Q&A: Reporting adverse effects of inhibitor medicines with ICD-10-CM

Q: A female patient previously had an inpatient stay due to severe enteritis, secondary to the use of Keytruda for colon cancer that she was diagnosed with six months earlier. While she has had positive results from the medication, she presented to the emergency room last week due to severe abdominal bloating and cramping with diarrhea and stools with large amounts of mucous. She was dehydrated and hypokalemic upon inpatient admission and has a history of depression. Her discharge note indicates that her potassium levels and dehydration are normalized. How would this encounter be reported in ICD-10-CM?

A: The ICD-10-CM codes for this encounter are:

• K52.1, toxic gastroenteritis and colitis
• T45.AX5D, adverse effect of immune checkpoint inhibitors and immunostimulant drugs, subsequent encounter
• C18.9, malignant neoplasm of colon, unspecified
• F32.A, depression, unspecified

The enteritis is the focus of care, but we are also looking to see if the medication use is a poisoning or adverse effect as well as what conditions are still present since resolved conditions are not coded.

Nothing in the report says she accidentally took too much of the Keytruda or that she was accidentally given too much. Therefore, this patient was taking the drug as ordered and simply had adverse effects of the drug. The known adverse effects include dehydration and hypokalemia, and on discharge, it indicates that her potassium and dehydration were normalized. These adverse effects would then be considered resolved and not coded. The only adverse effect still present is the enteritis.

At this point, coders should use the 2025 ICD-10-CM Alphabetic Index to find the correct code for enteritis due to drugs. Looking under “enteritis” and “drug-induced” will lead to ICD-10-CM code K52.1. Coders need to check the guidelines for sequencing because if there is an adverse effect, the primary condition is reported first, followed by a code from categories T36-T50 (poisoning by, adverse effects of, and underdosing of drugs, medicaments, and biological substances). Keytruda is a PD-1 inhibitor, leading to subcategory T45.A- (poisoning by, adverse effect of, and underdosing of immune checkpoint inhibitors and immunostimulant drugs). Additionally, this is not the first encounter for this patient’s enteritis due to the use of Keytruda and would be coded as a subsequent encounter. As a result, code T45.AX5D would be reported following the code for the enteritis.  

Nowhere does it say that the patient no longer has colon cancer, and because there's no indication that the malignant neoplasm of the colon has been excised, we are still going to code the neoplasm. Finally, the patient’s depression would need to be coded as well.

Editor’s note: This question and answer were excerpted from the DecisionHealth webinar, “Ensure Compliance with the Final 2025 ICD-10 Code Update,” presented by Brandi Whitemyer, RN, CDIP, COS-C, HCS-D, HCS-O, an independent home health and hospice coding consultant with expertise in post-acute billing and coding education, and was originally published on JustCoding.