M86.661
BillableOther chronic osteomyelitis, right tibia and fibula
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M86.661 an HCC code?
Yes. M86.661 maps to Bone/Joint/Muscle Infections/Necrosis under the CMS-HCC V28 risk adjustment model (and Bone/Joint/Muscle Infections/Necrosis 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 M86.661
For M86.661 to 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 M86.661 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M86.661 is the ICD-10-CM diagnosis code for other chronic osteomyelitis, right tibia and fibula. A chronic bone infection affecting the right shin bone (tibia) and the smaller bone next to it (fibula) that has been ongoing. M86.661 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering other osteopathies (m86-m90).
Under the CMS-HCC V28 risk adjustment model, M86.661 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.209. Under the older V24 model, M86.661 mapped to the same category but with a base RAF weight of 0.482 — 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.
This code specifically indicates the right lower leg; do not use if the infection is bilateral or on the left side. Because M86.661 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 M86.661 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifically indicates the right lower leg; do not use if the infection is bilateral or on the left side
- •Document whether this is a recurrent infection or a continuation of a previous osteomyelitis episode
Clinical Significance
Chronic osteomyelitis of the right tibia and fibula indicates a severe, persistent bone infection in weight-bearing bones that significantly affects mobility and quality of life. This condition requires intensive medical management and carries high risk for complications including pathological fractures and treatment failure.
Documentation Requirements
- ✓Explicit documentation of chronic osteomyelitis diagnosis
- ✓Clear specification of right tibia and fibula involvement
- ✓Evidence supporting chronicity through time course or recurrence
- ✓Radiological findings consistent with chronic osteomyelitis
- ✓Microbiological cultures or biopsy results when available
- ✓Documentation of current or previous antibiotic therapy
- ✓Assessment of functional limitations and mobility impact
- ✓Presence of sequestrum, involucrum, or draining sinuses if applicable
Commonly Confused Codes
- •M86.661 — Acute osteomyelitis of right tibia/fibula lacks chronic time element
- •M87.064 — Aseptic necrosis of right tibia is bone death without bacterial infection
- •M86.162 — Acute osteomyelitis of right tibia/fibula represents initial infection phase
- •M25.561 — Pain in right lower leg doesn't indicate active bone infection