ICD-10 Classification of Obesity: Can Someone Please Explain?

The International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) is a widely-used coding of diseases and signs, symptoms, abnormal findings, complaints, social circumstances and external causes of injury or diseases, as classified by the World Health Organization (WHO).

Classification for obesity can be found in Chapter IV (Endocrine, nutritional and metabolic diseases) under the subsection Obesity and Other Hyperalimentation (Codes E66.0 to E66.9).

These are defined as follows:

E66.0 Obesity due to excess calories

E66.1 Drug-induced obesity

E66.2 Extreme obesity with alveolar hypoventilation (Pickwickian syndrome)

E66.8 Other obesity (Morbid obesity)

E66.9 Obesity, unspecified (Simple obesity NOS)

Here is why I do not understand this classification:

Well firstly, as far as I know, every form of weight gain requires an imbalance between energy intake and output. This means that all forms of obesity result from “excess” calories – i.e, if you define “excess” as exceeding caloric expenditure. Code E66.0 would therefore apply to all patients with obesity.

Weight gain in E66.1, which refers to drug-induced obesity, is of course also related to caloric “excess” – whether this results from hyperphagia (e.g. clozapine) or reduced metabolic rate (e.g. beta-blockers). When do you decide that someone is gaining weight solely because of a medication? More often than not there are other predisposing factors (e.g. sedentariness, depression, etc.).

Why the “coders” decided to make a special case for the rather rare alveolar hypoventilation (E66.2) but ignored all other major complications of severe obesity (e.g. heart failure, sleep apnea, osteoarthrits, etc.), at least to me, makes no sense.

But things get even more confusing!

E66.8 refers to “other obesity” – does this mean obesity not caused by caloric excess? Then none of my patients would ever qualify for this code. Or does E66.8 apply only to “morbid obesity” – which is not a WHO definition, unless they actually mean Class III obesity – then what about Class II obesity in case of a patient who is “morbid” because he has type 2 diabetes? Would this person qualify?

To fully add to the confusion, you can also choose code E66.9, i.e. “unspecified” or “simple” obesity – again, do they mean not resulting from caloric excess? Or do they perhaps mean patients with no complications? Or does ”simple” refer to cases where the cause is not “complicated” (as in, “this fellow “simply” eats too much”)?

Overall, to me this classification makes no sense whatsoever. It is neither consistently based on etiology nor is it clinically meaningful in any sense of the word.

Take for example the case of someone with a BMI of 42 due to “simple” overeating who gains another 40 lbs after starting on clozapine resulting in numerous complications including alveolar hypoventilation. Would this patient’s obesity have to be classified as E66.0, E66.1; E66.2 or E66.8? Or, would this “simply” be E66.9. Or perhaps all of the above?

None of this makes any sense to me!

If there is anyone out there who can help me interpret these codes and show me how to use them to classify my patients in a way that would be clinically relevant, I’d be more than grateful.

In the meantime, I will continue using my own classification of obesity (which we now internally refer to as the “Edmonton Obesity Classification”). Although this system is not meant to provide insights into etiology, at least it allows me to clinically grade the amount of weight and to stage the extent of disease in every patient.

Edmonton, Alberta