New BMI Codes for FY 2027

Every October brings a number of ICD-10-CM updates, but one small change this year is worth slowing down for. Effective October 1, 2026, CMS added two new codes at the underweight end of the BMI scale, and if your coders and physicians haven’t talked about it yet, this is a good time to start.

What's new

For years, coders have had granular BMI codes across the overweight and obese range, but only one code, Z68.1 (Body mass index [BMI] 19.9 or less, adult), to capture the entire underweight population.  That changes this year.  CMS is applying the same specificity to underweight patients that’s long existed for obesity. That specificity matters clinically. A BMI of 15 and a BMI of 19 tell very different stories about a patient’s nutritional risk, and now it matters for coding too.

ICD-10-CM Codes

The rule that hasn't changed

New codes are the headline. The guideline underneath them is what actually determines whether you can report them at all, and it’s the same rule that’s applied to BMI coding for years: A BMI code can only be assigned when there is an associated, reportable diagnosis, such as obesity, underweight status, or a related nutritional condition, documented by the patient’s provider.

BMI alone is a data point, not a diagnosis. It’s calculated from height and weight, often captured by a nurse or a device, and it isn’t something a coder can assign on its own. It has to be attached to a condition the provider has actually diagnosed.

That means Z68.18 and Z68.19 only get reported when the provider has documented something like malnutrition, underweight status or another condition consistent with that BMI value, not simply because the number appeared in the vitals flowsheet.

When BMI fluctuates during the stay

One item of note on the existing guideline is worth a refresher, since it comes up often on multi-day admissions: when a patient’s BMI is documented at more than one value during the same encounter for the same associated condition, assign the code for the most severe value, per Coding Clinic guidance. A patient admitted at a BMI of 17.9 who improves to 19.1 by discharge is still coded at the more severe end of that range, because that’s the value reflecting the acuity of the condition being treated. One exception worth flagging: BMI codes are not assigned during pregnancy.

What to confirm before assigning Z68.18 or Z68.19

• Is there a documented, reportable diagnosis linked to the BMI value, not just the number itself?
• If BMI was recorded more than once during the stay, have you identified the most severe value tied to the associated condition?
• Is the diagnosis coming from the provider, not just a nursing assessment or vitals entry?
• Does the documentation distinguish underweight status from malnutrition?

These are related, but not interchangeable, findings.  That last question deserves the most attention. A low BMI is often a clue that something more clinically significant is going on, and determining whether it rises to a diagnosis, and whether the documentation supports one, gets complicated fast.

Key Takeaways

• Two new FY 2027 codes, Z68.18 and Z68.19, give underweight adult BMI the same specificity long available for obesity, expanding on the single Z68.1 code.
• A BMI code can be assigned only when the provider documents an associated, reportable diagnosis; the BMI value alone is not enough.
• When BMI is documented at more than one value during an encounter, the most severe value tied to the associated condition is coded, and BMI codes are never assigned during pregnancy.
• Underweight status and malnutrition are related but not interchangeable, so documentation must clearly support whichever diagnosis is reported.

Works Cited

• ICD-10-CM Official Guidelines for Coding and Reporting, Section I.C.21.c.3
• CMS FY 2027 ICD-10-CM code updates, effective October 1, 2026
• AHA Coding Clinic guidance on BMI reporting with fluctuating values