Gastroenterology Coding Alert

Question: Our physician has recently started performing gastric bypass surgeries and we’re trying to get a handle on how to calculate BMI. Can you advise?

Codify Subscriber

Answer: Both ICD-10 and the Centers for Disease Control and Prevention (CDC) describe BMI as being “a person’s weight in kilograms divided by the square of height in meters.” So, the calculation for a 5’5” adult that weighs 150 pounds (times a factor of 703 to convert from standard US measurements to metric), would look like this:

(150 ÷ 652) x 703 = 24.96

As a coder, though, you won’t have to get out your calculator whenever you need to know a patient’s BMI. An easy way to calculate this is to go to the BMI calculator available on the CDC website at https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html.

Unfortunately, the ICD-10 codes do not correspond exactly with the CDC’s definitions of underweight, normal, overweight, or obese. The BMI code set Z68.1 (Body mass index (BMI) 19.9 or less, adult), for example, covers adults designated by the CDC as underweight and normal, while the normal range continues through Z68.1 to Z68.24 (Body mass index (BMI) 24.0-24.9, adult). And a CDC designation of obese corresponds with both the Z68.3- and Z68.4- codes, as can be seen in the following table:

This, however, is not a problem for coders. If the patient’s BMI is also documented, then the specific code from Z68 should be coded. Naturally, the BMI codes also link with the overweight and obesity codes found in E66. In fact, a note accompanying E66 in ICD-10 tells you to “use additional code to identify body mass index (BMI) if known.”