An article published by Smith MG, et al. in the Orphanet Journal of Rare Diseases compares two methods to estimate the number of cases of two rare conditions (fragile X syndrome (FXS) and muscular dystrophy (MD)): the basic surveillance through active medical record review and a linked administrative data file.
Active surveillance through medical record abstraction is time consuming and costly. Moreover, passive surveillance for rare conditions that are identified through simple searches of diagnosis codes can lead to an overestimation of cases. The authors aimed to refine the numeric estimates rare conditions by applying capture-recapture methodology to passively collected data.
The major problem of the capture-recapture method applied to passively collected data is the postulate assuming that “multiple sources each have incomplete data on the number of true cases”. In reality, in most of the sources, the number of cases tends to be overestimated due to the entries of misdiagnosed or unconfirmed patients. In order to obtain a better estimate from the passive collection system, the authors first filtered the cases as follows: “cases with at least one in-patient diagnostic code or two outpatient diagnostic codes, and if necessary, restricting to diagnoses made by specialists, who are most likely to diagnose true cases”. Applying this data restriction algorithm successfully led to underestimates for both MD and FXS. The authors subsequently applied the capture- recapture method and reached a very close estimate of numbers of cases obtained with active data collection.
The authors concluded that applying capture-recapture method to passively collected surveillance data for rare health conditions produced an estimate of the number of true cases that was similar to that obtained through active data collection. It can be an useful method to estimate the number of cases of rare conditions in a defined population where administrative data are available, but active data collection is complicated.