Classifying Multimorbidity Using Drug Concepts via the Rx‐Risk Comorbidity Index: Methods & Comparative Cross‐sectional Study

Abstract

ObjectiveTo update a method to identify comorbid conditions using only medication information in circumstances where diagnosis codes may be under‐captured, such as in single‐specialty EHRs, and to compare the distribution of comorbidities across Rx‐Risk vs. other traditional comorbidity indices.MethodsUsing First Databank (FDB), RxNorm, and its web‐based clients, RxNav and RxClass, we mapped Drug Concept Unique Identifiers (RxCUIs), National Drug Codes (NDCs), and Anatomical Therapeutic Chemical (ATC) codes to Rx‐Risk, a medication‐focused comorbidity index. In established RA and OA cohorts within the Rheumatology Informatics System for Effectiveness (RISE) registry, we then compared Rx‐Risk with other comorbidity indices, including the Charlson Comorbidity Index, Rheumatic Disease Comorbidity Index (RDCI), and Elixhauser.ResultsWe identified 965 unique ingredient RxCUIs representing the 46 Rx‐Risk comorbidity categories. After excluding dosage form and ingredient related RxCUIs, 80,911 unique associated RxCUIs were mapped to the index. Additionally,187,024 unique NDCs and 354 ATC codes were obtained and mapped to the index categories. When compared to traditional comorbidity indices in the RA cohort, the median score (median/25th,75th percentiles) for Rx‐Risk: 6.00 (2,9), was much greater than for Charlson: 0 (0,0), RDCI: 0 (0,0), and Elixhauser: 1 (1,1). Analyses of the OA cohort yielded similar results. For patients with a Charlson score of 0 (85% of total), both the RDCI and Elixhauser were close to 1, but the Rx‐Risk score ranged from 0 to 16 or more.ConclusionsThe misclassification and under ascertainment of comorbidities in single‐specialty EHRs can largely be overcome by using a medication‐focused comorbidity index.This article is protected by copyright. All rights reserved.