Bedard NA, Carender CN, DeMik DE, et al. - This study was conducted to examine the effect of transitioning from International Classification of Diseases, Ninth Revision (ICD-9) to ICD-10 on reported complications rates following primary total knee arthroplasty. Researchers evaluated the Humana administrative claims database from 2-years prior to October 1, 2015 (ICD-9 cohort) and for 1-year after this date (ICD-10 cohort) to distinguish all total knee arthroplasty (TKA) procedures. Using the respective coding systems, complications occurring within 6 months of surgery were captured. They compared the incidence of each complication between cohorts applying risk ratios and 95% confidence intervals. In the ICD-9 cohort, there were 19,009 TKAs in the ICD-10 cohort and 38,172 TKAs. The results exhibited that the transition from ICD-9 to ICD-10 coding has altered the reported incidence of complications following TKA. These outcomes are likely due to the added complexity of ICD-10 which is joint and laterality specific. It is necessary to acknowledge the variations between coding systems as this data is used for quality initiatives, risk adjustment models, and clinical research. The results demonstrated that thoughtful methodology will be necessary when ICD-9 and ICD-10 data are being analyzed simultaneously.