Ankylosing spondylitis localized to the sacral and sacrococcygeal region — the lowest segment of the spine where the sacrum articulates with the coccyx.

Documentation tips

What should appear in the chart to support M45.8.

Source · Editorial brief grounded in 5 cited references ↓

• Provider must explicitly name the sacral or sacrococcygeal region as the AS-affected site — generic 'low back pain with AS' is insufficient to assign M45.8 over M45.9.
• Record imaging findings that confirm sacral/sacrococcygeal involvement: plain film evidence of bony ankylosis, CT showing sclerosis or fusion at the sacrococcygeal joint, or MRI bone marrow edema at the sacrum.
• Document HLA-B27 status and current inflammatory marker levels (CRP, ESR) — these support medical necessity and validate the AS diagnosis against audit scrutiny.
• If disease extends to additional spinal regions in the same encounter, consider whether M45.0 (multiple sites) more accurately reflects the documented clinical picture.
• Note whether the patient has a prior AS diagnosis established by rheumatology; include the specialist's documented region-specific findings to support the site-specific code.

Related CPT procedures

Procedure codes commonly billed with M45.8. 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 M45.8 and adjacent codes.

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

• Assigning M45.8 when documentation only states 'sacroiliac joint involvement' — sacroiliac arthritis in AS is not automatically synonymous with sacral/sacrococcygeal region disease; confirm the provider's language maps to this specific region.
• Using M45.8 instead of M45.A8 when the condition is non-radiographic axial spondyloarthritis — M45.A8 was introduced for nr-axSpA and is a distinct code; radiographic confirmation distinguishes the two.
• Defaulting to M45.9 (unspecified) when the record does contain regional documentation, leaving specificity on the table and risking downcoding or payer queries.
• Ignoring the Excludes1 for juvenile ankylosing spondylitis (M08.1) — if the patient is a minor with juvenile-onset disease, M45.8 is excluded; code M08.1 instead.
• Coding M45.8 alongside M43.2- (spinal fusion NEC) without verifying applicability — the Excludes1 on M43.2 references ankylosing spondylitis, which can trigger an edit.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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