Objective: To evaluate the success of masking study participants to treatment allocation using sham intravitreal injections. Methods: Eyes were randomized to receive sham injections pluspromptlaser, intravitreal ranibizumab injections plus prompt laser, intravitreal ranibizumab injections plus deferred laser, or intravitreal triamcinolone acetonide injections plus prompt laser up to every 16 weeks with sham injections intermittently. All eyes could receive treatment orshamas often as every 4 weeks. Participants with 2 study eyes had 1 eye randomized to sham plus prompt laser and 1eye randomized to a real injection group. Sham injections were performed by pressing the syringe hub against the conjunctiva to mimic a real injection. Laser treatment was not masked. At the 1-year visit, participants were asked if they believed that the injections received during the study were real, sham, or sometimes real and sometimes sham. Results: Among 423 participants with 1 study eye, the correct assignment was stated by 9.9% of the sham plus prompt laser group, 88.0% of the ranibizumab plus prompt laser group, 89.6% of the unmasked ranibizumab plus deferred laser group, and 44.0% of the triamcinolone plus prompt laser group. Among 112 participants with 2 study eyes, the correct assignment was stated for 24.1% of the sham plus prompt laser eyes. Conclusions: Successful masking of an intravitreal injection can be accomplished when a sham injection procedure carefully mimics a real injection procedure. Masking seems less successful when one eye is receiving a real injection and the other eye is receiving a sham injection or when an individual eye receives both real and sham injections.
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