Comparison of Diagnostic Accuracy for Lower-Extremity Amputation Codes During the ICD-9 and ICD-10 Eras in a High-Risk Population of Patients With Diabetes

Abstract

In fiscal year (FY) 2016, the U.S. transitioned to the ICD-10, which includes eight times more amputation procedure codes than the ICD-9. To evaluate trends over time, researchers need assurance that differences over time are not influenced by a change in the coding system. Thus, we aimed to compare the diagnostic accuracy for ascertaining amputations performed during the ICD-9 era compared with during the ICD-10 era, using medical record reviewas a gold standard. This information will be valuable for others who use electronic health records to study lower-extremity amputationand whose data span this transition. The source population was veterans who had a procedure code (ICD or Current Procedural Terminology [CPT]) for an initial toe or ray amputation (hereafter referred to as toe amputations) between FY 2005 and FY 2016 and a diagnosis of diabetes or prescription for a diabetesmedication in the year prior to their toe amputation (1). We randomly sampled 150 veterans from the parent study (1)who had initial toe amputations in FY 2014 and follow-up through the end of FY 2015 (during the ICD-9 era) and 150 veterans who had initial toe amputations in FY 2016 (during the ICD10 era). The outcome of interest was an ipsilateral amputation (atany level) in the year after the toe amputation. Procedure codes were extracted from the Veterans Affairs (VA) Corporate Data Warehouse, which includes data from the VA electronic medical record. To maximize sensitivity for the parent study, we relied on both ICD and CPT codes for identification of amputation procedures (1); there was no change in CPT coding during the study years. Coders reviewed clinical notes (the gold standard) to find information on amputations from the date of the initial toe amputation to 365 days later. Individuals missing information about the initial toe amputation were excluded (n 5 3). The first author (coder 1) developed the abstraction protocol by reviewing all clinical notes for 23 individuals (12 from the ICD-9 era and 11 from the ICD-10 era) in the year after the initial toe amputation. Coders 2 and 3 first coded 10 records (5 from each time period) that were initially coded by coder 1 to ensure consistent coding. Coder 1 reviewed ;10% of the remaining records for quality control. To identify amputations, coders searched for note titles including the words “operation,” “operative,” “surgery,” “podiatry,” “procedure,” or “vascular.” If no relevant notes with the above terms were found, all other noteswere reviewed for evidence of an amputation or postsurgicalhealing.Coder2 reviewed154records and coder 3 reviewed 235 records (including 94 records reviewed by both). Of the 64 records reviewed by coder 1, 33 were also reviewed by coder 2 and 49 were also reviewed by coder 3. Coder 1 resolved any differences in coding between coders 2 and 3 and made a final determination. k, a measure of intercoder agreement, ranged from 0.77 (for coder1vs. coder 2) to0.96 (for coder1vs. coder 3). Weassesseddiagnostic accuracy in the two eras by sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), misclassification, and their respective95%CIs, according to standard formulas, using chart review as a gold standard. Based on medical record review, 49 of 149 (32.9%) subjects in the ICD-9 sample and 47 of 148 (31.8%) in the ICD10 sample had a subsequent ipsilateral amputation. Based on procedure codes, the percentage classified as having a subsequent ipsilateral amputation was