What is ICD-10 code C91.50?

C91.50 is the ICD-10-CM diagnosis code for Adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. A blood cancer (adult T-cell lymphoma/leukemia) caused by HTLV-1 virus infection that has not achieved remission from treatment. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C91.50 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C91.50 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. C91.50 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. C91.50 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Which HCC does C91.50 map to under V28?

C91.50 maps to CMS-HCC V28 HCC 19 (Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

What documentation supports C91.50?

Use C91.50 when the provider wording supports adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. Check the FY2026 tabular notes and the full code path before the final code choice. For this DOS, capture the provider's current assessment and the care action documented for adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. Keep the checklist tied to the note. Do not swap C91.50 with C91.51. The FY2026 tabular describes C91.51 as adult T-cell lymphoma/leukemia (HTLV-1-associated), in remission. Follow the documented detail. For C91.50, 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. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. One mnemonic element does not establish that a diagnosis is reportable.

2026 ICD-10-CM C91.50: Adult T-cell

C91.50 maps to CMS-HCC V28 19. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools

ICD-10-CM Code View

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Code lookupC91.50

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of lymphoid, hematopoietic and related tissue (C81-C96)

C91.50

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

Adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission

A blood cancer (adult T-cell lymphoma/leukemia) caused by HTLV-1 virus infection that has not achieved remission from treatment.

Buddy Insight

Adult T-cell lymphoma/leukemia (ATLL) caused by Human T-cell Lymphotropic Virus type 1 (HTLV-1) is an aggressive malignancy with multiple clinical subtypes (acute, lymphoma, chronic, smoldering).

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 19

Myelodysplastic Syndromes, Mu...

Code-level coefficient reference

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

N/A

—

Not risk-adjusted for this model/period.

Not mapped

ESRD/PACE

N/A

—

Not risk-adjusted for this model/period.

Not mapped

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

C91Lymphoid leukemia C91.5Adult T-cell lymphoma/leukemia (HTLV-1-associated)C91.50Adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission

Inclusion Terms

Official

Adult T-cell lymphoma/leukemia (HTLV-1-associated) with failed remission
Adult T-cell lymphoma/leukemia (HTLV-1-associated) NOS
Acute variant of adult T-cell lymphoma/leukemia (HTLV-1-associated)Inherited from C91.5
Chronic variant of adult T-cell lymphoma/leukemia (HTLV-1-associated)Inherited from C91.5
Lymphomatoid variant of adult T-cell lymphoma/leukemia (HTLV-1-associated)Inherited from C91.5
Smouldering variant of adult T-cell lymphoma/leukemia (HTLV-1-associated)Inherited from C91.5

Excludes 2

Official

Kaposi's sarcoma of lymph nodes (C46.3)Inherited from C81-C96
secondary and unspecified neoplasm of lymph nodes (C77.-)Inherited from C81-C96
secondary neoplasm of bone marrow (C79.52)Inherited from C81-C96
secondary neoplasm of spleen (C78.89)Inherited from C81-C96

Related Codes

Official

C91.51Adult T-cell lymphoma/leukemia (HTLV-1-associated), in remission C91.52Adult T-cell lymphoma/leukemia (HTLV-1-associated), in relapse

Includes

Official

No Includes notes are included in this display for C91.50. Check the code and parent instructions in the Code Book.

Excludes 1

Official

personal history of leukemia (Z85.6)Inherited from C91

Code First

Official

No Code First sequencing instructions are included in this display for C91.50. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for C91.50. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for C91.50. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

Documentation must confirm HTLV-1 positivity by serology and/or viral load testing.

The clinical subtype (acute, lymphoma, chronic, smoldering) should be specified as it significantly impacts prognosis and treatment.

Active disease status must be explicitly stated, along with evidence of organ involvement (skin lesions, hypercalcemia, lymphadenopathy, hepatosplenomegaly).

CD4+CD25+ T-cell immunophenotype should be confirmed.

MEAT Support

HCC Buddy guidance

Documentation must confirm HTLV-1 positivity by serology and/or viral load testing.

The clinical subtype (acute, lymphoma, chronic, smoldering) should be specified as it significantly impacts prognosis and treatment.

Active disease status must be explicitly stated, along with evidence of organ involvement (skin lesions, hypercalcemia, lymphadenopathy, hepatosplenomegaly).

CD4+CD25+ T-cell immunophenotype should be confirmed.

Audit Caution

HCC Buddy guidance

HTLV-1 carrier status alone does not warrant this code -

only assign when ATLL has developed. The smoldering subtype may have minimal symptoms but still requires this code when diagnosed. Do not confuse HTLV-1 with HTLV-2 or HIV; the viral etiology must be specifically documented. Some providers may use the older term 'ATL' without specifying HTLV-1 -

query if the association is not documented.

Common Mistakes

HCC Buddy guidance

C84.40-C84.49 (peripheral T-cell lymphoma, not otherwise specified) does not capture the HTLV-1 association.

C91.60 (T-cell prolymphocytic leukemia) is a different T-cell malignancy without viral etiology.

C86.5 (angioimmunoblastic T-cell lymphoma) and other mature T-cell lymphomas should be distinguished.

C91.90 (lymphoid leukemia, unspecified) loses the critical HTLV-1 association.

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 C91.50 an HCC code?

Yes. C91.50 (Adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission) maps to HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 1.798. 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: C91.50 is billable and maps to V28 HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: C91.50
Description: Adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission
HCC (V28): HCC 19 — Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers
RAF reference coefficient: 1.798
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers

1.798

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 C91.50 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for C91.50

For C91.50, 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

C91.50 is the ICD-10-CM diagnosis code for adult t-cell lymphoma/leukemia (htlv-1-associated) not having achieved remission. A blood cancer (adult T-cell lymphoma/leukemia) caused by HTLV-1 virus infection that has not achieved remission from treatment. C91.50 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lymphoid, hematopoietic and related tissue (c81-c96).

Under the CMS-HCC V28 risk adjustment model, C91.50 maps to Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers (HCC 19) with a source-labeled community, non-dual, aged reference coefficient of 1.798. No V24 mapping is shown for C91.50; use the applicable model and payment year when reviewing the V28 mapping. 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.

Use C91.50 when the provider wording supports adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. Check the FY2026 tabular notes and the full code path before the final code choice. For C91.50, 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 C91.50 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use C91.50 when the provider wording supports adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. Check the FY2026 tabular notes and the full code path before the final code choice.
•For this DOS, capture the provider's current assessment and the care action documented for adult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission. Keep the checklist tied to the note.
•Do not swap C91.50 with C91.51. The FY2026 tabular describes C91.51 as adult T-cell lymphoma/leukemia (HTLV-1-associated), in remission. Follow the documented detail.
•C91.50 maps to V28 HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers. The mapping does not replace the documentation review. The code still has to match the note.
•Ask Buddy to show the official notes for C91.50 or compare its code path with C91.51.

Clinical Significance

Adult T-cell lymphoma/leukemia (ATLL) caused by Human T-cell Lymphotropic Virus type 1 (HTLV-1) is an aggressive malignancy with multiple clinical subtypes (acute, lymphoma, chronic, smoldering). The 'not in remission' status indicates active disease, which in the acute and lymphoma subtypes carries a particularly poor prognosis with median survival of less than one year. HTLV-1 is endemic in specific geographic regions including Japan, Caribbean, Central Africa, and parts of South America.

Documentation Requirements

✓Documentation must confirm HTLV-1 positivity by serology and/or viral load testing.
✓The clinical subtype (acute, lymphoma, chronic, smoldering) should be specified as it significantly impacts prognosis and treatment.
✓Active disease status must be explicitly stated, along with evidence of organ involvement (skin lesions, hypercalcemia, lymphadenopathy, hepatosplenomegaly).
✓CD4+CD25+ T-cell immunophenotype should be confirmed.

Commonly Confused Codes

•C84.40-C84.49 (peripheral T-cell lymphoma, not otherwise specified) does not capture the HTLV-1 association.
•C91.60 (T-cell prolymphocytic leukemia) is a different T-cell malignancy without viral etiology.
•C86.5 (angioimmunoblastic T-cell lymphoma) and other mature T-cell lymphomas should be distinguished.
•C91.90 (lymphoid leukemia, unspecified) loses the critical HTLV-1 association.