2026 ICD-10-CM Diagnosis Code K59.1Functional diarrhea

ICD-10-CM Codes›K00–K95›K55-K64›K59

ICD-10-CM K59.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

K59.1 is a billable ICD-10-CM diagnosis code for functional diarrhea. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 391 through 392. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other specified and unspecified gastrointestinal disorders.

Code Identity

ICD-10-CM Code
K59.1
Billable Status
Yes — Valid for Submission
Code Describes
Functional diarrhea
Short Description
Functional diarrhea
Same as the full description in the CMS dataset.
Parent Code
K59
Other functional intestinal disorders

Code Classification

ChapterK00–K95Diseases of the digestive system
SectionK55-K64Other diseases of intestines
CategoryK59Other functional intestinal disorders
This CodeK59.1Functional diarrhea

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Functional diarrhea

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Type 1 Excludes

  • diarrhea NOS R19.7
  • irritable bowel syndrome with diarrhea K58.0

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR DIG025
Other specified and unspecified gastrointestinal disorders
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Diarrhea

Diarrhea is loose, watery stools (bowel movements). You have diarrhea if you have loose stools three or more times in one day. Acute diarrhea is diarrhea that lasts a short time. It is a common problem. It usually lasts about one or two days, but it may last longer. Then it goes away on its own.

The full article covers:

  • What is diarrhea?
  • What causes diarrhea?
  • Who is at risk for diarrhea?
  • What other symptoms might I have with diarrhea?
  • When do I need to see a health care provider for diarrhea?
  • How is the cause of diarrhea diagnosed?
  • What are the treatments for diarrhea?
  • Can diarrhea be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert K59.1 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
564.5 Functional diarrhea
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About K59.1Overview

What is the ICD-10 code for functional diarrhea?

The ICD-10-CM code for functional diarrhea is K59.1 (sometimes written as K591). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

Is K59.1 (Other functional intestinal disorders) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report functional diarrhea on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does K59.1 group to?

When functional diarrhea is the principal diagnosis on an inpatient stay, it groups to MS-DRG 391, 392, with relative weights from 0.7796 to 1.2683 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of K59.1?

Under the General Equivalence Mappings, functional diarrhea converts to ICD-9-CM 564.5 (functional diarrhea). The mapping is a direct match.