Validity of a procedure to identify patients with chronic idiopathic thrombocytopenic purpura in the Danish National Registry of Patients

BACKGROUND Administrative data may be useful for epidemiological studies of chronic idiopathic thrombocytopenic purpura (ITP). However, the quality of the recorded diagnoses needs evaluation. AIM We evaluated the validity in predicting chronic ITP of the International Classification of Diseases (ICD)-10 diagnoses of ITP in the Danish National Registry of Patients (NRP). METHODS We used the NRP to identify patients with ITP, according to code D69.3, from January 1, 1996 to December 31, 2007. We defined chronic ITP as lasting longer than 6 months by including only patients with 2 or more hospital ITP diagnoses over longer than 6 months. We confirmed diagnoses by evaluating each candidate chronic ITP patient's medical chart and estimating the positive predictive value (PPV) and 95% confidence interval (CI) of the recorded NRP diagnostic code. RESULTS We identified 513 patients with chronic ITP in the NRP. We were able to retrieve the charts of 439. After evaluation of the charts, 410 patients were deemed to have a valid diagnosis of chronic ITP, yielding a PPV of 0.93 (95% CI: 0.91-0.96). CONCLUSION The validity of this procedure to identify chronic ITP patients was high. The NRP is valid for epidemiological studies of patients with chronic ITP.