ICD 10 Coding Alert

Know how the height/weight calculation impacts your coding choices.

You're probably familiar with the basic concept of body mass index (BMI). But do you know how it is calculated? More important, do you know how to use it when coding for obesity and overweight in adults or children?

Whether you're encountering the concept of BMI for the first time, or you're looking to refresh your understanding of how it relates to weight disorders, you'll find all the information you need if you read on.

Know BMI and How to Calculate It

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,  Chelle Johnson, CPMA, CPC, CPCO, CPPM, CEMC, AAPC Fellow, billing/credentialing/auditing/coding coordinator at County of Stanislaus Health Services Agency in Modesto, California, tells coders, "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.

JoAnne M. Wolf, RHIT, CPC, CEMC, AAPC Fellow, Coding Manager at Children's Health Network in Minneapolis, Minnesota, also points out that many electronic medical records (EMRs) will now provide the calculation as well, though she also recommends that "if only the height and weight were documented, I would not recommend assigning a BMI Z code since the BMI was not documented." So, coders should discuss with their providers the best way to handle the calculation in the record.

Reminder: Because they are still growing, children's BMIs are calculated differently. Instead of being a straight height and weight calculation, children's BMIs are expressed as a percentile relative to other children of the same age and sex. In ICD-10, remember to use BMI pediatric codes for persons 2-20 years of age and the adult codes for persons 21 years of age or older.

Know How BMI Affects Code Choice

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, according to Wolf. "If the patient's BMI is also documented, then the specific code from Z68 should be coded," Wolf tells coders.

Know the Other Codes That Link with BMI

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."

But, as with the BMI codes, assigning these codes can pose other problems. "If a clinician documents 'overweight,'" Wolf notes, it is simple enough to code E66.3 (Overweight). But, Wolf goes on, "I do not recommend adding an obesity or overweight diagnosis code just from a BMI calculation." Instead, Wolfonly recommends "coding a diagnosis that the provider actually documented in the medical record."

Johnson agrees, citing the following example: "There may be times that a patient has a high BMI but is not considered overweight or obese, as is the case with some athletes due to muscularity." Consequently, Johnson advises coders to make sure they read the assessment provided by their physician before assigning the code.

Additionally, the CDC does not define what constitutes morbid, or severe, obesity. The consensus opinion in the medical community, however, is that individuals with a BMI greater than 40 would fall into this category, tying Z68.4- to E66.01 (Morbid (severe) obesity due to excess calories) and E66.2 (Morbid (severe) obesity with alveolar hypoventilation). But, as both Wolf and Johnson stress, the diagnosis must be left to the provider.

And as for individuals in the obese category, ICD-10 provides two codes that ascribe causes to the obesity - E66.0- (Obesity due to excess calories) and E66.1 (Drug-induced obesity) - and two general codes - E66.8 (Other obesity) and E66.9 (Obesity, unspecified) - that do not. This can create yet one more coding headache.

Coding caution: Wolf reminds coders that E66.8 and E66.9 are not interchangeable codes. "E66.8 can be used if the provider documented a specific type of obesity not described by any of the other ICD-10-CM codes in this category," Wolf points out, adding that "E66.9 should be used if the clinician documents only 'obesity.'"

And, Johnson adds, "It would be a rare occurrence that the unspecified code would be used, as the physician should always be addressing a BMI of 30.0 of higher with a plan of care that should identify the cause and how to address the condition."