Caveat emptor: The accuracy of claims data in appendicitis research.

Background: International Classification of Disease, ninth/tenth revision codes are used to identify patients with appendicitis and classify severity of disease for research and hospital reimbursement. We sought to determine accuracy of International Classification of Disease, ninth/tenth revision codes in classifying appendicitis as uncomplicated versus complicated (defined as perforated, necrotic, or abscess) compared with the clinical gold standard: surgeon characterization of the appendix in the operative report.
Methods: Retrospective review of operative reports and discharge International Classification of Disease, ninth/tenth revision codes for patients ≥18 years old who underwent noninterval, nonincidental appendectomy between January 2012 and December 2019 at a tertiary referral center. Sensitivity, specificity, and positive predictive value were calculated for International Classification of Disease, ninth/tenth revision codes to classify appendicitis accurately as complicated compared with surgeon description. ICD-9/10 codes and surgeon description were categorized into complicated/uncomplicated based on the American Association for the Surgery of Trauma grading system.
Results: In the study, 1,495 patients with acute appendicitis underwent appendectomy. Per surgeon description, 200 (13%) were complicated and 1,295 (87%) uncomplicated. Compared with surgeon description, discharge International Classification of Disease, ninth/tenth revision codes did not accurately identify complicated appendicitis: sensitivity = 0.68, positive predictive value = 0.77. As a sensitivity analysis, the cohort was stratified by public versus private payers, and the results did not change.
Conclusion: International Classification of Disease, ninth/tenth revision codes do not accurately identify surgeon-described complicated appendicitis. Nearly one-third of the cases of complicated appendicitis were coded as uncomplicated. Such misclassification negatively impacts reimbursement for complicated appendicitis care and could lead to misleading results in research and quality improvement activities that rely on these codes.
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