Validity of psoriatic arthritis and capture of disease modifying antirheumatic drugs in the health improvement network.

Purpose: The aims of this study are to examine the validity of diagnostic codes for psoriatic arthritis in The Health Improvement Network (THIN) and to examine the agreement between General Practitioner (GP) report and prescription records for disease modifying antirheumatic drugs (DMARDs).

Methods: Questionnaires were sent to the GPs of 100 randomly selected patients with at least one medical record code for psoriatic arthritis. The positive predictive value (PPV) for a GP confirmed diagnosis was calculated, and alternative algorithms were examined to determine which method resulted in the highest PPV.

Results: The PPV for a single code for psoriatic arthritis was 85% (95%CI: 75.8-91.7%). Adding a prescription for a DMARD increased the PPV to 91% but with a substantial loss in sensitivity. Agreement between GPs and prescription data for use of an oral DMARD was 69%.

Conclusions: The diagnosis codes for psoriatic arthritis used in THIN are valid. All prescriptions for DMARDs may not be accounted for in THIN.