Question 1

What is ICD-10 code M66.321?

Accepted Answer

M66.321 is the ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, right upper arm. The flexor tendons in the right upper arm spontaneously tear or rupture without a specific injury or trauma. M66.321 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering disorders of synovium and tendon (m65-m67).

M66.321 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.

Confirm the rupture location is specifically the upper arm (not shoulder or forearm) as documented.

Question 2

Is M66.321 an HCC code?

Accepted Answer

No. M66.321 is a billable ICD-10-CM code but does not map to any payment HCC category under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can still be reported on Medicare Advantage encounter data submissions, but it does not contribute to the beneficiary's Risk Adjustment Factor (RAF) score and therefore does not affect the risk-adjusted capitation payment to the plan. V28 reached 100% phase-in for payment year 2026.

Question 3

Is M66.321 billable?

Accepted Answer

Yes. M66.321 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. M66.321 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Question 4

What is the V28 vs V24 difference for M66.321?

Accepted Answer

M66.321 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither community model rewards M66.321 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's V28/V24 community RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a community payment HCC.

Question 5

What should coders know when reporting M66.321?

Accepted Answer

Confirm the rupture location is specifically the upper arm (not shoulder or forearm) as documented Document any associated weakness or loss of function in the right arm for clinical correlation

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M66.321

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M66.321 ICD-10-CM Code: Spontaneous rupture of flexor tendons, right upper arm

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M66.321

Is M66.321 an HCC code?

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