M45.4 ICD-10-CM Code: Ankylosing spondylitis of thoracic region
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FY 2026 Apr update / Diseases of the musculoskeletal system and connective tissue (M00-M99) / Spondylopathies (M45-M49)
M45.4
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceAnkylosing spondylitis of thoracic region
A chronic inflammatory condition where the bones in the thoracic (mid-back) region of the spine become fused together, causing stiffness and reduced mobility.
Buddy Insight
Ankylosing spondylitis of the thoracic region significantly affects respiratory function through chest wall restriction and costovertebral joint involvement.
CMS-HCC V28
MappedHCC 93
RAF 0.617
CMS-HCC V24
MappedHCC 40
RAF 0.421
ACA/HHS
MappedHCC 56
Varies by metal level
ESRD/PACE
MappedHCC 40
RAF 0.058
RXHCC
MappedHCC 83
RAF 0.222
Code Book Path
Inclusion Terms
Official- Rheumatoid arthritis of spine
Excludes 2
Official- Behçet's disease (M35.2)
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for M45.4 in this effective period.
Excludes 1
Official- arthropathy in Reiter's disease (M02.3-)
- juvenile (ankylosing) spondylitis (M08.1)
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for M45.4 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for M45.4 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for M45.4 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is M45.4 an HCC code?
Yes. M45.4 (Ankylosing spondylitis of thoracic region) maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under V24), with a community non-dual aged RAF of 0.617. It is billable for payment year 2026.
Coder answer: M45.4 is billable and maps to V28 HCC 93, Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- M45.4
- Description
- Ankylosing spondylitis of thoracic region
- HCC (V28)
- HCC 93 — Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders
- RAF
- 0.617
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.
Work M45.4 in the Code Book — tabular path, V28 RAF, and MEAT checklist →
MEAT Criteria for M45.4
For M45.4 to 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 M45.4 during that encounter, not just copy-forwarded from a problem list.
Coder workflow notes
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What This Code Means
M45.4 is the ICD-10-CM diagnosis code for ankylosing spondylitis of thoracic region. A chronic inflammatory condition where the bones in the thoracic (mid-back) region of the spine become fused together, causing stiffness and reduced mobility. M45.4 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, M45.4 maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders (HCC 93) with a community, non-dual, aged base RAF weight of 0.617. Under the older CMS-HCC V24 model, M45.4 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 40) with a community, non-dual, aged base RAF weight of 0.421. 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.
Verify the specific spinal region affected; M45.4 is specifically for thoracic involvement only. Because M45.4 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 M45.4 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify the specific spinal region affected; M45.4 is specifically for thoracic involvement only
- •Ensure ankylosing spondylitis is confirmed by clinical findings and imaging; do not code based on symptoms alone
Clinical Significance
Ankylosing spondylitis of the thoracic region significantly affects respiratory function through chest wall restriction and costovertebral joint involvement. This location commonly causes reduced chest expansion and may lead to restrictive lung disease requiring pulmonary monitoring.
Documentation Requirements
- ✓Documentation of ankylosing spondylitis with thoracic spine involvement
- ✓Evidence of costovertebral and costochondral joint inflammation
- ✓Chest expansion measurements and respiratory function tests
- ✓Assessment of thoracic spine mobility and kyphosis
- ✓Documentation of breathing difficulties or chest pain
- ✓Evaluation for restrictive lung disease
- ✓Treatment plan addressing thoracic involvement and respiratory function
Commonly Confused Codes
- •M45.5: Ankylosing spondylitis of thoracolumbar region (includes lumbar component)
- •M47.814: Spondylosis without myelopathy, thoracic region (degenerative condition)
- •J98.6: Disorders of diaphragm (primary respiratory condition)
- •M94.0: Chondrocostal junction syndrome (localized costochondral problem)
- •M46.04: Spinal enthesopathy, thoracic region (different inflammatory process)