Titmus Fly Test Limitations for Stereopsis Assessment
The Titmus Fly test is NOT reliable for assessing true stereopsis because it contains significant monocular cues that produce false-positive results in up to 6% of patients, making it unsuitable for accurate clinical assessment of either fine or coarse stereopsis. 1
Key Problems with the Titmus Fly Test
False-Positive Results Undermine Clinical Utility
The Titmus Fly test produces false-positive responses in 6% of patients with strabismus who cannot have true stereopsis, specifically those with deviations greater than 20 prism diopters where stereopsis is impossible 1
The test's monocular clues allow patients to identify the "correct" answer without actually using binocular disparity cues, particularly problematic in patients with childhood-onset strabismus who have been repeatedly exposed to the test 2
Sensitivity is only 79% with specificity of just 26% when properly evaluated with modified presentation techniques designed to eliminate false positives 2
The Fly Test Measures Only Coarse Stereopsis (3000 arcseconds)
The Titmus Fly component specifically tests coarse stereopsis at 3000 arcseconds of arc, not fine stereopsis 3
Fine stereopsis requires testing at much smaller disparities (typically 40-400 arcseconds), which the Fly plate cannot assess 1
For fine stereopsis assessment, the Titmus Circles (levels 5-9) or other tests like the Preschool Randot test at 400 arcseconds or better are required 1
Clinical Recommendations
When the Fly Test Must Be Used
If the Titmus Fly is the only available test for patients with manifest strabismus, modify the presentation by retesting with polarizers of matching orientation for both eyes to verify the patient is responding to true stereoscopic cues rather than monocular clues 3
This modified two-step protocol increases sensitivity from 81% to 90% by eliminating false-negative "pass" results 3
Superior Alternative Tests
The Frisby test produces zero false-positive results and is particularly useful for rapid assessment of whether stereopsis is present or absent 1
The Preschool Randot test at 400 arcseconds or finer is valuable for quantifying true stereopsis in both children and adults 1
Critical Clinical Pitfall
- True stereopsis is rare when horizontal deviation exceeds 4 prism diopters, so any positive Fly test result in a patient with strabismus should be viewed with extreme skepticism 1