Estimating the prevalence of diagnosed diabetes in a health district of Wales: The importance of using primary and secondary care sources of ascertainment with adjustment for death and migration

Aims: To determine diagnosed diabetic prevalence within our district (population 434 398) in 1996 using data from two sources. Methods: A general practice audit comprising data on patients with diabetes from 61 (82%) of 74 general practices was linked to a record linkage-derived patient index in which data from secondary care and other sources underwent a process of probability matching to identify records relating to the same patient and to flag those with diabetes. By linking this dataset to a mortality dataset, patients known to have died before 1996 could be excluded. Age and sex- stratified emigration rates were applied to those identified by the hospital dataset for each year from 1991 onwards. Results: A total of 386988 residents (89.1%) were listed with a general practitioner participating in the audit, of whom 6050 patients were identified as having diabetes in 1996; a prevalence rate of 1.56%. From the hospital-based source, 7639 patients were identified who were alive in 1996, a period prevalence of 1.76%. By combining the two sources, and extrapolating the general practice audit to the population as a whole, a total of 10 530 patients were identified of whom 8735 were confirmed as still resident within South Glamorgan during 1996. This represented a period prevalence of between 2.01% to 2.42%. By applying age and sex-stratified migration rates to the diabetic population identified by hospital sources, a diagnosed diabetic population of 10,004 was identified, a prevalence of 2.3%. Conclusions: This study demonstrates that to calculate the true prevalence of diagnosed diabetes from health sources, it is necessary to use both primary and secondary care sources.