Axial spondyloarthritis of the lumbar spine with active inflammation confirmed by MRI or clinical criteria but without the structural sacroiliac joint damage visible on plain radiographs that defines ankylosing spondylitis.

Documentation tips

What should appear in the chart to support M45.A6.

Source · Editorial brief grounded in 6 cited references ↓

• Record the treating physician's explicit diagnosis of 'non-radiographic axial spondyloarthritis' — do not infer this code from a generic 'inflammatory back pain' note.
• Document MRI findings (sacroiliac joint bone marrow edema, periarticular inflammation) alongside the explicit statement that plain radiographs show no definitive sacroiliitis.
• Specify 'lumbar region' as the primary location in the assessment; if lumbosacral involvement is also described, evaluate whether M45.A7 or M45.AB better captures the documented anatomy.
• Record HLA-B27 status, CRP, and ESR results in the note — these support medical necessity for biologics tied to this diagnosis and may be required in prior authorization submissions.
• Note the duration and pattern of back pain (inflammatory pattern: worse with rest, better with activity, morning stiffness >30 minutes) to substantiate the inflammatory spondyloarthritis diagnosis versus mechanical low back pain.

Related CPT procedures

Procedure codes commonly billed with M45.A6. 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.A6 and adjacent codes.

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

• Coding M45.6 (ankylosing spondylitis, lumbar region) when plain X-rays are normal — radiographic sacroiliitis is required for AS; use M45.A6 when imaging is MRI-only positive.
• Defaulting to M46.8 (other specified inflammatory spondylopathies) or M54.51 (vertebrogenic low back pain) out of habit — M45.A6 has been the correct specific code since October 1, 2021.
• Using M45.A0 (unspecified sites) when the provider has documented lumbar involvement — always code to the highest level of specificity supported by documentation.
• Selecting M45.A7 (lumbosacral region) when the note says 'lumbar' — lumbosacral implies involvement at the L5-S1 junction and below; lumbar alone maps to M45.A6.
• Failing to verify payer system acceptance of M45.A6 before submitting — some payers were slow to update their edits for codes introduced in the FY2022 cycle; a rejection is a payer-system issue, not a code error.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

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