M45.5
BillableAnkylosing spondylitis of thoracolumbar region
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M45.5 an HCC code?
Yes. M45.5 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 M45.5
For M45.5to 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 M45.5 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M45.5 is the ICD-10-CM diagnosis code for ankylosing spondylitis of thoracolumbar region. A chronic inflammatory condition affecting the thoracolumbar region (where the mid-back and lower back meet) where spinal bones progressively fuse and stiffen. M45.5 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, M45.5 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, M45.5 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.
This code spans two regions; confirm documentation specifies thoracolumbar involvement rather than isolated thoracic or lumbar. Because M45.5 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 M45.5 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code spans two regions; confirm documentation specifies thoracolumbar involvement rather than isolated thoracic or lumbar
- •Document the extent of fusion and functional limitations for complete clinical picture
Clinical Significance
Ankylosing spondylitis of the thoracolumbar region affects the critical junction between mid-back and lower back, often causing severe functional impairment and kyphosis. This location experiences high mechanical stress and frequently leads to significant disability and pain.
Documentation Requirements
- ✓Documentation of ankylosing spondylitis involving thoracolumbar junction
- ✓Imaging evidence of inflammatory changes at T12-L1 region
- ✓Assessment of thoracolumbar kyphosis and spinal deformity
- ✓Evaluation of functional capacity and disability level
- ✓Documentation of pain patterns specific to thoracolumbar region
- ✓Assessment of impact on respiratory and truncal function
- ✓Treatment response and disease activity at junction
Commonly Confused Codes
- •M45.4 — Ankylosing spondylitis of thoracic region (misses lumbar component)
- •M45.6 — Ankylosing spondylitis lumbar region (misses thoracic component)
- •M47.815 — Spondylosis without myelopathy, thoracolumbar region (degenerative)
- •M54.15 — Radiculopathy, thoracolumbar region (nerve symptoms only)
- •M46.05 — Spinal enthesopathy, thoracolumbar region (different inflammatory condition)