Background: The Health Improvement Network (THIN) is a new medical records database that contains records from general practices some of which have or continue to participate in the General Practice Research Database (GPRD) and others that never participated in GPRD. We sought to replicate in THIN well-established associations from the medical literature and to compare results from the GPRD practices to the non-GPRD practices within THIN.
Methods: Using THIN data from 1986-2003, we conducted case-control studies of associations between diseases (e.g., hypertension and stroke) and between diseases and drugs (e.g., aspirin and colon cancer). Conditional logistic regression was used to calculate odds ratios adjusted for potential confounders. Differences between GPRD and non-GPRD practices were assessed by testing for a statistical interaction by practice type in each outcome-exposure association.
Results: We observed the expected positive associations (p < 0.05) of stroke with hypertension and diabetes mellitus; of myocardial infarction with hypertension, hypercholesterolemia, obesity, and smoking; and of peptic ulcer disease with aspirin, NSAIDs, and potassium. We observed the expected negative associations (p < 0.05) of colorectal cancer with aspirin, NSAIDs, and cox-2 inhibitors. The expected protective effect of aspirin use for myocardial infarction was not observed. In all cases, the results obtained from the GPRD practices were similar to the results obtained from the non-GPRD practices, only being statistically different for the associations of myocardial infarction with diabetes and aspirin use.
Conclusions: THIN data that are collected outside of the GPRD appear as valid as the data collected as part of the GPRD.