The Validity of Using ICD-9 Codes for Identifying Venous Thromboembolism Following Below Knee Surgery

Category: Other Introduction/Purpose: To determine the incidence of venous thromboembolism (VTE) after surgeries performed below the knee, previous studies have used the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) procedure codes to identify a DVT (deep vein thrombosis) or PE (pulmonary embolism) event. The purpose of this study was (1) to determine the incidence of VTE in a large cohort of patients who had undergone surgery below the knee, and (2) to assess the validity of the ICD-9-CM procedure codes used. Methods: Current Procedural Terminology codes and ICD-9-CM codes were used to identify patients who had undergone below knee surgery and a reported VTE. The medical records of 21,904 patients were evaluated to assess the validity of the VTE ICD-9-CM code against the documentation of DVT or PE in radiology reports. Positive predictive value (PPV) and sensitivity of ICD-9-CM codes were calculated. Results: The incidence of VTE was 2.5% (552/21,904). Of those patients identified as having VTE, 344 (62.3%) had a DVT, 130 (23.6%) had a PE, and 78 (14.1%) were recorded as having both a DVT and PE. The PPV of ICD-9-CM coding were 55.7% (327/587) for DVT, 84.4% (108/128) for PE, 60.2% (59/98) for DVT/PE combined, and 60.8% (494/813) for VTE in total. The resulting sensitivity scores were 95.1% (327/344) for DVT, 83.1% (108/130) for PE, 75.6% (59/78) for DVT/PE combined, and 89.5% (494/552) for VTE in total. Conclusion: The overall rate of VTE after below knee surgery was 2.5%. This study found that ICD-9 code use had an 89.5% sensitivity rate for VTE. Using ICD-9 codes to search for a VTE demonstrated a low PPV, however, because only 60.8% could be verified by a radiological study. By comprehensively assessing the incidence of VTE after below knee surgery, we illustrated a potential source of inaccuracy in the use of retrospective ICD-9 based data capture methodologies.