Modified Test Protocol Improves Sensitivity of the Stereo Fly Test

Stereoacuity measurement is a common element of pediatric ophthalmic examinations. Although the Stereo Fly Test is routinely used to establish the presence of coarse stereopsis (3000 arcsecs), it often yields a false negative "pass" due to learned responses and non-stereoscopic cues. We developed and evaluated a modified Stereo Fly Test protocol aimed at increasing sensitivity, thus reducing false negatives.

The Stereo Fly Test was administered according to manufacturer instructions to 321 children aged 3-12 years. Children with a "pass" outcome (n = 147) were re-tested wearing glasses fitted with polarizers of matching orientation for both eyes to verify that they were responding to stereoscopic cues (modified protocol). The response to the standard Stereo Fly Test was considered a false negative (pass) if the child still pinched above the plate after disparity cues were eliminated. Randot^® Preschool Stereoacuity and Butterfly Tests were used as gold standards.

Sensitivity was 81% (95% CI: 0.75 - 0.86) for standard administration of the Stereo Fly Test (19% false negative "pass"). The modified protocol increased sensitivity to 90% (95% CI: 0.85 - 0.94). The modified two-step protocol is a simple and convenient way to administer the Stereo Fly Test with increased sensitivity in a clinical setting.