Issues with randomizing second eyes in a large trial to evaluate preoperative medical testing prior to cataract surgery

Document Type

Journal Article

Publication Date



Investigative Ophthalmology and Visual Science






Purpose. To examine the statistical and practical implications of including 2nd eye surgeries in a clinical trial to evaluate the impact of routine preoperative medical testing prior to cataract surgery on major medical events occurring perioperatively or within 7 days of surgery. Methods. Different options are identified and logistics and analysis issues surrounding each option are discussed. Results. In order to detect a modest difference in rates of rare major medical events between the tested and untested groups, 20,000 surgeries must be randomized. About 30% of surgeries during this time period are estimated to be done on 2nd eyes. Different options for dealing with 2nd eye surgery are: exclusion of patients who plan to undergo surgery in both eyes during the study period, exclusion of 2nd eye surgeries that occur during the trial, exclusion of 2nd eye surgeries done within a specified interval of 1st surgery, and rerandomization of the 2nd eye surgery. Conclusions. The final decision was to exclude 2nd eye surgeries done within 28 days of the 1st eye, but to rerandomize all other 2nd eye surgeries. Analysis can be done using Generalized Estimating Equations and the association between outcomes estimated with pairwise odds ratios. An anticipate small positive correlation is likely to have minimal impact on power and effective sample size.

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