How to control measurement variation in estimating the mean intraocular pressure reducing effect at a specific point in time.

OBJECTIVE: To clarify how to control measurement variation in estimating the mean intraocular pressure reducing effect at a specific point in time. METHODS: A set of 629 open-angle glaucoma or ocular hypertensive patients, eligible for treatment in both eyes, previously untreated, were reanalyzed at 3 or 6 months of intraocular pressure (IOP) reducing treatment. The trial design had parallel treatment groups; one group receives treatment A and the other group receives treatment B. The standard deviation (SD) of the IOP response variable was calculated based on a single eye per patient, mean of right and left eye (bilaterally identical treatment within each group) or difference between right and left eye (paired-eye difference within each group; one eye receives active treatment and the other eye receives placebo). The effect of using mean of replicated recordings on the SD of the patient group mean IOP was also studied. The results were generalized by applying a measurement variation model. RESULTS: The "paired-eye difference within each group" design was clearly most effective; the SD of the IOP response variable was 2.2 mm Hg. Using mean of the right and left eye in a "design with bilaterally identical treatment in each group" was the next best solution; the SD of the IOP was 3.0 mm Hg. The SD of the IOP was 3.2 to 3.4 mm Hg when based on a single eye per patient. The minimization of the SD gave important reduction of number of patients required to enroll in a new trial. Using a single IOP recording per patient produced similar SD of the patient group mean IOP as using mean of replicated IOP recordings when measured with Goldmann tonometry. CONCLUSION: Calculating the mean IOP by using measurements from both eyes of the patient gives important reduction of the variability.