M04.9
BillableAutoinflammatory syndrome, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M04.9 an HCC code?
Yes. M04.9 maps to Reactive and Specified Arthropathies under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under V24).
HCC Category Mapping
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 M04.9
For M04.9to 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 M04.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M04.9 is the ICD-10-CM diagnosis code for autoinflammatory syndrome, unspecified. A group of inflammatory conditions where the body's immune system causes recurring inflammation, but the specific type has not been determined. M04.9 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering autoinflammatory syndromes (m04).
Under the CMS-HCC V28 risk adjustment model, M04.9 maps to Reactive and Specified Arthropathies (HCC 94) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, M04.9 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 40) with a community, non-dual, aged base RAF weight of 0.307. 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 this code only when the specific autoinflammatory syndrome cannot be identified; consider querying the provider for more specific diagnosis. Because M04.9 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 M04.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use this code only when the specific autoinflammatory syndrome cannot be identified; consider querying the provider for more specific diagnosis
- •Review documentation for symptoms like recurrent fever, rashes, or joint pain to help identify the specific syndrome
Clinical Significance
This represents autoinflammatory syndrome when the specific type is not specified or determined. This code indicates a confirmed autoinflammatory process but lacks specificity, suggesting need for further evaluation to identify the exact syndrome for optimal treatment planning and monitoring.
Documentation Requirements
- ✓Documentation confirming autoinflammatory syndrome diagnosis
- ✓Evidence of recurrent inflammatory episodes
- ✓Exclusion of infectious and autoimmune causes
- ✓Clinical features consistent with autoinflammatory process
- ✓Laboratory evidence of systemic inflammation
- ✓Justification for why specific syndrome type cannot be determined
- ✓Documentation of diagnostic workup performed
- ✓Treatment response to anti-inflammatory therapies
Commonly Confused Codes
- •Specific autoinflammatory syndrome codes (M04.1, M04.2, M04.8) when type is known
- •M35.9 — Systemic involvement of connective tissue, unspecified (autoimmune vs autoinflammatory)
- •D89.9 — Disorder involving the immune mechanism, unspecified
- •R50.9 — Fever, unspecified (symptom vs syndrome diagnosis)
- •M79.3 — Panniculitis, unspecified