In their recently published study, Bozic and colleagues [1] analyzed the effect of 3 different thromboprophylaxis regimens on the 30-day incidence of acute venous thromboembolism (VTE) after total knee arthroplasty, using the proprietary Perspective database. The authors used a risk-adjusted multivariate model that included a propensity score (for aspirin use) to compare outcomes associated with 3 principal thromboprophylactic regimens: aspirin alone, low–molecular-weight heparins, and warfarin. Among the International Classification of Diseases, Ninth Revision, Clinical Modification codes they used to define the outcome of interest, VTE, they included the code V12.51 in their definition of both “any VTE” and “proximal DVT.” Unfortunately, this code is used to indicate a “personal history of venous thrombosis or embolism,” not acute VTE. To determine how frequently this code is used, we analyzed the records of 237 935 cases that underwent knee arthroplasty in the State of California and found that there were 3755 cases (1.55%) with a discharge code of V12.51 for the index surgical hospitalization. Assuming the Perspective cohort that was analyzed is similar to the California cohort, the effect of including V12.51 in the definition of VTE leads to 2 errors: a significantly higher observed incidence of acute VTE and, more importantly, a potential bias against warfarin and/or low–molecular-weight heparin thromboprophylaxis because physicians are more likely to use these agents rather than aspirin in patients with a history of VTE. We suggest that the authors repeat their otherwise comprehensive analysis after eliminating V12.51 as an outcome code for VTE.