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K35.209 ICD-10-CM Code: Acute appendicitis with generalized peritonitis, without abscess, unspecified as to perforation

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

Coding at a Glance

Tabular directives
1 inclusion term

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 v43, Appendix B.

  • MS-DRG 371 — MAJOR GASTROINTESTINAL DISORDERS AND PERITONEAL INFECTIONS WITH MCC (MDC 06)
  • MS-DRG 372 — MAJOR GASTROINTESTINAL DISORDERS AND PERITONEAL INFECTIONS WITH CC (MDC 06)
  • MS-DRG 373 — MAJOR GASTROINTESTINAL DISORDERS AND PERITONEAL INFECTIONS 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 K35.209 in the official ICD-10-CM tabular list, quoted as published.

Inclusion Terms

Alternative terms the tabular list files under this code.

  • (Acute) appendicitis with generalized peritonitis NOS

Coder workflow for K35.209

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

Before you code K35.209

  1. Unspecified does not mean incorrect. When the record gives no greater specificity, K35.209 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, the presence or absence of the associated condition. 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).

    ReviewK35.200, K35.201

    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 — K35.209 is appropriate when the documentation goes no further.

    ReviewK35.200, K35.201

Consider K35.209. 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).
The associated condition or complication
Whether the associated condition the title names is documented; the “with” convention presumes some links, and a provider statement that the conditions are unrelated defeats it (Guidelines I.A.15).
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).

ReviewK35.200, K35.201

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

Acute appendicitis with generalized peritonitis, without abscess, unspecified as to perforation is a billable ICD-10-CM diagnosis code (K35.209).

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 (3)

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 (FY2026). 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 K35.209 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 3 Excludes1 notes: K37 — Unspecified appendicitis (via K35.2.-), K65 — Peritonitis (via K35.-), K65 — Peritonitis (via K35.2.-).

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

Referenced by 1 Excludes2 note: K63.1 — Perforation of intestine (nontraumatic) (via K35.2.-).

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

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 371 (MDC 06), DRG 372 (MDC 06), DRG 373 (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); DIG016 — Peritonitis and intra-abdominal abscess.

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.

K31.89 — Other diseases of stomach and duodenum, K31.9 — Disease of stomach and duodenum, unspecified, K35.200 — Acute appendicitis with generalized peritonitis, without perforation or abscess, K35.201 — Acute appendicitis with generalized peritonitis, with perforation, without 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, K35.30 — 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, K35.80 — Unspecified 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, +3 more

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical categories (Appendicitis and other appendiceal conditions, Peritonitis and intra-abdominal abscess).

A18.31 — Tuberculous peritonitis, A54.85 — Gonococcal peritonitis, A74.81 — Chlamydial peritonitis, K35.2 — Acute appendicitis with generalized peritonitis, K35.20 — Acute appendicitis with generalized peritonitis, without abscess, K35.200 — Acute appendicitis with generalized peritonitis, without perforation or abscess, K35.201 — Acute appendicitis with generalized peritonitis, with perforation, without abscess, 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, K35.80 — Unspecified acute appendicitis, K35.89 — Other acute appendicitis, K35.890 — Other acute appendicitis without perforation or gangrene, K35.891 — Other acute appendicitis without perforation, with gangrene, +44 more

Same Index main term, other category

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

A06.89 — Other amebic infections (amebic), A18.31 — Tuberculous peritonitis (tuberculous), A18.32 — Tuberculous enteritis (tuberculous), A36.89 — Other diphtheritic complications (diphtheritic), A50.08 — Early visceral congenital syphilis (syphilitic, congenital), A52.74 — Syphilis of liver and other viscera (syphilitic), A54.85 — Gonococcal peritonitis (gonococcal), A74.81 — Chlamydial peritonitis (chlamydial), E85.0 — Non-neuropathic heredofamilial amyloidosis (periodic, familial), K36 — Other appendicitis (healed), K37 — Unspecified appendicitis, K57.00 — Diverticulitis of small intestine with perforation and abscess without bleeding (with or following, diverticular disease, small intestine), K57.01 — Diverticulitis of small intestine with perforation and abscess with bleeding (with or following, diverticular disease, small intestine, with, bleeding), K57.20 — Diverticulitis of large intestine with perforation and abscess without bleeding (with or following, diverticular disease, large intestine), K57.21 — Diverticulitis of large intestine with perforation and abscess with bleeding (with or following, diverticular disease, large intestine, with, bleeding), K57.40 — Diverticulitis of both small and large intestine with perforation and abscess without bleeding (with or following, diverticular disease, small intestine, with, large intestine), K57.41 — Diverticulitis of both small and large intestine with perforation and abscess with bleeding (with or following, diverticular disease, large intestine, with, small intestine, with bleeding), K57.80 — Diverticulitis of intestine, part unspecified, with perforation and abscess without bleeding (with or following, diverticular disease), K57.81 — Diverticulitis of intestine, part unspecified, with perforation and abscess with bleeding (with or following, diverticular disease, with bleeding), K65.0 — Generalized (acute) peritonitis (acute), +15 more

Contextual Map

Every relationship of K35.209 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 K35.209 with these 3 related codes in Claim Check

Hierarchy

Referenced by Excludes1 notes

  • K37 — Unspecified appendicitis[Excludes1](via K35.2.-): “unspecified appendicitis with peritonitis (K35.2-, K35.3-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
  • K65 — Peritonitis[Excludes1](via K35.-): “peritonitis with or following appendicitis (K35.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
  • K65 — Peritonitis[Excludes1](via K35.2.-): “acute appendicitis with generalized peritonitis (K35.2-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Excludes2 notes

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. 5,744 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

  • Appendicitis (pneumococcal) (retrocecal), acute (catarrhal) (fulminating) (obstructive) (retrocecal) (suppurative), with, peritonitis NEC, generalized[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Appendicitis (pneumococcal) (retrocecal), with, peritonitis NEC, generalized[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Peritonitis (adhesive) (bacterial) (fibrinous) (hemorrhagic) (idiopathic) (localized) (perforative) (primary) (with adhesions) (with effusion), with or following, appendicitis, generalized[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (19)

Change history

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 FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Coding guidelines Official source data
ICD-10-CM Official Guidelines for Coding and Reporting (FY2026), quoted by section Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 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 — v43 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

Change history

  • FY2024 — October 1, 2023
    Added to the code set
    Acute appendicitis with generalized peritonitis, without abscess, unspecified as to perforation
    FY2024 changes

Nearest Codes in This Family

Official ICD-10-CM classifications closest to K35.209 in its code family, with their registry titles.

View all codes in the K35 family