1. Estimation of the prevalence of diagnosed diabetes from primary care and secondary care source data: comparison of record linkage with capture- recapture analysis J. N. Harvey, L. Craney, D. Kelly Group members: Suzette Poliquin, Eduardo da Silveira, Eric Lam, Susie Lau

2. Background Information Objectives: 1. Compare GP records and hospital records with capture-recapture analysis in determining the prevalence of type 1 and type 2 diabetes in a UK white population (population based surveillance) 2.Assess whole population trends in diabetes prevalence and treatment by comparison with previous studies

3. Target Population Total population from county of Clwyd, North Wales, defined by postal code and resident as of March 1998  total population 418,200 people > 99% white

4. Method of Sampling Data obtained from: A) 3 District General Hospitals Patient Administration Computer Systems (PAS) patients coded as having diabetes (ICD 9 code 250) Hospital diabetes clinics -type of diabetes coded at this stage Diabetes nurses records of patients having attended diabetic clinic in previous 15 years

5. Method of Sampling Cont’d B) 74 Practices (Primary Care) list of diabetic patients was supplied reference date - March 1998 must be resident in catchment area Case Definition Case Definition: Type 1 diabetes = diagnosis before age 40, on insulin treatment within 1 year from diagnosis ( WHO criteria ) Exclusions Exclusions: Intermediate glucose intolerance and gestational diabetes

6. Data Base Validation One year after starting the study, further inquiries made of all patients whose name did not appear in more than one source or type of diabetes and treatment not listed Used hospital and primary care records and the NHS Wales administrative Register to eliminate those who did not have diabetes, had died or were not resident in the catchment area on the reference date of March 1998

7. Capture - recapture Analysis Two source model used: Hospital source: combination of PAS system, hospital clinics, diabetes nurses records Primary Care Source: combined lists from the general practitioners

8. Capture-recapture Cont’d Predicted number of cases (N) calculated from a formula (LaPorte) to give the overall unadjusted prevalence for all patients. Age adjusted prevalence rates calculated for each five year age by gender and diabetes type

9. Critique 1. Study design and sampling method appropriate for the research question? -Yes, however missing patients referred to tertiary care, homeless people 2. Sampling frame appropriate? -Yes for comparing source linkage and capture-recapture analysis - No for establishing trends

10. Critique 3. Sample size adequate? -Yes for county Clwyd -Representativeness of North Wales? 4. Objectives, suitable and standard criteria used for measurement of outcome? - WHO criteria: Type 1 diabetes = diagnosis before age 40, on insulin treatment within 1 year from diagnosis -Over estimation of type 1 diabetics

11. Critique 5. Outcome measured in an unbiased fashion? -Information bias, missing type 2 diabetics not having consulted a physician or not treated in hospital 6. Is the response rate adequate? Refusers described? -Response rate adequate (8,877) -Refusers were considered to be the unclassified(301), no further description given

12. Critique 7. Estimates of prevalence or incidence given with CI in detail by subgroups? Using the capture-recapture methodology, age specific prevalence for type 1 and type 2 diabetes was given by gender, with the 95% confidence intervals.

13. Figure 1 Age and gender specific prevalence of type 1 diabetes calculated by capture-recapture analysis. Error bars represent 95% confidence intervals. Harvey et al, Journal of Epidemiology and Community Health 2002; 56:18-23

14. Figure 2 Age and gender specific prevalence of type 2 diabetes calculated by capture-recapture analysis. Error bars represent 95% confidence intervals Harvey et al, Journal of Epidemiology and Community Health 2002;56:18-23

15. Critique 8. Implications of results for action or intervention Effective monitoring is important for development of disease prevention programs; allocation of health-care resources i.e. Type 1 diabetes in men between age 35 and 45 Type 2 diabetes in men after age 45