[Validation of secondary data. Strengths and limitations]

Medical records databases (such as the General Practice Research Database-GPRD) and administrative databases (such as German Statutory Health Insurance (SHI) claims data) are useful sources for pharmacoepidemiology and health services research. However, these data are not primarily collected for research purposes. Validation studies are needed to examine their completeness and accuracy depending on the corresponding research question. This article reviews strategies for checks of internal consistency within the data from one SHI as well as between data from several SHIs and possibilities of internal data validation. Descriptive analyses of consistency can help to determine the integrity of data. The aim of internal validation is to separate uncertain from true cases based on information from secondary data alone or to reproduce known associations within the database. In addition external validation of secondary data is desirable using original prescriptions, medical records, hospital discharge letters and/or patient or physician interviews as a gold standard. A considerable number of external validation studies of diagnostic coding have been conducted within the GPRD. In contrast, such validation studies of German SHI claims data are mostly lacking and are urgently needed in the near future.