M45.9
BillableAnkylosing spondylitis of unspecified sites in spine
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M45.9 an HCC code?
Yes. M45.9 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease 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 M45.9
For M45.9to 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 M45.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M45.9 is the ICD-10-CM diagnosis code for ankylosing spondylitis of unspecified sites in spine. Ankylosing spondylitis affecting the spine when the specific location or extent of involvement is not documented or specified. M45.9 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering spondylopathies (m45-m49).
Under the CMS-HCC V28 risk adjustment model, M45.9 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 93) with a community, non-dual, aged base RAF weight of 0.175. Under the older V24 model, M45.9 mapped to the same category but with a base RAF weight of 0.307 — 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 only when the specific spinal region cannot be determined from documentation. Because M45.9 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 M45.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use only when the specific spinal region cannot be determined from documentation
- •Query the provider for more specific anatomical location when possible to assign a more precise code
Clinical Significance
Unspecified site ankylosing spondylitis indicates confirmed diagnosis without documentation of specific spinal regions involved. This chronic inflammatory condition requires lifelong monitoring and treatment with biologics or targeted synthetic DMARDs to prevent spinal fusion and disability.
Documentation Requirements
- ✓Confirmed diagnosis of ankylosing spondylitis
- ✓Clinical criteria satisfaction per rheumatology guidelines
- ✓Imaging findings consistent with spondyloarthropathy
- ✓Inflammatory back pain characteristics documented
- ✓Disease activity measures when available
- ✓Treatment history and current medications
- ✓Functional status assessment
- ✓Monitoring for extra-articular complications
Commonly Confused Codes
- •M45.6 — Lumbar region when specific location is documented
- •M45.7 — Lumbosacral when both regions are mentioned
- •M46.9 — Unspecified inflammatory spondylopathy without ankylosing features
- •M48.10 — Ankylosing hyperostosis which is non-inflammatory
- •M54.9 — Dorsalgia when inflammatory component is established