C40.31
BillableMalignant neoplasm of short bones of right lower limb
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C40.31 an HCC code?
Yes. C40.31 maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers 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 C40.31
For C40.31to 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 C40.31 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C40.31 is the ICD-10-CM diagnosis code for malignant neoplasm of short bones of right lower limb. Cancer that starts in the small bones of the right leg or foot, such as the ankle or foot bones. C40.31 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of bone and articular cartilage (c40-c41).
Under the CMS-HCC V28 risk adjustment model, C40.31 maps to Lymphoma and Other Cancers (HCC 21) with a community, non-dual, aged base RAF weight of 0.671. Under the older V24 model, C40.31 mapped to the same category but with a base RAF weight of 0.675 — 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.
Confirm right side laterality is documented; this code is specific to right lower limb short bones. Because C40.31 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 C40.31 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Confirm right side laterality is documented; this code is specific to right lower limb short bones
- •Short bones include tarsals and metatarsals; ensure distinction from long bone malignancies
Clinical Significance
Primary malignant neoplasm of the short bones (tarsals, metatarsals) of the right lower limb. Foot and ankle bone cancers are rare and may present as persistent swelling or pain often mistaken for trauma or infection. Accurate laterality coding is important for surgical planning and risk adjustment documentation.
Documentation Requirements
- ✓Right-side laterality explicitly documented
- ✓Specific short bone involved (calcaneus, talus, navicular, cuboid, cuneiform, metatarsal)
- ✓Histologic type from biopsy (chondrosarcoma, Ewing sarcoma, osteosarcoma)
- ✓Primary vs. metastatic origin confirmed
- ✓Imaging findings (radiograph, MRI, CT) supporting the diagnosis
- ✓Treatment plan and current disease status
Commonly Confused Codes
- •C40.32 — Short bones of LEFT lower limb; laterality differentiator
- •C40.30 — Short bones of UNSPECIFIED lower limb; avoid when right side is documented
- •C40.21 — Long bones of right lower limb; different anatomic axis
- •C49.21 — Malignant neoplasm of connective and soft tissue of right lower limb; for soft tissue sarcomas, not bone