Validation of venous thromboembolism diagnoses in patients receiving rivaroxaban or warfarin in The Health Improvement Network

PURPOSE: To describe the effect that validation of venous thromboembolism (VTE) coded entries in the health improvement network (THIN) has on incidence rates of VTE among a cohort of rivaroxaban/warfarin users. METHODS: Among 36 701 individuals with a first prescription for rivaroxaban/warfarin between 2012 and 2015, we performed a two‐step VTE case identification process followed by a two‐step case validation process involving manual review of patient records. A valid case required a coded entry for VTE at some point after their first rivaroxaban/warfarin prescription with evidence of referral/hospitalization either as a coded entry or entered as free text. Positive predictive values (PPVs) with 95% confidence intervals (CIs) were calculated using validated cases as the gold standard. Incidence rates were calculated per 1000 person‐years with 95% CIs. RESULTS: We identified 2166 patients with a coded entry of VTE after their initial rivaroxaban/warfarin prescription; incidence rate of 45.31 per 1000 person‐years (95% CI: 43.49‐47.22). After manual review of patient records including the free text, there were 712 incident VTE cases; incidence rate of 14.90 per 1000 person‐years (95% CI: 13.85‐16.02). The PPV for coded entries of VTE alone was 32.9%, and the PPV for coded entries of VTE with a coded entry of referral/hospitalization was 39.8%; this increased to 69.6% after manual review of coded clinical entries in patient records. CONCLUSIONS: Among rivaroxaban/warfarin users in THIN, valid VTE case identification requires manual review of patient records including the free text to prevent outcome misclassification and substantial overestimation of VTE incidence rates.