Ankylosing spondylitis localized to the thoracolumbar junction — the transitional zone where the thoracic spine meets the lumbar spine (approximately T12–L1) — characterized by chronic inflammatory involvement, progressive ossification, and potential fusion at that spinal segment.

Documentation tips

What should appear in the chart to support M45.5.

Source · Editorial brief grounded in 6 cited references ↓

• Specify 'thoracolumbar region' or 'T12-L1 junction' explicitly in the assessment — vague terms like 'spine' will push coders toward M45.9 (unspecified) and may trigger a specificity flag.
• Record radiographic findings supporting sacroiliitis: plain film grade (bilateral ≥2 per modified New York criteria), MRI active inflammation, or CT findings of syndesmophytes at the thoracolumbar level.
• Document HLA-B27 status and inflammatory markers (CRP, ESR) in the note — these are standard clinical validation elements auditors and LCDs look for when M45.x codes are billed.
• Distinguish ankylosing spondylitis from non-radiographic axial spondyloarthritis in the provider's assessment; the two map to different M45 subcategories and have distinct imaging requirements.
• Note prior conservative treatment history (NSAIDs, biologics, physical therapy) when relevant to justifying advanced imaging or interventional procedures linked to this diagnosis.

Related CPT procedures

Procedure codes commonly billed with M45.5. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

Common coding pitfalls

The recurring mistakes coders make with M45.5 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

• Billing M45 (the parent, non-billable code) instead of M45.5 — M45 alone will be rejected; always carry the code to the 5th character.
• Confusing M45.4 (thoracic region) with M45.5 (thoracolumbar region) — if the provider documents involvement at T12–L1 or explicitly names the thoracolumbar junction, M45.5 is correct, not M45.4.
• Using M45.5 when the provider documents non-radiographic axial spondyloarthritis — that maps to M45.A5, a distinct code introduced with the M45.A expansion; do not conflate confirmed AS with nr-axSpA.
• Applying M45.5 when the record actually supports rheumatoid arthritis of the spine — per the Excludes1 note under M45, rheumatoid arthritis of the spine is excluded from M45 and should be coded elsewhere.
• Omitting secondary codes for extraspinal manifestations (e.g., uveitis, inflammatory bowel disease) that may be documented in the same encounter and affect the clinical picture for payers.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M45 (laterality / anatomic variants).