Historically, a body mass index (BMI) ≥ 40 has been defined as “morbid” or “extreme” obesity. The World Health Organization (WHO) categorizes obesity by three classes with Class 3 “morbid” or “severe” obesity as BMI ≥ 40.

The “Practical Guide to the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults” was developed by the National Institutes of Health (NIH) and the North American Association for the Study of Obesity using evidence-based methodology for assessing and treating overweight and obese patients.

This report introduced the term “clinically severe” obesity. Clinically severe obesity is defined as: BMI ≥ 40 or a BMI ≥ 35 with serious comorbid conditions that may warrant weight loss surgery. 

According to the NIH report, those diseases or conditions that denote serious comorbid conditions and high absolute risk are established coronary heart disease, other atherosclerotic diseases, type 2 diabetes, and sleep apnea. Three or more of the following risk factors also confer high absolute risk: hypertension, cigarette smoking, high low-density lipoprotein cholesterol, low high-density lipoprotein cholesterol, impaired fasting glucose, family history of early cardiovascular disease, and age (male ≥ 45 years, female ≥ 55 years).

The NIH definition conforms to the indications for bariatric surgery. The NIH further indicates that a BMI of 35-39.9 without comorbidity is “obesity.”

From a coding standpoint, ICD-10 code E66.01 includes both “morbid” and “severe” obesity. If morbid or severe obesity is documented for a patient with a BMI 35-39.9 and who has a serious comorbid-related condition, code E66.01 is assigned. On the other hand, if no serious comorbid condition is documented within the record, code E66.01 is not clinically valid.  

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