M86.659
BillableOther chronic osteomyelitis, unspecified thigh
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M86.659 an HCC code?
Yes. M86.659 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 M86.659
For M86.659 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 M86.659 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M86.659 is the ICD-10-CM diagnosis code for other chronic osteomyelitis, unspecified thigh. A chronic bone infection in the thigh bone that has persisted over time, where the specific side is not specified. M86.659 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, M86.659 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, M86.659 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.
Use this code only when the thigh location is documented but laterality (left/right) cannot be determined from the medical record. Because M86.659 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 M86.659 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
Clinical Significance
Chronic osteomyelitis represents a persistent bone infection that significantly impacts patient morbidity and healthcare utilization. This condition indicates a compromised immune status and bone integrity, requiring long-term antibiotic therapy and potentially surgical intervention, making it a high-risk condition for complications.
Documentation Requirements
- ✓Clear documentation of chronic osteomyelitis diagnosis
- ✓Specification of thigh location (femur involvement)
- ✓Evidence of chronicity through duration or recurrent episodes
- ✓Supporting diagnostic imaging (MRI, bone scan, or CT)
- ✓Laboratory evidence such as elevated ESR, CRP, or positive cultures
- ✓Documentation of antibiotic therapy or surgical treatment
- ✓Assessment of functional impact on patient mobility
- ✓Identification of underlying risk factors or comorbidities
Commonly Confused Codes
- •M86.10 — Acute osteomyelitis lacks the chronic time component and represents initial bone infection
- •M87.9 — Aseptic necrosis is bone death without infection, while osteomyelitis involves active bacterial infection
- •M86.20 — Subacute osteomyelitis represents intermediate stage between acute and chronic phases
- •M86.9 — Unspecified osteomyelitis lacks the chronic designation and anatomical specificity