AIM:: To investigate the association of intraocular pressure (IOP) asymmetry with undiagnosed primary glaucoma in rural and urban populations of south India. METHODS:: Chennai Glaucoma Study is a population-based cross-sectional study. The participants were adults, 40 years or older. The prevalence of primary glaucoma was defined using International Society of Geographical and Epidemiological Ophthalmology classification. IOP asymmetry was diagnosed when the difference in IOP between 2 eyes was >3 mm Hg. Analysis included only bilaterally phakic subjects. We excluded subjects with known glaucoma, secondary glaucoma, pseudoexfoliation, ocular trauma, and ocular surgery. RESULTS:: Of 6310 subjects, 3052 (48.4%) were rural residents. The mean age was 52 (standard deviation: 9.5) years. The prevalence of IOP asymmetry increased steadily with increasing patient age. The prevalence of undiagnosed primary glaucoma was 3.4% (95% confidence interval, 2.9-3.8). Undiagnosed primary glaucoma was 3 times more common in subjects with IOP asymmetry than without (odds ratio 3.08, 95% confidence interval, 2.1-4.3). The specificity of IOP asymmetry in detecting undiagnosed primary glaucoma was 92%. CONCLUSIONS:: IOP asymmetry of >3 mm Hg has limited accuracy for the detection of undiagnosed primary glaucoma. The criterion should be combined with other indicators of glaucoma to build the pretest probability of the disease.
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