M05.A ICD-10-CM Code: Abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis

M05.A is the ICD-10-CM diagnosis code for abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis. Rheumatoid arthritis in patients who have abnormal blood test results showing both rheumatoid factor and anti-citrullinated protein antibodies, indicating a more aggressive form of the disease. M05.A 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.A maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders (HCC 93) with a community, non-dual, aged base RAF weight of 0.617. M05.A was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. 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.

This code indicates dual positive serology (both RF and anti-CCP antibodies present). Because M05.A 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.