B46.1
BillableRhinocerebral mucormycosis
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is B46.1 an HCC code?
Yes. B46.1 maps to Opportunistic Infections under the CMS-HCC V28 risk adjustment model (and Opportunistic Infections 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 B46.1
For B46.1to 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 B46.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
B46.1 is the ICD-10-CM diagnosis code for rhinocerebral mucormycosis. A serious fungal infection caused by mucormycosis that affects the nasal passages and spreads to the brain and surrounding tissues. This is a life-threatening condition that typically occurs in people with weakened immune systems. B46.1 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering mycoses (b35-b49).
Under the CMS-HCC V28 risk adjustment model, B46.1 maps to Opportunistic Infections (HCC 6) with a community, non-dual, aged base RAF weight of 0.381. Under the older V24 model, B46.1 mapped to the same category but with a base RAF weight of 0.424 — 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.
Rhinocerebral mucormycosis is a specific site specification of mucormycosis - do not use the more general B46.9 code if the rhinocerebral location is documented. Because B46.1 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 B46.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Rhinocerebral mucormycosis is a specific site specification of mucormycosis - do not use the more general B46.9 code if the rhinocerebral location is documented
- •This code often requires additional documentation of the underlying immunocompromised state (such as diabetes, HIV, or hematologic malignancy) which should be coded separately to capture the complete clinical picture
Clinical Significance
Rhinocerebral mucormycosis is the most common and most recognizable form of mucormycosis, originating in the sinuses and rapidly invading through bone into the orbit and brain. It is strongly associated with diabetic ketoacidosis and carries mortality rates of 25-60%. The angioinvasive nature causes tissue necrosis, black eschar formation, and rapid progression requiring emergent surgical intervention.
Documentation Requirements
- ✓CT/MRI imaging showing sinus opacification with bone erosion and orbital/intracranial extension
- ✓Tissue biopsy from nasal/sinus debridement confirming Mucorales
- ✓Clinical findings: facial pain, nasal congestion, black necrotic eschar, cranial nerve palsies, proptosis
- ✓Underlying condition: diabetic ketoacidosis, blood glucose levels, hemoglobin A1c
- ✓Surgical debridement performed and extent documented
Commonly Confused Codes
- •B46.0 (Pulmonary mucormycosis) - lung involvement only; rhinocerebral involves sinuses, orbit, and brain
- •J32.9 (Chronic sinusitis, unspecified) - invasive fungal sinusitis is a surgical emergency, not chronic bacterial sinusitis
- •B46.5 (Mucormycosis, unspecified) - never use when rhinocerebral presentation is documented