Estimation of the prevalence of diagnosed diabetes Estimation of the prevalence of diagnosed diabetes from primary care and secondary care source data: from primary care and secondary care source data:

comparison of record linkage with capture-comparison of record linkage with capture-recapture analysisrecapture analysis

Estimation of the prevalence of diagnosed diabetes Estimation of the prevalence of diagnosed diabetes from primary care and secondary care source data: from primary care and secondary care source data:

comparison of record linkage with capture-comparison of record linkage with capture-recapture analysisrecapture analysis

J. N. Harvey, L. Craney, D. KellyJ. N. Harvey, L. Craney, D. Kelly

Group members:Group members:

Suzette Poliquin, Eduardo da Silveira, Eric Lam, Susie LauSuzette Poliquin, Eduardo da Silveira, Eric Lam, Susie Lau

Background InformationBackground InformationBackground InformationBackground 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

Target PopulationTarget 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

Method of Sampling Method of Sampling

Data obtained from:Data obtained from:

A) 3 District General HospitalsA) 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

Method of Sampling Cont’dMethod of Sampling Cont’d

B) B) 74 Practices (Primary Care) 74 Practices (Primary Care)• list of diabetic patients was supplied• reference date - March 1998 must be resident in

catchment area

Case DefinitionCase Definition:

Type 1 diabetes = diagnosis before age 40, on insulin treatment within 1 year from diagnosis (WHO criteria)

ExclusionsExclusions:

Intermediate glucose intolerance and gestational diabetes

Data Base ValidationData 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

Capture - recapture AnalysisCapture - recapture Analysis

Two source model used:Two source model used:

Hospital source: combination of PAS system, hospital clinics, diabetes nurses records

Primary Care Source: combined lists from the general practitioners

Capture-recapture Cont’dCapture-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

CritiqueCritique

1. Study design and sampling method 1. Study design and sampling method appropriate appropriate for the research question?for the research question?

-Yes, however missing patients referred to tertiary care, homeless people

2. Sampling frame appropriate?2. Sampling frame appropriate?

-Yes for comparing source linkage and capture-recapture analysis

- No for establishing trends

CritiqueCritique

3. Sample size adequate?3. Sample size adequate?

-Yes for county Clwyd

-Representativeness of North Wales?

4. Objectives, suitable and standard criteria used for 4. Objectives, suitable and standard criteria used for measurement of outcome?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

CritiqueCritique

5. Outcome measured in an unbiased fashion?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?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

CritiqueCritique

7. Estimates of prevalence or incidence given with 7. Estimates of prevalence or incidence given with CI in detail by subgroups?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 .

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

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

CritiqueCritique

8. Implications of results for action or intervention8. 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