Is ICD-9 Coding of Morbid Obesity Reliable in Patients Undergoing Total Knee Arthroplasty?

Morbid obesity is considered to have a stronger association with complications after total knee arthroplasty (TKA). Although the impact of obesity coding errors has been previously reported, the extent of coding inaccuracies with respect to morbid obesity is unclear. Therefore, the purpose of this study was to assess (1) the utility of coding in identifying morbid obesity and (2) the effects of morbid obesity on 90-day complications after TKA when morbid obesity was defined by both body mass index (BMI) and International Classification of Diseases 9th edition (ICD-9) coding. A total of 18,030 primary TKAs performed at a single institution from 2004 to 2014 were identified. Patients were defined as morbidly obese based on ICD-9 codes or by BMI recorded in the electronic medical record (EMR). Patients were defined as obese (ICD-9 codes 278.0, 278.00, 278.01, 278.03, 649.10-14, 793.91, V85.30-39, V85.41-45, V85.54) or morbidly obese (278.01, V85.41-45) by ICD-9 codes. Patient EMRs were also reviewed to identify obese and morbidly obese patients (BMI cutoffs of 30 and 40 kg/m², respectively). Complications between the cohorts were compared. Sensitivity and specificity were also calculated. Among the 2,880 surgeries performed in morbidly obese patients, a code for obesity was present in 1,618 (56.2%) surgeries, but only 57.9% (937) of these patients had a code specific for morbid obesity, with the rest having a code not specifying morbid obesity. The sensitivity and specificity of obesity coding were 34.5 and 96.0%, while that of morbid obesity were 32.5 and 96.7%, respectively (area under curve: 0.65 vs. 0.65, p = 0.214). A higher rate of complications was noted when patients were defined as morbidly obese by ICD-9 as when defined by EMR-reported BMI. Although morbidly obese patients are more likely to have a code for obesity compared with obese patients, these patients may not be correctly identified as morbidly obese due to a lack of specificity in the codes. These errors may lead to inadequate reimbursements, and may also overestimate the effect of morbid obesity on complications.