K51.313 maps to CMS-HCC V28 81. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools

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Code lookupK51.313

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the digestive system (K00-K95) / Noninfective enteritis and colitis (K50-K52)

K51.313

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Ulcerative (chronic) rectosigmoiditis with fistula

Chronic inflammation with ulcers in the rectum and sigmoid colon with an abnormal tunnel-like connection (fistula) formed between the bowel and another structure.

Buddy Insight

Ulcerative rectosigmoiditis with fistula represents an abnormal connection developing from the inflamed rectosigmoid region to adjacent structures.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 81

Ulcerative Colitis

Coefficient HCC 81: 0.244 (Community Non-Dual Aged (CNA))

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 048

HHS-HCC 048 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 35

Inflammatory Bowel Disease

Code-level coefficient reference

RXHCC

Mapped

HCC 67

Inflammatory Bowel Disease

Code-level coefficient reference

Code Book Path

Official

K51.3Ulcerative (chronic) rectosigmoiditis K51.31Ulcerative (chronic) rectosigmoiditis with complications K51.313Ulcerative (chronic) rectosigmoiditis with fistula

Inclusion Terms

Official

No inclusion terms are included in this display for K51.313. Check the code and parent instructions in the Code Book.

Excludes 2

Official

certain conditions originating in the perinatal period (P04-P96)Inherited from K00-K95
certain infectious and parasitic diseases (A00-B99)Inherited from K00-K95
complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from K00-K95
congenital malformations, deformations and chromosomal abnormalities (Q00-Q99)Inherited from K00-K95
endocrine, nutritional and metabolic diseases (E00-E88)Inherited from K00-K95
injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from K00-K95
neoplasms (C00-D49)Inherited from K00-K95
symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from K00-K95

Related Codes

Official

K51.311Ulcerative (chronic) rectosigmoiditis with rectal bleeding K51.312Ulcerative (chronic) rectosigmoiditis with intestinal obstruction K51.314Ulcerative (chronic) rectosigmoiditis with abscess K51.318Ulcerative (chronic) rectosigmoiditis with other complication K51.319Ulcerative (chronic) rectosigmoiditis with unspecified complications

Includes

Official

noninfective inflammatory bowel diseaseInherited from K50-K52

Excludes 1

Official

irritable bowel syndrome (K58.-)Inherited from K50-K52, K51
megacolon (K59.3-)Inherited from K50-K52, K51
Crohn's disease [regional enteritis] (K50.-)Inherited from K50-K52, K51

Code First

Official

No Code First sequencing instructions are included in this display for K51.313. Check the code and parent instructions in the Code Book.

Use Additional

Official

code to identify any associated fistulas, if applicable:Inherited from K51
anal fistula (K60.3-)Inherited from K51
anorectal fistula (K60.5-)Inherited from K51
rectal fistula (K60.4-)Inherited from K51
code to identify manifestations, such as:Inherited from K51
pyoderma gangrenosum (L88)Inherited from K51

Code Also

Official

No Code Also instructions are included in this display for K51.313. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

Confirmed ulcerative colitis of the rectum and sigmoid colon

Documentation of fistula type and location

Imaging confirmation of fistula tract

Provider statement attributing fistula to the rectosigmoiditis

MEAT Support

HCC Buddy guidance

Confirmed ulcerative colitis of the rectum and sigmoid colon

Documentation of fistula type and location

Imaging confirmation of fistula tract

Provider statement attributing fistula to the rectosigmoiditis

Audit Caution

HCC Buddy guidance

Not reconsidering whether the correct diagnosis is Crohn disease when fistula is present

Using separate codes for the fistula and the colitis instead of the combination code

Failing to document fistula location and type for clinical accuracy

Using K51.318 (other complication) when fistula has its own specific code

Common Mistakes

HCC Buddy guidance

K51.313 vs K50.113 — K50.113 is Crohn disease of large intestine with fistula; verify underlying diagnosis

K51.313 vs K60.3 — K60.3 is anal fistula not related to inflammatory bowel disease

K51.313 vs K51.013 — K51.013 is pancolitis with fistula; rectosigmoiditis is more limited

K51.313 vs K51.213 — K51.213 is proctitis with fistula; rectosigmoiditis includes sigmoid involvement

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is K51.313 an HCC code?

Yes. K51.313 (Ulcerative (chronic) rectosigmoiditis with fistula) maps to HCC 81, Ulcerative Colitis under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.244. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: K51.313 is billable and maps to V28 HCC 81, Ulcerative Colitis. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: K51.313
Description: Ulcerative (chronic) rectosigmoiditis with fistula
HCC (V28): HCC 81 — Ulcerative Colitis
RAF reference coefficient: 0.244
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 81, Ulcerative Colitis

0.244

ESRDHCC 35, Inflammatory Bowel Disease

Not separately weighted

RxHCCHCC 67, Inflammatory Bowel Disease

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work K51.313 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for K51.313

For K51.313, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Coder workflow notes

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What This Code Means

K51.313 is the ICD-10-CM diagnosis code for ulcerative (chronic) rectosigmoiditis with fistula. Chronic inflammation with ulcers in the rectum and sigmoid colon with an abnormal tunnel-like connection (fistula) formed between the bowel and another structure. K51.313 sits in the ICD-10-CM chapter for diseases of the digestive system (k00-k95), within the section covering noninfective enteritis and colitis (k50-k52).

Under the CMS-HCC V28 risk adjustment model, K51.313 maps to Ulcerative Colitis (HCC 81) with a source-labeled community, non-dual, aged reference coefficient of 0.244. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Verify fistula presence and location in documentation. For K51.313, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for K51.313 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Verify fistula presence and location in documentation
•Fistulas are a serious complication often requiring surgical intervention

Clinical Significance

Ulcerative rectosigmoiditis with fistula represents an abnormal connection developing from the inflamed rectosigmoid region to adjacent structures. This serious complication typically requires surgical evaluation and may prompt reassessment of whether the diagnosis should be reclassified as Crohn disease, since fistulas are more characteristic of Crohn disease.

Documentation Requirements

✓Confirmed ulcerative colitis of the rectum and sigmoid colon
✓Documentation of fistula type and location
✓Imaging confirmation of fistula tract
✓Provider statement attributing fistula to the rectosigmoiditis
✓Surgical consultation and management plan
✓Consideration of whether diagnosis should be reclassified

Commonly Confused Codes

•K51.313 vs K50.113: K50.113 is Crohn disease of large intestine with fistula; verify underlying diagnosis
•K51.313 vs K60.3: K60.3 is anal fistula not related to inflammatory bowel disease
•K51.313 vs K51.013: K51.013 is pancolitis with fistula; rectosigmoiditis is more limited
•K51.313 vs K51.213: K51.213 is proctitis with fistula; rectosigmoiditis includes sigmoid involvement