M35.81
BillableMultisystem inflammatory syndrome
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M35.81 an HCC code?
Yes. M35.81 maps to Septicemia, Sepsis, Systemic Inflammatory Response Syndrome/Shock under the CMS-HCC V28 risk adjustment model (and Septicemia, Sepsis, Systemic Inflammatory Response Syndrome/Shock 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 M35.81
For M35.81to 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 M35.81 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M35.81 is the ICD-10-CM diagnosis code for multisystem inflammatory syndrome. A condition where multiple body systems become inflamed at the same time, often affecting the heart, lungs, and other organs. This can occur in children and adults and may be related to infections or immune system problems. M35.81 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering systemic connective tissue disorders (m30-m36).
Under the CMS-HCC V28 risk adjustment model, M35.81 maps to Septicemia, Sepsis, Systemic Inflammatory Response Syndrome/Shock (HCC 2) with a community, non-dual, aged base RAF weight of 0.455. The V24 model used during the PY2024–PY2025 transition mapped M35.81 the same way and at the same RAF weight. 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.
Document which specific systems are involved (cardiac, pulmonary, gastrointestinal, etc.) for more accurate coding. Because M35.81 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 M35.81 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Document which specific systems are involved (cardiac, pulmonary, gastrointestinal, etc.) for more accurate coding
- •Distinguish from other systemic inflammatory conditions and document any triggering factors or associated diagnoses
Clinical Significance
Multisystem inflammatory syndrome represents a severe acute inflammatory condition requiring immediate intensive management and monitoring for life-threatening complications. This diagnosis carries high mortality risk and demands intensive care resources with multi-specialty involvement.
Documentation Requirements
- ✓Documentation of inflammatory involvement of multiple organ systems
- ✓Laboratory evidence of systemic inflammation (elevated inflammatory markers)
- ✓Clinical manifestations affecting cardiovascular, respiratory, GI, or neurologic systems
- ✓Exclusion of other causes of multisystem inflammation
- ✓Timeline and progression of inflammatory symptoms
- ✓Treatment response to anti-inflammatory or immunosuppressive therapy
- ✓Association with preceding infection or trigger if identified
Use Additional Code
Code First
- , if applicable, COVID-19 (U07.1)
Code Also
- any associated complications such as:
- acute hepatic failure (K72.0-)
- acute kidney failure (N17.-)
- acute myocarditis (I40.-)
- acute respiratory distress syndrome (J80)
- cardiac arrhythmia (I47-I49.-)
- pneumonia due to COVID-19 (J12.82)
- severe sepsis (R65.2-)
- viral cardiomyopathy (B33.24)
- viral pericarditis (B33.23)
Commonly Confused Codes
- •R65.20 — Severe sepsis without septic shock (infectious rather than inflammatory)
- •M32.10 — Systemic lupus erythematosus with organ involvement (chronic autoimmune)
- •T78.2 — Anaphylactic shock, unspecified (acute allergic reaction)
- •M35.9 — Systemic involvement of connective tissue, unspecified (less acute)
- •D89.9 — Disorder involving the immune mechanism, unspecified (less specific)