2026 ICD-10-CM Diagnosis Code K59.1Functional diarrhea
ICD-10-CM Codes›K00–K95›K55-K64›K59
- Billable — Valid for Submission
- Not Chronic
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
Code Classification
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
A type 1 excludes note is a pure excludes note. It means "NOT CODED HERE!" An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
- Diarrhea, diarrheal (disease) (infantile) (inflammatory) - R19.7
- functional - K59.1
- neurogenic - K59.1
External Cause of Injuries IndexGuidance
References for this code in the External Cause of Injuries Index.
- Diarrhea, diarrheal(disease) (infantile) (inflammatory)
- functional
- Diarrhea, diarrheal(disease) (infantile) (inflammatory)
- neurogenic
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
Code HistoryHistory
Questions About K59.1Overview
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.
Footnotes
[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.
