What is ICD-10 code M05.479?

M05.479 is the ICD-10-CM diagnosis code for Rheumatoid myopathy with rheumatoid arthritis of unspecified ankle and foot. Rheumatoid arthritis affecting the ankle and foot with muscle weakness or damage, when the specific foot (left or right) is not specified. M05.479 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. M05.479 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. M05.479 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. M05.479 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 M05.479?

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 M05.479, V28 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease at RAF 0.175 while V24 mapped to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease at RAF 0.307. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting M05.479?

Use only when laterality is truly unspecified or cannot be determined from documentation Review chart for any indication of bilateral involvement that would warrant M05.49 Because M05.479 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.

M05.479 ICD-10 Code: Rheumatoid myopathy

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupM05.479Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the musculoskeletal system and connective tissue (M00-M99) / Inflammatory polyarthropathies (M05-M14)

M05.479

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

Rheumatoid myopathy with rheumatoid arthritis of unspecified ankle and foot

Rheumatoid arthritis affecting the ankle and foot with muscle weakness or damage, when the specific foot (left or right) is not specified.

Buddy Insight

This diagnosis captures rheumatoid arthritis with myopathy affecting the ankle and foot when specific laterality cannot be determined.

CMS-HCC V28

Mapped

HCC 93

Rheumatoid Arthritis and Infl...

RAF 0.175

CMS-HCC V24

Mapped

HCC 40

Rheumatoid Arthritis and Infl...

RAF 0.307

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Mapped

HCC 40

Rheumatoid Arthritis and Infl...

RAF 0.0

RXHCC

Mapped

HCC 83

Rheumatoid Arthritis and Infl...

RAF 0.0

Code Trumping

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Code Book Path

Official

M05.4Rheumatoid myopathy with rheumatoid arthritis

M05.47Rheumatoid myopathy with rheumatoid arthritis of ankle and foot

M05.479Rheumatoid myopathy with rheumatoid arthritis of unspecified ankle and foot

Inclusion Terms

Official

Rheumatoid myopathy with rheumatoid arthritis, tarsus, metatarsus and phalanges

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for M05.479 in this effective period.

Related Child Codes

Official

M05.471Rheumatoid myopathy with rheumatoid arthritis of right ankle and foot

M05.472Rheumatoid myopathy with rheumatoid arthritis of left ankle and foot

Includes

Official

ICD-10-CM does not list Includes notes for M05.479 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for M05.479 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for M05.479 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for M05.479 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for M05.479 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Confirmed rheumatoid arthritis diagnosis with clinical or laboratory support

Documentation of ankle and foot involvement without laterality specification

Clinical evidence of myopathy with muscle weakness or atrophy

Explanation of why ankle/foot laterality cannot be determined

MEAT Support

HCC Buddy guidance

Confirmed rheumatoid arthritis diagnosis with clinical or laboratory support

Documentation of ankle and foot involvement without laterality specification

Clinical evidence of myopathy with muscle weakness or atrophy

Explanation of why ankle/foot laterality cannot be determined

Audit Caution

HCC Buddy guidance

Using this code when ankle/foot laterality is documented in the record

Failing to document why laterality cannot be determined

Missing the myopathy component in clinical documentation

Using when bilateral ankle/foot involvement should be coded separately

Common Mistakes

HCC Buddy guidance

M05.471 — Rheumatoid myopathy with RA of right ankle and foot (when laterality known)

M05.472 — Rheumatoid myopathy with RA of left ankle and foot (when laterality known)

M06.879 — Other specified rheumatoid arthritis of unspecified ankle and foot (lacks myopathy)

M25.579 — Pain in unspecified ankle and joints of unspecified foot (symptom only)

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is M05.479 an HCC code?

Yes. M05.479 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under V24).

HCC Category Mapping

V28HCC 93, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.175

V24HCC 40, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.307

ESRDHCC 40, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.000

RxHCCHCC 83, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.000

RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. 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.

MEAT Criteria for M05.479

For M05.479to 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 M05.479 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

M05.479 is the ICD-10-CM diagnosis code for rheumatoid myopathy with rheumatoid arthritis of unspecified ankle and foot. Rheumatoid arthritis affecting the ankle and foot with muscle weakness or damage, when the specific foot (left or right) is not specified. M05.479 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering inflammatory polyarthropathies (m05-m14).

Under the CMS-HCC V28 risk adjustment model, M05.479 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 93) with a community, non-dual, aged base RAF weight of 0.175. Under the older V24 model, M05.479 mapped to the same category but with a base RAF weight of 0.307, V28 recalibrated weights across the entire model. 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.

Use only when laterality is truly unspecified or cannot be determined from documentation. Because M05.479 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 M05.479 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use only when laterality is truly unspecified or cannot be determined from documentation
•Review chart for any indication of bilateral involvement that would warrant M05.49

Clinical Significance

This diagnosis captures rheumatoid arthritis with myopathy affecting the ankle and foot when specific laterality cannot be determined. The combination indicates severe systemic disease requiring comprehensive management of both inflammatory joint disease and muscle pathology affecting weight-bearing function.

Documentation Requirements

✓Confirmed rheumatoid arthritis diagnosis with clinical or laboratory support
✓Documentation of ankle and foot involvement without laterality specification
✓Clinical evidence of myopathy with muscle weakness or atrophy
✓Explanation of why ankle/foot laterality cannot be determined
✓Assessment of functional impact on gait, balance, and mobility
✓Exclusion of other causes of ankle/foot symptoms and muscle weakness
✓Treatment plan addressing both ankle/foot arthritis and systemic myopathy

Commonly Confused Codes

•M05.471: Rheumatoid myopathy with RA of right ankle and foot (when laterality known)
•M05.472: Rheumatoid myopathy with RA of left ankle and foot (when laterality known)
•M06.879: Other specified rheumatoid arthritis of unspecified ankle and foot (lacks myopathy)
•M25.579: Pain in unspecified ankle and joints of unspecified foot (symptom only)
•M19.079: Primary osteoarthritis, unspecified ankle and foot (different arthritis type)