ICD-10 Code K59.1 – Functional diarrhea (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code K59.1 – Functional diarrhea
What it is
K59.1 identifies functional diarrhea, meaning loose or frequent stools without an identified structural or inflammatory cause. It is used when the diagnosis is based on symptoms and clinical evaluation.
Clinical signs
Typical findings include recurrent loose stools, increased stool frequency, and a pattern that does not suggest infection, inflammation, or malabsorption. Clinical features vary; refer to documentation.
When to use this code
Use K59.1 when the provider documents functional diarrhea or a similar nonorganic diarrhea diagnosis. It fits cases where evaluation does not reveal another specific cause and the chart supports a functional disorder. If the record names another condition, code that diagnosis instead.
Do not use for
Do not use this code for diarrhea due to infection, inflammatory bowel disease, medication effects, or other documented organic causes. Check documentation if the cause is unclear.
Coding tip
Code K59.1 only when the provider clearly links the diarrhea to a functional diagnosis rather than a confirmed underlying disease.