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.