP measurement was made while patients were receiving their usual medications to lower IOP, and 3 IOP measurements were made at 8 am, 12 pm, and 4 pm after patients underwent washout of all IOP-lowering eyedrops.
Main Outcomes and Measures: The proportion of eyes in which the increase in IOP after washout, using the mean of the 3 measurements, differed by more than 0.5, 1.0, 1.5, or 2.0 mm Hg from the increase in IOP after washout using only 1 of the postwashout measurements. A proportion of 10% or less at the 1.5-mm Hg cutoff was considered clinically acceptable. The hypothesis was formulated after data collection but before the data were examined.
Results: The mean (SD) IOP before washout was 18.5 (4.0) mm Hg. The mean increase in IOP after washout, using the mean of the 3 measurements, was 5.3 (4.2) mm Hg. The percentage of eyes in which the increase in IOP using a single postwashout IOP differed from the increase in IOP using the mean of 3 measurements by more than 1.5 mm Hg was 35.1%, 25.6%, 34.2%, 30.0%, and 31.4% when the single measurement was made at 8 am, 12 pm, 4 pm, a randomly chosen single measure of those 3 times, and the time closest to that of the prewashout IOP, respectively. By logistic regression, the 12 pm postwashout IOP had the lowest proportion of eyes differing from the mean (P < .001) and thus most closely approximated the mean diurnal IOP.
Conclusions and Relevance: Although eliminating multiple IOP measurements would simplify the conduct of surgical trials in glaucoma, our data show that using a single IOP measurement after washout does not adequately approximate the mean of multiple IOP measurements.