2026 ICD-10-CM Diagnosis Code M05.AAbnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis
M05.A is a billable ICD-10-CM diagnosis code for abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 545 through 547.
For Medicare Advantage risk adjustment, M05.A maps to CMS-HCC Category 93 (Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders) under the V28 model, adding a risk factor of about 0.617 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
M05.A maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Code First
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first followed by the manifestation. Wherever such a combination exists, there is a "use additional code" note at the etiology code, and a "code first" note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Code History & ChangesHistory
New Code M05.A was added to the ICD-10-CM code set for FY 2026, effective October 1, 2025.
Replacement M05.A replaces the following previously assigned code(s):
- M05.9 - Rheumatoid arthritis with rheumatoid factor, unspecified
Questions About M05.AOverview
Is M05.A (Rheumatoid arthritis with rheumatoid factor) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does M05.A group to?
When abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 545, 546, 547, with relative weights from 0.8362 to 2.4817 depending on complications. Higher weights mean higher Medicare reimbursement.
What HCC is M05.A?
M05.A (abnormal rheumatoid factor and anti-citrullinated protein antibody with rheumatoid arthritis) maps to CMS-HCC Category 93 (Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders), commonly written as HCC 93, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 83.
Does M05.A risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, M05.A adds a risk adjustment factor of about 0.617 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.288 to 0.617 depending on the payment segment). HCC 93 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 93 category page.
