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K35.80 ICD-10-CM Code: Unspecified acute appendicitis

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
2 inclusion terms

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v44, Appendix B.

  • MS-DRG 393 — OTHER DIGESTIVE SYSTEM DIAGNOSES WITH MCC (MDC 06)
  • MS-DRG 394 — OTHER DIGESTIVE SYSTEM DIAGNOSES WITH CC (MDC 06)
  • MS-DRG 395 — OTHER DIGESTIVE SYSTEM DIAGNOSES WITHOUT CC/MCC (MDC 06)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for K35.80 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2027Effective: October 1, 2026

Trace:FY2027 changesChange historyRelease, file and checksum

Inclusion Terms

Alternative terms the tabular list files under this code.

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

Coder workflow for K35.80

MedCoder structured workflow — derived from this code’s own official record

Before you code K35.80

  1. Unspecified does not mean incorrect. When the record gives no greater specificity, K35.80 may be the appropriate code. Check the record for detail that supports a more specific sibling — in this subcategory the siblings differ by acute versus chronic. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

    ReviewK35.89

    See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

Choose the right path

  1. Does the record document the detail a more specific sibling code needs?
    Yes → Review the specific siblings in this subcategory.
    No → Continue — K35.80 is appropriate when the documentation goes no further.

    ReviewK35.89

Consider K35.80. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Acuity
Acute or chronic as documented; when both are documented and separate codes exist, both are reported with the acute code first (Guidelines I.B.8).
Any detail beyond this code’s title
What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: The provider documents the condition in the terms of this code’s title and records no further detail.

Coding question: Is a more specific sibling code supportable?

Path: Review the specific siblings in this subcategory and what each requires the record to state.

Reason: A more specific code needs documentation of the distinguishing element; without it the unspecified code is appropriate, and a provider query is the route to specificity (Guidelines I.A.9.b, I.B.18).

ReviewK35.89

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Unspecified acute appendicitis is a billable ICD-10-CM diagnosis code (K35.80).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (2)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Official Coding Guidelines

No excerpt in the ICD-10-CM Official Guidelines names this code specifically. Its chapter carries only this chapter-wide note:

Chapter 11: Diseases of the Digestive System (K00-K95)

Reserved for future guideline expansion

Verify Before Coding

  • CC as a secondary diagnosis (FY2027). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name K35.80 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: K65 — Peritonitis (via K35.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

MS-DRG Grouper Relationships (FY2027)

Potential MS-DRG participation — not a DRG assignment.

FY2027 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 24 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 3 MS-DRGs: DRG 393 (MDC 06), DRG 394 (MDC 06), DRG 395 (MDC 06).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):DIG009 — Appendicitis and other appendiceal conditions (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

K35.200 — Acute appendicitis with generalized peritonitis, without perforation or abscess, K35.201 — Acute appendicitis with generalized peritonitis, with perforation, without abscess, K35.209 — Acute appendicitis with generalized peritonitis, without abscess, unspecified as to perforation, K35.210 — Acute appendicitis with generalized peritonitis, without perforation, with abscess, K35.211 — Acute appendicitis with generalized peritonitis, with perforation and abscess, K35.219 — Acute appendicitis with generalized peritonitis, with abscess, unspecified as to perforation, K35.30 — Acute appendicitis with localized peritonitis, without perforation or gangrene, K35.31 — Acute appendicitis with localized peritonitis and gangrene, without perforation, K35.32 — Acute appendicitis with perforation, localized peritonitis, and gangrene, without abscess, K35.33 — Acute appendicitis with perforation, localized peritonitis, and gangrene, with abscess, K35.890 — Other acute appendicitis without perforation or gangrene, K35.891 — Other acute appendicitis without perforation, with gangrene, K36 — Other appendicitis, K37 — Unspecified appendicitis, K38.0 — Hyperplasia of appendix, K38.1 — Appendicular concretions, K38.2 — Diverticulum of appendix, K38.3 — Fistula of appendix, K38.8 — Other specified diseases of appendix, K38.9 — Disease of appendix, unspecified, +3 more

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Appendicitis and other appendiceal conditions).

K35.209 — Acute appendicitis with generalized peritonitis, without abscess, unspecified as to perforation, K35.21 — Acute appendicitis with generalized peritonitis, with abscess, K35.210 — Acute appendicitis with generalized peritonitis, without perforation, with abscess, K35.211 — Acute appendicitis with generalized peritonitis, with perforation and abscess, K35.219 — Acute appendicitis with generalized peritonitis, with abscess, unspecified as to perforation, K35.3 — Acute appendicitis with localized peritonitis, K35.30 — Acute appendicitis with localized peritonitis, without perforation or gangrene, K35.31 — Acute appendicitis with localized peritonitis and gangrene, without perforation, K35.32 — Acute appendicitis with perforation, localized peritonitis, and gangrene, without abscess, K35.33 — Acute appendicitis with perforation, localized peritonitis, and gangrene, with abscess, K35.89 — Other acute appendicitis, K35.890 — Other acute appendicitis without perforation or gangrene, K35.891 — Other acute appendicitis without perforation, with gangrene, K36 — Other appendicitis, K37 — Unspecified appendicitis, K38.0 — Hyperplasia of appendix, K38.1 — Appendicular concretions, K38.2 — Diverticulum of appendix, K38.3 — Fistula of appendix, K38.8 — Other specified diseases of appendix, +5 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main terms “Appendicitis”, “Gangrene, gangrenous”; these codes share that main term but sit in a different category of the Tabular List.

I73.1 — Thromboangiitis obliterans [Buerger's disease] (presenile), I89.1 — Lymphangitis (lymphangitis), I96 — Gangrene, not elsewhere classified, J04.0 — Acute laryngitis (laryngitis), J36 — Peritonsillar abscess (quinsy), J39.2 — Other diseases of pharynx (retropharyngeal), J85.0 — Gangrene and necrosis of lung (lung), K04.1 — Necrosis of pulp (tooth), K12.2 — Cellulitis and abscess of mouth (uvulitis), K14.0 — Glossitis (glossitis), K36 — Other appendicitis (healed), K37 — Unspecified appendicitis, K55.069 — Acute infarction of intestine, part and extent unspecified (mesentery), M27.3 — Alveolitis of jaws (alveolar), N44.8 — Other noninflammatory disorders of the testis (testis, noninfective), N45.1 — Epididymitis (epididymis), N45.2 — Orchitis (testis), N48.29 — Other inflammatory disorders of penis (penis), N48.89 — Other specified disorders of penis (penis, noninfective), N49.1 — Inflammatory disorders of spermatic cord, tunica vaginalis and vas deferens (vas deferens), +14 more

Contextual Map

Every relationship of K35.80 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run K35.80 with this related code in Claim Check

Hierarchy

Referenced by Excludes1 notes

  • K65 — Peritonitis[Excludes1](via K35.-): “peritonitis with or following appendicitis (K35.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2027

Clinical classification (CCSR)

MS-DRG Grouper

MDC crossing

  • MDC 06 — Diseases and Disorders of the Digestive System[MDC crossing]: “Diseases and Disorders of the Digestive System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 5,748 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2027

Index entries

  • Appendicitis (pneumococcal) (retrocecal), acute (catarrhal) (fulminating) (obstructive) (retrocecal) (suppurative)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2027
  • Gangrene, gangrenous (connective tissue) (dropsical) (dry) (moist) (skin) (ulcer), appendix[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2027

Nearest codes (19)

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2027 tabular list, index and tables, effective October 1, 2026 Release, file and checksum · Publisher’s page
Coding guidelines Official source data
ICD-10-CM Official Guidelines for Coding and Reporting (FY2027), quoted by section Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v44.0 (October 2026) Release, file and checksum · Publisher’s page
Inpatient payment groups Official source data
CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v44 Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "K35.80 — Unspecified acute appendicitis." ICD-10-CM FY2027. https://medcoder.ai/icd10/code/k35.80-unspecified-acute-appendicitis

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Unspecified acute appendicitis

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027, and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to K35.80 in its code family, with their registry titles.

View all codes in the K35 family