Ankylosing spondylitis involving two or more distinct regions of the spine simultaneously, captured as a single billable diagnosis.

Documentation tips

What should appear in the chart to support M45.0.

Source · Editorial brief grounded in 6 cited references ↓

• Name every spinal region involved (e.g., 'lumbar and thoracic') — vague language like 'diffuse spine' still supports M45.0 but region-specific documentation is stronger for audit defense.
• Record HLA-B27 status in the problem-oriented note; positive HLA-B27 is a key diagnostic marker that substantiates the AS diagnosis on payer review.
• Document radiographic evidence: sacroiliitis grade on plain film or MRI, syndesmophytes, bamboo spine appearance, or Kellgren-Lawrence equivalent findings for the sacroiliac joints.
• Note CRP and ESR values when elevated — inflammatory markers corroborate active disease and support medical necessity for biologics or surgical intervention.
• If the patient has both AS and Behçet's disease, list both M45.0 and M35.2 as separate diagnoses; the Excludes2 relationship permits dual coding.
• Document age at onset and disease duration to distinguish adult AS (M45.0) from juvenile ankylosing spondylitis (M08.1); the distinction determines which code is correct.

Related CPT procedures

Procedure codes commonly billed with M45.0. 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 M45.0 and adjacent codes.

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

• Using M45.9 (unspecified sites) when the provider note actually names multiple regions — M45.0 is the more specific and billable code when multi-region involvement is documented.
• Coding M45.0 for patients under 16 with juvenile-onset spondylitis — the Excludes1 rule requires M08.1 for juvenile ankylosing spondylitis, and M45.0 is invalid in that context.
• Assigning M45.0 for non-radiographic axial spondyloarthritis (nr-axSpA) — that condition has its own code, M45.A, and is clinically and radiographically distinct from AS.
• Applying M45.0 when only a single spinal region is documented — if the note says 'lumbar spine only,' use M45.6, not M45.0.
• Confusing AS with Reiter's-related arthropathy of the spine — arthropathy in Reiter's disease is an Excludes1 condition (M02.3-) and cannot be reported as M45.0.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

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