What is ICD-10 code M46.21?

M46.21 is the ICD-10-CM diagnosis code for Osteomyelitis of vertebra, occipito-atlanto-axial region. A bone infection (osteomyelitis) affecting the vertebrae in the upper neck region where the skull connects to the spine (occipito-atlanto-axial region). M46.21 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. M46.21 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. M46.21 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. M46.21 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 M46.21 map to under V28?

M46.21 maps to CMS-HCC V28 HCC 92 (Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

What documentation supports M46.21?

This rare location involves the C0-C2 vertebrae; ensure documentation clearly specifies this upper cervical region Infections in this region can be serious due to proximity to vital structures; verify if additional codes for complications are needed Because M46.21 maps to an HCC category, the documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. MEAT is generic documentation evidence, not a payer-specific rule.

2026 ICD-10-CM M46.21: Osteomyelitis of vertebra

M46.21 maps to CMS-HCC V28 92. A source-labeled RAF reference is available. Documentation must support MEAT. MEAT criteria · RAF Calculator · HCC coding software

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Code lookupM46.21Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the musculoskeletal system and connective tissue (M00-M99) / Spondylopathies (M45-M49)

M46.21

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

Osteomyelitis of vertebra, occipito-atlanto-axial region

A bone infection (osteomyelitis) affecting the vertebrae in the upper neck region where the skull connects to the spine (occipito-atlanto-axial region).

Buddy Insight

Osteomyelitis in the upper cervical spine is particularly dangerous due to proximity to the brainstem and potential for neurological complications.

CMS-HCC V28

Mapped

HCC 92

Bone/Joint/Muscle/Severe Soft...

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

Mapped

HCC 055

HHS-HCC 055 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 39

Bone/Joint/Muscle Infections/...

Code-level coefficient reference

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

M46Other inflammatory spondylopathies

M46.2Osteomyelitis of vertebra

M46.21Osteomyelitis of vertebra, occipito-atlanto-axial region

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for M46.21 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for M46.21 in this effective period.

Related Child Codes

Official

M46.20Osteomyelitis of vertebra, site unspecified

M46.22Osteomyelitis of vertebra, cervical region

M46.23Osteomyelitis of vertebra, cervicothoracic region

M46.24Osteomyelitis of vertebra, thoracic region

M46.25Osteomyelitis of vertebra, thoracolumbar region

Includes

Official

ICD-10-CM does not list Includes notes for M46.21 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for M46.21 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for M46.21 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for M46.21 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for M46.21 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation of bone infection specifically in occipito-atlanto-axial region

Clinical presentation consistent with upper cervical osteomyelitis

Imaging studies confirming infection in C1-C2 region

Neurological assessment for cranial nerve or brainstem involvement

MEAT Support

HCC Buddy guidance

Documentation of bone infection specifically in occipito-atlanto-axial region

Clinical presentation consistent with upper cervical osteomyelitis

Imaging studies confirming infection in C1-C2 region

Neurological assessment for cranial nerve or brainstem involvement

Audit Caution

HCC Buddy guidance

Confusing anatomical boundaries of occipito-atlanto-axial region

Missing documentation of specific upper cervical involvement

Not distinguishing from other cervical spine infections

Coding disc infection when vertebral osteomyelitis is present

Common Mistakes

HCC Buddy guidance

M4622 — Cervical osteomyelitis affects lower cervical vertebrae

G04.91 — Myelitis is spinal cord inflammation, not bone infection

M50.00 — Cervical disc disorder is degenerative, not infectious

A39.82 — Meningococcal arthritis affects joints, not vertebral bone

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 M46.21 an HCC code?

Yes. M46.21 (Osteomyelitis of vertebra, occipito-atlanto-axial region) maps to HCC 92, Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.479. 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: M46.21 is billable and maps to V28 HCC 92, Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: M46.21
Description: Osteomyelitis of vertebra, occipito-atlanto-axial region
HCC (V28): HCC 92 — Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis
RAF reference coefficient: 0.479
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 92, Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis

0.479

ESRDHCC 39, Bone/Joint/Muscle Infections/Necrosis

Not separately weighted

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 M46.21 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT Criteria for M46.21

For M46.21 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 M46.21 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

M46.21 is the ICD-10-CM diagnosis code for osteomyelitis of vertebra, occipito-atlanto-axial region. A bone infection (osteomyelitis) affecting the vertebrae in the upper neck region where the skull connects to the spine (occipito-atlanto-axial region). M46.21 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering spondylopathies (m45-m49).

Under the CMS-HCC V28 risk adjustment model, M46.21 maps to Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis (HCC 92) with a source-labeled community, non-dual, aged reference coefficient of 0.479. No V24 mapping is shown for M46.21; use the applicable model and payment year when reviewing the V28 mapping. 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. 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.

This rare location involves the C0-C2 vertebrae; ensure documentation clearly specifies this upper cervical region. Because M46.21 maps to an HCC category, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment. 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 M46.21 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•This rare location involves the C0-C2 vertebrae; ensure documentation clearly specifies this upper cervical region
•Infections in this region can be serious due to proximity to vital structures; verify if additional codes for complications are needed

Clinical Significance

Osteomyelitis in the upper cervical spine is particularly dangerous due to proximity to the brainstem and potential for neurological complications. This location requires urgent treatment and careful monitoring for spinal instability and neurological deterioration.