Missing laboratory test data in electronic general practice records: analysis of rheumatoid factor recording in the clinical practice research datalink.

PURPOSE: To investigate whether information from the literature could be used to identify periods of practice data in an electronic healthcare database during which rheumatoid factor (RF) test results are likely to be missing-not-at-random (MNAR). METHODS: RF tests recorded in the Clinical Practice Research Datalink (CPRD) were identified and defined as having a positive, negative or missing result. The proportion of positive test results was then calculated based on (i) complete-case analysis (ii) after restriction to tests from practice years with no missing test results and (iii) following multiple imputation of missing test results. The same three analyses were then carried out after excluding practice years with a proportion of positive tests incompatible with the missing completely at random (MCAR) assumption. RESULTS: We identified 127,969 RF test records, 30.4% of which did not have an associated test result. Among tests with results available, 19% were positive. Both multiple imputation of the 38,867 missing test results and restriction of the study population to the 491 practice years with complete data had little impact on the percentage of positive tests. Following exclusion of the 544 practice years in which data were likely to be MNAR the percentage of positive tests in all analyses decreased to ~7%. CONCLUSIONS: Recording of RF tests and RF test results in the CPRD is incomplete, with data likely to be MNAR in many practices. Exclusion of practice years with a high proportion of positive tests brought the distribution of positive tests in the study in line with the literature.