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C91.4 vs C94.3

C91.4 (Hairy cell leukemia) compared with C94.3 (Mast cell leukemia), from the official CMS tabular data.

Summary

These codes should not be reported together because C94.3’s tabular entry lists C91.4 under Excludes1.

Official rule
What is different?

Title wording.The titles differ in one word, “Hairy” (C91.4) versus “Mast” (C94.3): C91.4 is “Hairy cell leukemia”; C94.3 is “Mast cell leukemia”.

Registry fact

Can these codes be reported together?

No.C94.3’s entry carries an Excludes1 note covering C91.4: “leukemic reticuloendotheliosis (C91.4-)”. Both may be reported only when the documentation shows the two conditions are unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Official ruleGuide: Excludes1 vs Excludes2

Is there an Excludes1 relationship?

Yes.C94.3’s tabular entry: “leukemic reticuloendotheliosis (C91.4-)”. The note covers C91.4.

Official ruleGuide: Excludes1 vs Excludes2

Is there an Excludes2 relationship?

None.Neither entry carries an Excludes2 note that names the other code. C91.4 carries 4 Excludes2 notes and C94.3 carries 4 Excludes2 notes covering other codes; the side-by-side table quotes them.

Official rule

Is one more specific?

Not comparable.The codes sit in different categories; specificity is only comparable within one category.

Is one a parent or header code?

Both are headers.Both are non-billable headers, and neither contains the other.

Registry fact

Are there sequencing instructions?

None.Neither entry carries a Code First or Use Additional Code note that names the other.

Official rule

Are there other coding relationships?

None found.No laterality, encounter-phase, Table of Neoplasms, Table of Drugs and Chemicals, or history-versus-active-disease relationship links these codes.

MedCoder-derived

Official rule: a tabular instructional note or a section of the ICD-10-CM Official Guidelines, quoted as published. Registry fact: the codes’ own published attributes (titles, billable status, position in the hierarchy, code set). MedCoder-derived: a reading MedCoder computes from those facts; it is not itself a rule. How to read the labels

Can these codes be reported together?

Excludes1 — generally not reported together

Excludes1 conflict

C94.3
Mast cell leukemia
↔
C91.4
Hairy cell leukemia

What we found

No — do not report together.C94.3 carries an Excludes1 note covering C91.4: “leukemic reticuloendotheliosis (C91.4-)”

Sole exception: when the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

What to review

Report one of the two. Both may be reported only when the documentation shows the two conditions are unrelated to each other.

Guide: Excludes1 vs Excludes2Check these on a claim

Why it matters

An Excludes1 note means the two conditions are not ordinarily reported together for the same encounter, because the classification treats them as mutually exclusive forms of the same condition. The documented exception is when the two conditions are unrelated to each other.

Source / rule

CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a

Official guidance behind these answers

  • Section I.A.12.a — Excludes1 · applies to: Excludes1

    A type 1 Excludes note is a pure excludes note. It means “NOT CODED HERE!” An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition. An exception to the Excludes1 definition is the circumstance when the two conditions are unrelated to each other. If it is not clear whether the two conditions involving an Excludes1 note are related or not, query the provider. For example, code F45.8, Other somatoform disorders, has an Excludes1 note for "sleep related teeth grinding (G47.63)," because "teeth grinding" is an inclusion term under F45.8. Only one of these two codes should be assigned for teeth grinding. However psychogenic dysmenorrhea is also an inclusion term under F45.8, and a patient could have both this condition and sleep related teeth grinding. In this case, the two conditions are clearly unrelated to each other, and so it would be appropriate to report F45.8 and G47.63 together.

    ICD-10-CM Official Guidelines FY2026

Quoted from the ICD-10-CM Official Guidelines for Coding and Reporting in effect for the release shown. The tabular notes above are the code-level instruction; these sections are the convention that says how such a note is applied. Source document (CMS PDF) · Release and checksum

Side by side

C91.4Hairy cell leukemiaC94.3Mast cell leukemia
Billing statusNon-billable header

Report instead: C91.40, C91.41, C91.42

Non-billable header

Report instead: C94.30, C94.31, C94.32

ClassificationC00-D49 — Neoplasms (C00-D49)C00-D49 — Neoplasms (C00-D49)
DefinitionHairy cell leukemia is a non-billable ICD-10-CM category code (C91.4).Mast cell leukemia is a non-billable ICD-10-CM category code (C94.3).
Includes
Leukemic reticuloendotheliosis
—
Excludes1
personal history of leukemia (Z85.6)
leukemic reticuloendotheliosis (C91.4-)
myelodysplastic syndromes (D46.-)
personal history of leukemia (Z85.6)
plasma cell leukemia (C90.1-)
Excludes2
Kaposi's sarcoma of lymph nodes (C46.3)
secondary and unspecified neoplasm of lymph nodes (C77.-)
secondary neoplasm of bone marrow (C79.52)
secondary neoplasm of spleen (C78.89)
Kaposi's sarcoma of lymph nodes (C46.3)
secondary and unspecified neoplasm of lymph nodes (C77.-)
secondary neoplasm of bone marrow (C79.52)
secondary neoplasm of spleen (C78.89)

Notes are the code’s own tabular entry, quoted as published. A note that names the other code is the one the verdict above rests on.

Edit this comparisonCheck a whole claim

Derived from the official CMS ICD-10-CM tabular data (FY2027). A coding-rule comparison, not billing advice: payer-specific edits and medical-necessity policy are outside its scope. All data sources