M01.X79
BillableDirect infection of unspecified ankle and foot in infectious and parasitic diseases classified elsewhere
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M01.X79 an HCC code?
Yes. M01.X79 maps to Bone/Joint/Muscle/Severe Soft Tissue 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 M01.X79
For M01.X79to 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 M01.X79 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M01.X79 is the ICD-10-CM diagnosis code for direct infection of unspecified ankle and foot in infectious and parasitic diseases classified elsewhere. Joint infection in an unspecified ankle and foot caused by an infectious disease classified elsewhere. M01.X79 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering infectious arthropathies (m00-m02).
Under the CMS-HCC V28 risk adjustment model, M01.X79 maps to Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.479. Under the older CMS-HCC V24 model, M01.X79 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 39) with a community, non-dual, aged base RAF weight of 0.401. 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 ankle or foot joint is not documented or when bilateral involvement is present. Because M01.X79 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 M01.X79 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 ankle or foot joint is not documented or when bilateral involvement is present
- •Must be accompanied by a secondary diagnosis code for the underlying infectious or parasitic disease
Clinical Significance
This code captures direct ankle and foot joint infection secondary to classified infectious or parasitic diseases when laterality is not documented, representing incomplete clinical documentation. The condition poses significant risk for mobility impairment and requires comprehensive treatment of multiple small joints.
Documentation Requirements
- ✓Documentation of ankle and foot joint infection without laterality specification
- ✓Required underlying infectious or parasitic disease code as primary diagnosis
- ✓Evidence that ankle/foot infection is direct extension of classified disease
- ✓Clinical examination findings consistent with infectious ankle/foot arthropathy
- ✓Laboratory markers supporting infectious and inflammatory processes
- ✓Imaging studies demonstrating ankle and foot joint involvement
- ✓Microbiological studies from joint samples when accessible
- ✓Treatment protocol addressing both systemic infection and foot/ankle function
Commonly Confused Codes
- •M00.979 — Pyogenic arthritis, unspecified ankle and foot (bacterial infection not from classified disease)
- •M25.579 — Pain in unspecified ankle and foot (joint symptoms without infectious cause)
- •M01.X71 — Direct infection of right ankle and foot (when laterality is documented)
- •M01.X72 — Direct infection of left ankle and foot (when laterality is documented)