D81.6 ICD-10-CM Code: Major histocompatibility complex class I deficiency

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for D81.6 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on D81.6 itself; “inherited from” names the category or block whose note applies here.

This page already reflects the FY2027 tabular note taking effect October 1, 2026.

Coder workflow for D81.6

MedCoder structured workflow — derived from this code’s own official record

Before you code D81.6

1. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with D81.6. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

   See the official tabular notes

Choose the right path

1. Does the documentation support a condition named in D81.6’s Excludes1 note?
   Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
   No → Continue.

   ReviewD80.0, M35.9, D71

Consider D81.6. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Severity

Mild, moderate or severe as the provider states it; severity is not inferred from the clinical picture.

The associated condition or complication

Whether the associated condition the title names is documented; the “with” convention presumes some links, and a provider statement that the conditions are unrelated defeats it (Guidelines I.A.15).

Official instructions as workflow

• Excludes1 — check before selecting D81.6(3 notes)

  Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with D81.6: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

  CompareD80.0, M35.9, D71

  See the official tabular notes · Guidelines I.A.12.a

• Excludes2 — not part of D81.6(1 note)

  Coding workflow: The conditions named in this note are not included in D81.6. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareB20

  See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (2)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Deficiency, deficient, major histocompatibility complex, class I
• Syndrome, bare lymphocyte

Official Coding Guidelines

No excerpt in the ICD-10-CM Official Guidelines names this code specifically. Its chapter carries only this chapter-wide note:

Don’t confuse D81.6 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of D81.6 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• D80.0— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of autosomal recessive agammaglobulinemia (Swiss type) moves the case to D80.0; the two are not reported together unless documented as unrelated.

  Full comparison: D81.6 vs D80.0 →

• M35.9— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of autoimmune disease (systemic) NOS moves the case to M35.9; the two are not reported together unless documented as unrelated.

  Full comparison: D81.6 vs M35.9 →

• D71— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of functional disorders of polymorphonuclear neutrophils moves the case to D71; the two are not reported together unless documented as unrelated.

  Full comparison: D81.6 vs D71 →

• D81.9— Specificity relationship (Review)

  Overlapping diagnoses in the same category.

  Overlapping codes in the same category — the documentation decides which is the more specific fit.

  What moves you there: A record that documents no more than “Combined immunodeficiency, unspecified” takes D81.9; specificity in the record moves it back here.

  Full comparison: D81.6 vs D81.9 →

• B20— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of human immunodeficiency virus [HIV] disease adds B20; it is not part of this code and both may be reported when both are documented.

  Full comparison: D81.6 vs B20 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name D81.6 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 3 Excludes1 notes across 2 chapters: D72 — Other disorders of white blood cells (via D81.-), D84.81 — Immunodeficiency due to conditions classified elsewhere (via D81.-), E79 — Disorders of purine and pyrimidine metabolism (via D81.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 44 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 3 MS-DRGs: DRG 808 (MDC 16), DRG 809 (MDC 16), DRG 810 (MDC 16).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):BLD008 — Immunity disorders (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

D80.4 — Selective deficiency of immunoglobulin M [IgM], D80.5 — Immunodeficiency with increased immunoglobulin M [IgM], D80.6 — Antibody deficiency with near-normal immunoglobulins or with hyperimmunoglobulinemia, D80.7 — Transient hypogammaglobulinemia of infancy, D80.8 — Other immunodeficiencies with predominantly antibody defects, D80.9 — Immunodeficiency with predominantly antibody defects, unspecified, D81.0 — Severe combined immunodeficiency [SCID] with reticular dysgenesis, D81.1 — Severe combined immunodeficiency [SCID] with low T- and B-cell numbers, D81.2 — Severe combined immunodeficiency [SCID] with low or normal B-cell numbers, D81.4 — Nezelof's syndrome, D81.7 — Major histocompatibility complex class II deficiency, D81.82 — Activated Phosphoinositide 3-kinase Delta Syndrome [APDS], D81.89 — Other combined immunodeficiencies, D81.9 — Combined immunodeficiency, unspecified, D82.0 — Wiskott-Aldrich syndrome, D82.1 — Di George's syndrome, D82.2 — Immunodeficiency with short-limbed stature, D82.3 — Immunodeficiency following hereditary defective response to Epstein-Barr virus, D82.4 — Hyperimmunoglobulin E [IgE] syndrome, D82.8 — Immunodeficiency associated with other specified major defects, +23 more

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Common Variable and Combined Immunodeficiencies) for risk-adjusted payment.

D81.0 — Severe combined immunodeficiency [SCID] with reticular dysgenesis, D81.1 — Severe combined immunodeficiency [SCID] with low T- and B-cell numbers, D81.2 — Severe combined immunodeficiency [SCID] with low or normal B-cell numbers, D81.30 — Adenosine deaminase deficiency, unspecified, D81.31 — Severe combined immunodeficiency due to adenosine deaminase deficiency, D81.32 — Adenosine deaminase 2 deficiency, D81.39 — Other adenosine deaminase deficiency, D81.5 — Purine nucleoside phosphorylase [PNP] deficiency, D81.7 — Major histocompatibility complex class II deficiency, D81.82 — Activated Phosphoinositide 3-kinase Delta Syndrome [APDS], D81.89 — Other combined immunodeficiencies, D81.9 — Combined immunodeficiency, unspecified, D83.0 — Common variable immunodeficiency with predominant abnormalities of B-cell numbers and function, D83.1 — Common variable immunodeficiency with predominant immunoregulatory T-cell disorders, D83.2 — Common variable immunodeficiency with autoantibodies to B- or T-cells, D83.8 — Other common variable immunodeficiencies, D83.9 — Common variable immunodeficiency, unspecified

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Specified Immunodeficiencies and White Blood Cell Disorders

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Immunity disorders).

D81.0 — Severe combined immunodeficiency [SCID] with reticular dysgenesis, D81.1 — Severe combined immunodeficiency [SCID] with low T- and B-cell numbers, D81.2 — Severe combined immunodeficiency [SCID] with low or normal B-cell numbers, D81.3 — Adenosine deaminase [ADA] deficiency, D81.30 — Adenosine deaminase deficiency, unspecified, D81.31 — Severe combined immunodeficiency due to adenosine deaminase deficiency, D81.32 — Adenosine deaminase 2 deficiency, D81.39 — Other adenosine deaminase deficiency, D81.4 — Nezelof's syndrome, D81.5 — Purine nucleoside phosphorylase [PNP] deficiency, D81.7 — Major histocompatibility complex class II deficiency, D81.810 — Biotinidase deficiency, D81.818 — Other biotin-dependent carboxylase deficiency, D81.819 — Biotin-dependent carboxylase deficiency, unspecified, D81.82 — Activated Phosphoinositide 3-kinase Delta Syndrome [APDS], D81.89 — Other combined immunodeficiencies, D81.9 — Combined immunodeficiency, unspecified, D82.0 — Wiskott-Aldrich syndrome, D82.1 — Di George's syndrome, D82.2 — Immunodeficiency with short-limbed stature, +64 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Deficiency, deficient”; these codes share that main term but sit in a different category of the Tabular List.

D74.0 — Congenital methemoglobinemia (NADH-methemoglobin reductase), D75.A — Glucose-6-phosphate dehydrogenase (G6PD) deficiency without anemia (glucose-6-phosphate dehydrogenase, without anemia), D80.0 — Hereditary hypogammaglobulinemia (gammaglobulin in blood, hereditary), D80.1 — Nonfamilial hypogammaglobulinemia (gammaglobulin in blood), D80.2 — Selective deficiency of immunoglobulin A [IgA] (immunity, IgA), D80.3 — Selective deficiency of immunoglobulin G [IgG] subclasses (immunity, IgG), D80.4 — Selective deficiency of immunoglobulin M [IgM] (immunity, IgM), D80.6 — Antibody deficiency with near-normal immunoglobulins or with hyperimmunoglobulinemia (antibody with, hyperimmunoglobulinemia), D80.8 — Other immunodeficiencies with predominantly antibody defects (kappa-light chain), D80.9 — Immunodeficiency with predominantly antibody defects, unspecified (immunity, humoral), D82.0 — Wiskott-Aldrich syndrome (immunity, cell-mediated, with thrombocytopenia and eczema), D84.1 — Defects in the complement system (C1 esterase inhibitor), D84.89 — Other immunodeficiencies (immunity, cell-mediated), D84.9 — Immunodeficiency, unspecified (immunity), E20.9 — Hypoparathyroidism, unspecified (parathyroid), E23.0 — Hypopituitarism (gonadotropin), E23.2 — Diabetes insipidus (vasopressin), E25.0 — Congenital adrenogenital disorders associated with enzyme deficiency (21-hydroxylase), E27.1 — Primary adrenocortical insufficiency (corticoadrenal, primary), E27.40 — Unspecified adrenocortical insufficiency (corticoadrenal), +165 more

Contextual Map

Every relationship of D81.6 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run D81.6 with these 3 related codes in Claim Check

Hierarchy

• D50-D89 — Chapter 3: Diseases of the Blood and Blood-forming Organs and Certain Disorders Involving the Immune Mechanism (D50-D89) (D50-D89)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D80-D89 — Certain disorders involving the immune mechanism[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes1 notes

• D72 — Other disorders of white blood cells[Excludes1](via D81.-): “immunity disorders (D80-D89)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• D84.81 — Immunodeficiency due to conditions classified elsewhere[Excludes1](via D81.-): “certain disorders involving the immune mechanism (D80-D83, D84.0, D84.1, D84.9)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• E79 — Disorders of purine and pyrimidine metabolism[Excludes1](via D81.-): “combined immunodeficiency disorders (D81.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• BLD008 — Immunity disorders[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

Risk adjustment (CMS-HCC)

• HCC 114 — Common Variable and Combined Immunodeficiencies [CMS-HCC]: “Common Variable and Combined Immunodeficiencies — supersedes HCC 115 (Specified Immunodeficiencies and White Blood Cell Disorders)”— CMS-HCC V28 · 2026

MS-DRG Grouper

• CC — Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (CC).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
• DRG 808 — MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITH MCC[MS-DRG]: “MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITH MCC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 809 — MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITH CC[MS-DRG]: “MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITH CC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 810 — MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITHOUT CC/MCC[MS-DRG]: “MAJOR HEMATOLOGICAL AND IMMUNOLOGICAL DIAGNOSES EXCEPT SICKLE CELL CRISIS AND COAGULATION DISORDERS WITHOUT CC/MCC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• MDC 16 — Diseases and Disorders of Blood, Blood Forming Organs and Immunologic Disorders[MDC crossing]: “Diseases and Disorders of Blood, Blood Forming Organs and Immunologic Disorders — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 1,614 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Deficiency, deficient, major histocompatibility complex, class I[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Syndrome, bare lymphocyte[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (20)

• D81 — Combined immunodeficiencies[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.0 — Severe combined immunodeficiency [SCID] with reticular dysgenesis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.1 — Severe combined immunodeficiency [SCID] with low T- and B-cell numbers[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.2 — Severe combined immunodeficiency [SCID] with low or normal B-cell numbers[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.3 — Adenosine deaminase [ADA] deficiency[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.30 — Adenosine deaminase deficiency, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.31 — Severe combined immunodeficiency due to adenosine deaminase deficiency[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D81.32 — Adenosine deaminase 2 deficiency[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 12 more

Change history (3)

• FY2027 — Excludes2 note will be added[Change history]— CMS release files (code change ledger) · icd10cm-fy2027
• and 2 more

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Coding guidelines Official source data

ICD-10-CM Official Guidelines for Coding and Reporting (FY2026), quoted by section Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Risk adjustment Official source data

2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to D81.6 in its code family, with their registry titles.

• D81.31 — Severe combined immunodeficiency due to adenosine deaminase deficiency
• D81.32 — Adenosine deaminase 2 deficiency
• D81.39 — Other adenosine deaminase deficiency
• D81.4 — Nezelof's syndrome
• D81.5 — Purine nucleoside phosphorylase [PNP] deficiency
• D81.7 — Major histocompatibility complex class II deficiency
• D81.8 — Other combined immunodeficiencies
• D81.81 — Biotin-dependent carboxylase deficiency
• D81.810 — Biotinidase deficiency
• D81.818 — Other biotin-dependent carboxylase deficiency

View all codes in the D81 family