2026 ICD-10-CM Diagnosis Code K35.80Unspecified acute appendicitis

ICD-10-CM Codes›K00–K95›K35-K38›K35

ICD-10-CM K35.80
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

K35.80 is a billable ICD-10-CM diagnosis code for unspecified acute appendicitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 393 through 395. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Appendicitis and other appendiceal conditions.

Code Identity

ICD-10-CM Code
K35.80
Billable Status
Yes — Valid for Submission
Code Describes
Unspecified acute appendicitis
Short Description
Unspecified acute appendicitis
Same as the full description in the CMS dataset.
Parent Code
K35.8
Other and unspecified acute appendicitis

Code Classification

ChapterK00–K95Diseases of the digestive system
SectionK35-K38Diseases of appendix
CategoryK35Acute appendicitis
This CodeK35.80Unspecified acute appendicitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Abscess of appendix
  • Acute appendicitis
  • Acute appendicitis with appendix abscess
  • Acute appendicitis without peritonitis
  • Acute fulminating appendicitis
  • Acute gangrenous appendicitis
  • Acute obstructive appendicitis
  • Acute phlegmonous appendicitis
  • Acute suppurative appendicitis
  • Complicated appendicitis
  • Gangrenous appendicitis
  • Purulent enteritis
  • Suppurative appendicitis

Tabular List NotesGuidance

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

Inclusion Terms

  • Acute appendicitis NOS
  • Acute appendicitis without (localized) (generalized) peritonitis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Appendicitis(pneumococcal) (retrocecal)
      • acute (catarrhal) (fulminating) (obstructive) (retrocecal) (suppurative)
    • Gangrene, gangrenous(connective tissue) (dropsical) (dry) (moist) (skin) (ulcer)
      • appendix

Clinical ClassificationClinical

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

CCSR DIG009
Appendicitis and other appendiceal conditions
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Appendicitis

The appendix is a small, tube-like organ attached to the first part of the large intestine. It is located in the lower right part of the abdomen. It has no known function. A blockage inside of the appendix causes appendicitis. The blockage leads to increased pressure, problems with blood flow, and inflammation.

Read the full article at MedlinePlus

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

Convert K35.80 to ICD-9-CMHistory

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

ICD-9-CM
540.9 Acute appendicitis NOS
Approximate The match is approximate rather than exact.

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 K35.80Overview

Is K35.80 (Other and unspecified acute appendicitis) a billable code?

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

What MS-DRG does K35.80 group to?

When unspecified acute appendicitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 393, 394, 395, with relative weights from 0.6490 to 1.5993 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of K35.80?

Under the General Equivalence Mappings, unspecified acute appendicitis converts to ICD-9-CM 540.9 (acute appendicitis NOS). The mapping is approximate, so confirm the match fits the documentation.

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.