What is ICD-10 code C91.42?

C91.42 is the ICD-10-CM diagnosis code for Hairy cell leukemia, in relapse. A rare blood cancer (hairy cell leukemia) with hair-like abnormal cells that has returned after a period of remission. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C91.42 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C91.42 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. C91.42 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. C91.42 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.

What is the V28 vs V24 difference for C91.42?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For C91.42, V28 maps to Bladder, Colorectal, and Other Cancers at RAF 0.363 while V24 mapped to Lymphoma and Other Cancers at RAF 0.675. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting C91.42?

The fifth character '2' indicates relapse - ensure documentation shows cancer has recurred Document timing and circumstances of relapse when available Because C91.42 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM C91.42: Hairy cell leukemia

ICD-10-CM Code View

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Code lookupC91.42Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C91.42

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

Hairy cell leukemia, in relapse

A rare blood cancer (hairy cell leukemia) with hair-like abnormal cells that has returned after a period of remission.

Buddy Insight

Hairy cell leukemia in relapse indicates recurrence of disease after a prior period of documented remission.

CMS-HCC V28

Mapped

HCC 22

Bladder, Colorectal, and Othe...

RAF 0.363

CMS-HCC V24

Mapped

HCC 10

Lymphoma and Other Cancers

RAF 0.675

ACA/HHS

Mapped

HCC 10

HHS-HCC 10

Varies by metal level

ESRD/PACE

Mapped

HCC 10

Lymphoma and Other Cancers

RAF 0.111

RXHCC

Mapped

HCC 21

Hodgkin Lymphoma and Other Ca...

RAF 0.410

Code Book Path

Official

C91Lymphoid leukemia

C91.4Hairy cell leukemia

C91.42Hairy cell leukemia, in relapse

Inclusion Terms

Official

Leukemic reticuloendotheliosis

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C91.42 in this effective period.

Related Child Codes

Official

C91.40Hairy cell leukemia not having achieved remission

C91.41Hairy cell leukemia, in remission

Includes

Official

ICD-10-CM does not list Includes notes for C91.42 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for C91.42 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C91.42 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C91.42 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C91.42 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation must establish prior remission and current evidence of disease recurrence, including rising hairy cells in blood or bone marrow, returning cytopenias, or splenic re-enlargement.

The interval since last remission, BRAF V600E mutation status (if tested), and planned treatment approach should be recorded.

Updated immunophenotyping at relapse helps confirm diagnosis and rule out other entities.

MEAT Support

HCC Buddy guidance

Documentation must establish prior remission and current evidence of disease recurrence, including rising hairy cells in blood or bone marrow, returning cytopenias, or splenic re-enlargement.

The interval since last remission, BRAF V600E mutation status (if tested), and planned treatment approach should be recorded.

Updated immunophenotyping at relapse helps confirm diagnosis and rule out other entities.

Audit Caution

HCC Buddy guidance

Ensure relapse is documented as recurrence after a prior remission, not simply persistent disease. HCL can relapse many years after initial treatment, so a thorough history review is essential. Late relapses (more than 5 years) may respond well to retreatment with the same purine analog, while early relapses may require alternative approaches -

this clinical context informs accurate coding.

Common Mistakes

HCC Buddy guidance

C91.40 (HCL not in remission) is for disease that never achieved remission.

C91.41 (HCL in remission) applies when disease is controlled.

C91.12 (B-CLL in relapse) should not be used for relapsed HCL.

C91.Z2 (other lymphoid leukemia in relapse) is less specific and should be avoided when HCL is confirmed.

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 C91.42 an HCC code?

Yes. C91.42 (Hairy cell leukemia, in relapse) maps to Bladder, Colorectal, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers under V24), with a community non-dual aged RAF of 0.363. It is billable for payment year 2026.

Coder answer: C91.42 is billable and maps to V28 HCC 22, Bladder, Colorectal, and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: C91.42
Description: Hairy cell leukemia, in relapse
HCC (V28): HCC 22 — Bladder, Colorectal, and Other Cancers
RAF: 0.363
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 22, Bladder, Colorectal, and Other Cancers

0.363

V24HCC 10, Lymphoma and Other Cancers

0.675

ESRDHCC 10, Lymphoma and Other Cancers

0.111

RxHCCHCC 21, Hodgkin Lymphoma and Other Cancers

0.410

Each model's RAF is its CMS base weight for that model's standard population, so 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 segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

Work C91.42 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C91.42

For C91.42 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

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

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed C91.42 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C91.42 is the ICD-10-CM diagnosis code for hairy cell leukemia, in relapse. A rare blood cancer (hairy cell leukemia) with hair-like abnormal cells that has returned after a period of remission. C91.42 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, C91.42 maps to Bladder, Colorectal, and Other Cancers (HCC 22) with a community, non-dual, aged base RAF weight of 0.363. Under the older CMS-HCC V24 model, C91.42 maps to Lymphoma and Other Cancers (HCC 10) with a community, non-dual, aged base RAF weight of 0.675. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

The fifth character '2' indicates relapse - ensure documentation shows cancer has recurred. Because C91.42 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. 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, RAF weights, and MEAT documentation criteria for C91.42 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•The fifth character '2' indicates relapse - ensure documentation shows cancer has recurred
•Document timing and circumstances of relapse when available

Clinical Significance

Hairy cell leukemia in relapse indicates recurrence of disease after a prior period of documented remission. Relapsed HCL may present with recurrent cytopenias, re-enlargement of the spleen, or bone marrow re-infiltration. Second remissions are achievable with the same purine analog or alternative agents (rituximab, vemurafenib for BRAF V600E-mutated cases), though duration of subsequent remissions may be shorter.

Documentation Requirements

✓Documentation must establish prior remission and current evidence of disease recurrence, including rising hairy cells in blood or bone marrow, returning cytopenias, or splenic re-enlargement.
✓The interval since last remission, BRAF V600E mutation status (if tested), and planned treatment approach should be recorded.
✓Updated immunophenotyping at relapse helps confirm diagnosis and rule out other entities.

Commonly Confused Codes

•C91.40 (HCL not in remission) is for disease that never achieved remission.
•C91.41 (HCL in remission) applies when disease is controlled.
•C91.12 (B-CLL in relapse) should not be used for relapsed HCL.
•C91.Z2 (other lymphoid leukemia in relapse) is less specific and should be avoided when HCL is confirmed.