Question 1

What is ICD-10 code M66.329?

Accepted Answer

M66.329 is the ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, unspecified upper arm. The flexor tendons in the upper arm spontaneously tear or rupture, but the specific side (left or right) is not documented or specified. M66.329 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).

M66.329 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.

Query the provider for laterality documentation to assign a more specific code when possible.

Question 2

Is M66.329 an HCC code?

Accepted Answer

No. M66.329 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 M66.329 billable?

Accepted Answer

Yes. M66.329 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. M66.329 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 M66.329?

Accepted Answer

M66.329 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 M66.329 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 M66.329?

Accepted Answer

Query the provider for laterality documentation to assign a more specific code when possible Use only when bilateral involvement is documented or when side cannot be determined

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M66.329

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M66.329 ICD-10-CM Code: Spontaneous rupture of flexor tendons, unspecified upper arm

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M66.329

Is M66.329 an HCC code?

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Work M66.329 in HCC Buddy