K51.90: Ulcerative colitis, unspecified, without complications

The ICD-10-CM code for ulcerative colitis when the record documents neither the extent of colonic involvement nor any complication, given in the Alphabetic Index as the default under the main term Colitis, ulcerative.

Billable

FY2027

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At a glance

Code Family

K51: Ulcerative colitis

Condition

Ulcerative colitis

HCC Mapping (v28)

HCC 81: Ulcerative Colitis, the category every billable code in K51 maps to.

DRG Grouping

MS-DRG 385, 386 or 387 within MDC 06 under the FY2027 MS-DRG v44 grouper, depending on the complications and comorbidities also reported.

Family Specificity

Forty nine billable codes on two axes, the fourth character for extent of colonic involvement and the fifth or sixth character for complication.

Effective Date

Valid in the FY2026 code set and unchanged in the FY2027 update.

Which K51 code do I assign?

Category K51 splits on two axes in sequence: the fourth character fixes the extent of colonic involvement, the fifth and sixth characters fix whether a complication is documented and which one it is. K51.90 sits at the residual end of both axes and is correct only where the record names neither the extent nor a complication.

When documentation says…

Code

Pancolitis, inflammation reaching through the entire colon

K51.00

Ulcerative (chronic) pancolitis without complications. Backwash ileitis is an inclusion term at K51.0. Ulcerative (chronic) pancolitis NOS is an inclusion term at K51.00. The ACG places pancolitis within extensive colitis, Montreal E3, disease beyond the splenic flexure.

Ulcerative proctitis, disease limited to the rectum

K51.20

Ulcerative (chronic) proctitis without complications. Ulcerative (chronic) proctitis NOS is an inclusion term here. The ACG defines proctitis, Montreal E1, as disease within 18 cm of the anal verge, distal to the rectosigmoid junction.

Ulcerative rectosigmoiditis

K51.30

Ulcerative (chronic) rectosigmoiditis without complications. Ulcerative (chronic) rectosigmoiditis NOS is an inclusion term here. The Alphabetic Index routes the subterm rectosigmoiditis under Colitis, ulcerative to the main term Rectosigmoiditis, ulcerative.

Left sided colitis or left hemicolitis

K51.50

Left sided colitis without complications. Left hemicolitis is an inclusion term at K51.5. The ACG defines left sided colitis, Montreal E2, as disease extending from the sigmoid to the splenic flexure.

Ulcerative colitis with rectal bleeding at this encounter

K51.911

Ulcerative colitis, unspecified with rectal bleeding. The sixth character carries the complication: 1 for rectal bleeding, 2 for intestinal obstruction, 3 for fistula, 4 for abscess, 8 for another documented complication. The same six characters repeat under every extent subcategory in K51.

Ulcerative colitis with a complication the note does not name

K51.919

Ulcerative colitis, unspecified with unspecified complications. This code requires that a complication be documented. It is not the code for a record that is silent on complications, which is K51.90.

Ulcerative colitis, extent and complication not documented

K51.90

Ulcerative colitis, unspecified, without complications. The Alphabetic Index gives K51.90 as the default under the main term Colitis, ulcerative (chronic). Where the provider names an extent that has no subcategory of its own, the index routes the subterm specified type NEC to K51.80.

Tabular notes

EXCLUDES1

Category K51 carries one Excludes1 note, for Crohn's disease [regional enteritis] (the K50 codes). Category K50 carries the reciprocal note for ulcerative colitis (the K51 codes). An Excludes1 note means the two are not reported together for the same condition. Where the record cannot separate them, ICD-10-CM provides K52.3 for indeterminate colitis rather than dual coding. Colonic inflammatory bowel disease unclassified is an inclusion term at K52.3. Block K50-K52 adds two further Excludes1 notes that reach K51.90: irritable bowel syndrome (K58.-) and megacolon (the K59.3 codes). Irritable bowel syndrome (K58.9) is therefore not reported with K51.90 for the same condition. The Index sends toxic megacolon to K59.31, which falls under the same note.

USE ADDITIONAL

Category K51 carries two Use additional code instructions. The first reads code to identify any associated fistulas, if applicable: anal fistula (K60.3-), anorectal fistula (K60.5-), rectal fistula (the K60.4 codes). The second reads code to identify manifestations, such as: pyoderma gangrenosum (L88). Subcategories K60.3, K60.4 and K60.5 each carry the reciprocal instruction to code first, if applicable, Crohn's disease (K50.-) or ulcerative colitis (the K51 codes). The ulcerative colitis code is sequenced first under that instruction and the fistula code second. A documented fistula moves the encounter to sixth character 3, which is K51.913 at unspecified extent. The fistula instruction is therefore not reached from K51.90 itself. The manifestations instruction is reached from K51.90, because pyoderma gangrenosum is not one of the four named complications in K51.

FY2027: EFFECTIVE 1 OCTOBER 2026

K51.90 is valid in the FY2026 code set and unchanged in the FY2027 update, which took effect on 1 October 2026. The FY2027 code and tabular addenda add, delete or revise nothing in category K51 or in block K50-K52. Block K50-K52 continues to carry the Includes note reading noninfective inflammatory bowel disease.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

K51.90 maps to CMS-HCC v28 category HCC 81, Ulcerative Colitis.

All 49 billable codes in category K51 map to HCC 81. The mapping does not turn on the extent of disease. The seven intestinal obstruction codes, with sixth character 2, also map to HCC 78: Intestinal Obstruction/Perforation.

CMS-HCC v28 keeps the two inflammatory bowel diseases in separate categories. HCC 80 is Crohn's Disease (Regional Enteritis) and covers the 28 billable codes in K50. HCC 81 is Ulcerative Colitis.

CMS-HCC v28 is fully phased in at 100 percent as of 1 January 2026.

For Medicare Advantage risk adjustment the diagnosis must be documented as actively managed each calendar year. The code must appear on a dated encounter note rather than only on a problem list.

MS-DRG v44 (FY2027)

MS-DRG 385: Inflammatory bowel disease with MCC.

MS-DRG 386: Inflammatory bowel disease with CC.

MS-DRG 387: Inflammatory bowel disease without CC or MCC.

Inpatient admissions with K51.90 as principal diagnosis group to MS-DRG 385, 386 or 387 within MDC 06 under the FY2027 MS-DRG v44 grouper. The three-way split is driven by the complications and comorbidities reported as secondary diagnoses.

WHERE SPECIFICITY IS LOST

Assigning K51.90 where the record names the extent discards it. Pancolitis codes to K51.00, proctitis to K51.20, rectosigmoiditis to K51.30 and left sided colitis to K51.50.

Assigning K51.90 where a complication is documented discards the complication. Rectal bleeding codes to K51.911, intestinal obstruction to K51.912, fistula to K51.913 and abscess to K51.914.

Assigning K51.919 where the note is simply silent on complications reverses the axis. K51.919 requires that a complication be documented without being named.

Assigning K52.9 for unspecified colitis where the record supports ulcerative colitis drops the HCC 81 mapping, because no code in category K52 maps to a CMS-HCC v28 category. It also moves the inpatient case from MS-DRG 385, 386 or 387 to MS-DRG 391 or 392, esophagitis, gastroenteritis and miscellaneous digestive disorders.

Assigning K50.90 for Crohn's disease where the record supports ulcerative colitis moves the case from HCC 81 to HCC 80. The two conditions carry reciprocal Excludes1 notes and are not reported together for the same condition.

Medicare and commercial payers

K51.90 is a valid billable code and is not rejected as an incomplete code.

There is no National Coverage Determination specific to ulcerative colitis diagnosis coding.

Section I.B.18 of the FY2027 ICD-10-CM Official Guidelines directs that an unspecified code is assigned when it is the code that most accurately reflects what is known about the patient's condition at that encounter.

SEVERITY DESIGNATION (MS-DRG V44.0)

K51.90 is a CC. It appears in Appendix G, diagnoses defined as complications or comorbidities, in the ICD-10 MS-DRG Definitions Manual Version 44.0. As a secondary diagnosis it can move a case to a with-CC tier where the DRG pair offers one.

Documentation checklist to support K51.90

Chart elements that support code selection and reduce specificity-driven denials.

A provider statement of ulcerative colitis, rather than colitis alone, because unspecified colitis codes to K52.9.

The extent of colonic involvement, named as pancolitis, proctitis, rectosigmoiditis or left sided colitis, which selects the fourth character.

An explicit statement that no complication is present, or the absence of any documented complication, which is what separates K51.90 from K51.919.

Where a complication is documented, which one it is: rectal bleeding, intestinal obstruction, fistula, abscess or another named complication.

The colonoscopy or flexible sigmoidoscopy report, with the proximal extent of mucosal inflammation and the histology from mucosal biopsies.

The appearance of the ileocecal valve and the terminal ileum, which the ACG notes are not ulcerated or stenotic in ulcerative colitis.

Extraintestinal manifestations where present, such as pyoderma gangrenosum, enteropathic arthropathy or primary sclerosing cholangitis, to support the Use additional code instruction at K51.

Current therapy and any anemia workup, including systemic steroids, immunosuppressive biologics, hemoglobin and iron studies.

For outpatient encounters, a confirmed diagnosis only. Probable or suspected ulcerative colitis is not coded in the outpatient setting.

FAQ

Is K51.90 a billable code for FY2027?

Yes. K51.90 is a valid billable ICD-10-CM code at the fifth character. It is valid in the FY2026 code set and unchanged in the FY2027 update, which took effect on 1 October 2026. The parent K51.9 is not billable, because two codes sit below it. The record should show a provider statement of ulcerative colitis with no extent named and no complication documented.

What is the difference between K51.90 and K51.919?

The difference is whether a complication is documented at all. K51.90 reads without complications and applies where the record documents no complication. K51.919 reads with unspecified complications and applies where the record states that a complication is present but does not name which one. A note that is simply silent on complications supports K51.90, not K51.919. Note that K51.99 is not a valid ICD-10-CM code. The unspecified-extent complication codes carry six characters, K51.911 through K51.919.

When should I use K52.9, K50.90 or K52.3 instead of K51.90?

Use K52.9, noninfective gastroenteritis and colitis unspecified, where the provider documents colitis without naming it as ulcerative. Colitis NOS is an inclusion term at K52.9. Use K50.90, Crohn's disease unspecified without complications, where the record supports Crohn's disease. K50 and K51 carry reciprocal Excludes1 notes and are not reported together for the same condition. Use K52.3, indeterminate colitis, where colonic inflammatory bowel disease cannot be classified as either one. Irritable bowel syndrome (K58.9) is excluded from the whole K50-K52 block by an Excludes1 note at block level. Diverticular disease codes such as K57.92 carry no Excludes note against K51 and may be reported alongside it when both are documented.

Does K51.90 map to a CMS-HCC v28 category?

Yes. K51.90 maps to HCC 81, Ulcerative Colitis, under CMS-HCC v28. All 49 billable codes in category K51 map to that same category. The mapping does not turn on extent. The seven intestinal obstruction codes, with sixth character 2, also map to HCC 78: Intestinal Obstruction/Perforation. Crohn's disease sits in a separate v28 category, HCC 80, Crohn's Disease (Regional Enteritis). CMS-HCC v28 is fully phased in at 100 percent as of 1 January 2026.

Which MS-DRG does K51.90 group to?

Inpatient admissions with K51.90 as principal diagnosis group to MS-DRG 385, 386 or 387 within MDC 06 under the FY2027 MS-DRG v44 grouper. The three read inflammatory bowel disease with MCC, with CC, and without CC or MCC. Substituting K52.9 for unspecified colitis moves the case to MS-DRG 391 or 392, esophagitis, gastroenteritis and miscellaneous digestive disorders.

What must the note document to support a more specific K51 code?

The note must document two things, one for each axis. The first is the extent of colonic involvement: pancolitis, proctitis, rectosigmoiditis, left sided colitis or another named extent. The second is the complication, if any: rectal bleeding, intestinal obstruction, fistula, abscess or another named complication. Naming the extent alone moves the code from K51.90 to K51.00, K51.20, K51.30, K51.50 or K51.80. Two companion codes are drawn from the same note when ongoing therapy is documented. Z79.620 covers long term use of an immunosuppressive biologic. Z79.899 covers other long term drug therapy. One subcategory in the family is not an extent at all. K51.4 covers inflammatory polyps of colon and carries its own Excludes2 note for adenomatous polyp of colon (D12.6), polyposis of colon (D12.6) and polyps of colon NOS (K63.5).

How is K51.90 sequenced in the inpatient and outpatient settings?

K51.90 may be the principal diagnosis where ulcerative colitis is the condition established after study to be chiefly responsible for the admission. It is a secondary diagnosis where another condition accounts for the encounter. Where an anal, rectal or anorectal fistula is also reported, subcategories K60.3, K60.4 and K60.5 instruct the coder to code the ulcerative colitis first. In the inpatient setting a diagnosis documented at discharge as probable or suspected is coded as though confirmed, under UHDDS rules. In the outpatient setting only a confirmed diagnosis is coded.

What changed for K51.90 in FY2027?

K51.90 is unchanged in the FY2027 update, effective 1 October 2026, which adds, deletes or revises nothing in category K51. The code was not revised in the FY2026 annual release, which took effect on 1 October 2025. No codes were added to or deleted from category K51. The April 1, 2026 mid-year addendum added no new diagnosis codes. Its one Chapter 11 tabular change sits at K60, where the Excludes1 note that excluded category K61 was deleted.

Related codes

Codes in the same family as K51.90. Only codes with a published page render as links.

Sibling codes in the K51 family

K51.00: Ulcerative (chronic) pancolitis without complications.
K51.011: Ulcerative (chronic) pancolitis with rectal bleeding.
K51.012: Ulcerative (chronic) pancolitis with intestinal obstruction.
K51.013: Ulcerative (chronic) pancolitis with fistula.
K51.014: Ulcerative (chronic) pancolitis with abscess.
K51.018: Ulcerative (chronic) pancolitis with other complication.
K51.019: Ulcerative (chronic) pancolitis with unspecified complications.
K51.20: Ulcerative (chronic) proctitis without complications.
K51.211: Ulcerative (chronic) proctitis with rectal bleeding.
K51.212: Ulcerative (chronic) proctitis with intestinal obstruction.
K51.213: Ulcerative (chronic) proctitis with fistula.
K51.214: Ulcerative (chronic) proctitis with abscess.
K51.218: Ulcerative (chronic) proctitis with other complication.
K51.219: Ulcerative (chronic) proctitis with unspecified complications.
K51.30: Ulcerative (chronic) rectosigmoiditis without complications.
K51.311: Ulcerative (chronic) rectosigmoiditis with rectal bleeding.
K51.312: Ulcerative (chronic) rectosigmoiditis with intestinal obstruction.
K51.313: Ulcerative (chronic) rectosigmoiditis with fistula.
K51.314: Ulcerative (chronic) rectosigmoiditis with abscess.
K51.318: Ulcerative (chronic) rectosigmoiditis with other complication.
K51.319: Ulcerative (chronic) rectosigmoiditis with unspecified complications.
K51.40: Inflammatory polyps of colon without complications.
K51.411: Inflammatory polyps of colon with rectal bleeding.
K51.412: Inflammatory polyps of colon with intestinal obstruction.
K51.413: Inflammatory polyps of colon with fistula.
K51.414: Inflammatory polyps of colon with abscess.
K51.418: Inflammatory polyps of colon with other complication.
K51.419: Inflammatory polyps of colon with unspecified complications.
K51.50: Left sided colitis without complications.
K51.511: Left sided colitis with rectal bleeding.
K51.512: Left sided colitis with intestinal obstruction.
K51.513: Left sided colitis with fistula.
K51.514: Left sided colitis with abscess.
K51.518: Left sided colitis with other complication.
K51.519: Left sided colitis with unspecified complications.
K51.80: Other ulcerative colitis without complications.
K51.811: Other ulcerative colitis with rectal bleeding.
K51.812: Other ulcerative colitis with intestinal obstruction.
K51.813: Other ulcerative colitis with fistula.
K51.814: Other ulcerative colitis with abscess.
K51.818: Other ulcerative colitis with other complication.
K51.819: Other ulcerative colitis with unspecified complications.
K51.911: Ulcerative colitis, unspecified with rectal bleeding.
K51.912: Ulcerative colitis, unspecified with intestinal obstruction.
K51.913: Ulcerative colitis, unspecified with fistula.
K51.914: Ulcerative colitis, unspecified with abscess.
K51.918: Ulcerative colitis, unspecified with other complication.
K51.919: Ulcerative colitis, unspecified with unspecified complications.

Additional codes to consider

Codes outside the K51 family considered alongside K51.90.

Companion: code alongside when documented

L88: Pyoderma gangrenosum.
M07.60: Enteropathic arthropathies, unspecified site.
K83.01: Primary sclerosing cholangitis.
L52: Erythema nodosum.
H20.9: Unspecified iridocyclitis.
M45.9: Ankylosing spondylitis of unspecified sites in spine.
Z79.620: Long term (current) use of immunosuppressive biologic.
Z79.52: Long term (current) use of systemic steroids.
Z79.899: Other long term (current) drug therapy.
K60.30: Anal fistula, unspecified.
K60.40: Rectal fistula, unspecified.
K60.50: Anorectal fistula, unspecified.

Consider instead

K51.919: Ulcerative colitis, unspecified with unspecified complications.
K51.911: Ulcerative colitis, unspecified with rectal bleeding.
K51.912: Ulcerative colitis, unspecified with intestinal obstruction.
K51.913: Ulcerative colitis, unspecified with fistula.
K51.914: Ulcerative colitis, unspecified with abscess.
K51.918: Ulcerative colitis, unspecified with other complication.
K51.00: Ulcerative (chronic) pancolitis without complications.
K51.20: Ulcerative (chronic) proctitis without complications.
K51.30: Ulcerative (chronic) rectosigmoiditis without complications.
K51.50: Left sided colitis without complications.
K51.80: Other ulcerative colitis without complications.
K51.40: Inflammatory polyps of colon without complications.
K50.90: Crohn's disease, unspecified, without complications.
K52.3: Indeterminate colitis.
K52.9: Noninfective gastroenteritis and colitis, unspecified.
A09: Infectious gastroenteritis and colitis, unspecified.
K58.9: Irritable bowel syndrome, unspecified.
K55.9: Vascular disorder of intestine, unspecified.
K62.5: Hemorrhage of anus and rectum.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software