Validation of self-reported diagnosis of diabetes in the 1946 British birth cohort
The aim of this study was to validate self-reported diabetes and age at diagnosis among a sample of the British population, using general practitioners (GPs) as the reference standard. Using data from the Medical Research Council National Survey of Health and Development (NSHD), self-reported diabetes was determined either in response to a direct question at five follow-ups between 1977 and 2010, or from other self-reported medical information. A validation questionnaire was sent to the GP for all participants who reported a diagnosis of diabetes and gave permission to contact their GP (172). The validity of self-reported diabetes was assessed by calculating the percentage of self-reported diabetes cases that were confirmed by their GP, i.e. the positive predictive value (PPV). The difference between self-reported and GP-confirmed age at diagnosis was analysed with a Bland-Altman plot. Completed questionnaires were obtained from 157 GPs (91.2%). Of these, 149 confirmed their patient self-reported diabetes diagnosis (PPV=94.9%). Results were similar when self-reported diabetes was assessed by responses to direct questions only (PPV=95.4%). The average difference between self-reported and GP-reported age at diagnosis was 0.6 years (95% CI 0.2-1.1). We conclude that among the British population questionnaires are a valid method to assess GP-diagnosed diabetes, as measured by responses to a direct question or by patient-reported medical information.