ICD-10-CM 2027
ICD-10 code for functional diarrhea
From the official ICD-10-CM alphabetic index entry “Diarrhea, diarrheal (disease) (infantile) (inflammatory) › functional”. Page updated September 29, 2026.
About coding functional diarrhea
The ICD-10-CM code for functional diarrhea is K59.1 (Functional diarrhea). The official index lists 2 more specific codes, so check the documentation for details such as type, cause, site or severity before settling on K59.1.
Within K59 (other functional intestinal disorders), K59.1 is specifically for functional diarrhea. Related codes cover constipation (K59.0), neurogenic bowel, not elsewhere classified (K59.2), megacolon, not elsewhere classified (K59.3), anal spasm (K59.4), other specified functional intestinal disorders (K59.8) and functional intestinal disorder, unspecified (K59.9).
Don't report K59.1 together with the conditions in its Excludes1 note: diarrhea NOS (R19.7) and irritable bowel syndrome with diarrhea (K58.0).
It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.
More specific functional diarrhea codes (official index)
Don't confuse with (Excludes1 for K59.1)
Common questions about functional diarrhea ICD-10 codes
What is the ICD-10 code for functional diarrhea?
- K59.1 — Functional diarrhea. More specific codes apply when the documentation supports them: K91.89, F45.8.
What is the ICD-10 code for psychogenic functional diarrhea?
- F45.8 — Other somatoform disorders.
Is K59.1 billable?
- Yes. K59.1 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).
Can K59.1 be used as a principal diagnosis?
- Yes. The Medicare Code Editor doesn't restrict K59.1 as a principal diagnosis.
Is K59.1 a CC or MCC?
- No. K59.1 is neither a CC nor an MCC under MS-DRG v44.0.