Objective: To assess the extent to which incidence rates calculated for common ocular diseases by using claims data may be overestimated according to the length of the disease-free look-back period used in the analysis.

Methods: Retrospective longitudinal cohort analysis. Methods: Billing records of 2457 persons continuously enrolled for 11 years in a managed-care network were searched for International Classification of Diseases (ICD-9-CM) diagnoses of cataract, open-angle glaucoma (OAG), nonexudative age-related macular degeneration (ARMD), and nonproliferative diabetic retinopathy (NPDR) at eye-care visits in the first half of 2001, the second half of 2010, and 2011. For each condition, incidence rates calculated by using "look-back" periods ranging from 0.5-9 years were compared with best estimates from a gold-standard period of 9.5 years.

Results: With a 1-year disease-free look-back period, incidence was overestimated by 260% for cataract, 135% for OAG, 209% for ARMD, and 300% for NPDR. Expanding the disease-free look-back period to 3 years resulted in a reduction of incidence overestimation to 40% for cataract, 14% for OAG, 45% for ARMD, and 100% for NPDR. A 5-year look-back period yielded incidence rates that were overestimated by <30% for all 4 conditions.

Conclusions: In our claims-data analysis of 4 common ocular conditions, a disease-free interval ≤1 year insufficiently distinguished newly diagnosed from pre-existing disease, resulting in grossly overestimated incidence rates. Using look-back periods of 3-5 years, depending on the specific diagnosis, yielded considerably more accurate estimates of disease incidence.