Objective: The present investigation used data from the Verona Diabetes Study to verify a main assumption of the capture-recapture method (source independence) and to characterize the subgroup of known diabetic patients missed by all sources whose number is estimated by the capture-recapture method.
Research design and methods: The Verona Diabetes Study identified 7,148 type 2 diabetic patients on 31 December 1986 using 3 different sources: family physicians, a diabetes center, and a drug prescription database. Completeness of ascertainment was estimated with traditional methods based on the hypergeometric distribution and with a log-linear model.
Results: Identification sources were not independent because the drug prescription database was positively related to family physicians and negatively related to the diabetes center (P < 0.001). Thus, completeness of ascertainment was overestimated (87.5% [95% CI 86.3-88.8]) when using only family physicians and the drug prescription database and underestimated (45.9% [43.9-48.1]) when using only the diabetes center and the drug prescription database. Because of characteristics contributing to variable "catchability" (probability of ascertainment), the estimated proportion of ascertainment increased with increasing time since diagnosis from 65.6% in the first tertile (<6 years) to 91.5% in the third tertile (>12 years); moreover, the ascertainment was estimated to be nearly complete (97.9%) for insulin-treated patients and scanty (28.9%) for diet-treated patients.
Conclusions: Because identification sources are likely to be dependent, the capture-recapture method should be used with caution in diabetes epidemiology and possibly when at least 3 sources are available. The subgroup of diabetic patients whose existence is inferred by this technique likely consists of newly diagnosed patients with mild disease severity.