Non-radiographic axial spondyloarthritis localized to the sacral and sacrococcygeal region, where inflammatory sacroiliitis is confirmed by MRI or clinical/lab criteria but structural damage is absent on plain radiographs.
Verified May 8, 2026 · 7 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 10
- Region
- Spine
Documentation tips
What should appear in the chart to support M45.A8.
Source · Editorial brief grounded in 7 cited references ↓
- Record the specific spinal region by name — 'sacral and sacrococcygeal region' — not just 'low back' or 'SI joint inflammation'; vague regional language forces a fallback to M45.A0.
- Document MRI findings explicitly: active bone marrow edema on STIR or contrast sequences at the sacroiliac joint confirms inflammatory sacroiliitis without radiographic change.
- State that plain radiographs were obtained and did not demonstrate grade ≥2 bilateral sacroiliitis; this single sentence is what separates M45.A8 from M45.8 and protects against a payer challenge.
- Include HLA-B27 result and inflammatory markers (CRP, ESR) in the note — these support nr-axSpA classification and are frequently required for biologic prior authorizations tied to this code.
- If the clinician uses terms like 'pre-radiographic ankylosing spondylitis' or 'early axial SpA,' query for clarification; those phrases map to M45.A8 only if radiographic criteria are explicitly unmet.
Related CPT procedures
Procedure codes commonly billed with M45.A8. 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.A8 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Coding M45.8 (ankylosing spondylitis, sacral region) instead of M45.A8 when the record shows MRI sacroiliitis only — M45.8 requires radiographic structural damage meeting modified NY criteria; if X-rays are negative or not diagnostic, M45.A8 is correct.
- Defaulting to M45.A0 (unspecified sites) when the provider has clearly documented sacral or sacrococcygeal involvement — always code to the highest specificity the record supports.
- Using parent code M45.A, which is non-billable; only the site-specific child codes (M45.A0 through M45.AB) are valid for claims submission.
- Conflating nr-axSpA with non-specific inflammatory back pain codes (M54.5x range) — M45.A8 requires an established clinical diagnosis of axial spondyloarthritis, not just inflammatory back pain symptoms.
- Failing to update the code when a patient progresses from nr-axSpA to radiographic axial SpA (ankylosing spondylitis); once X-rays confirm structural sacroiliitis, transition to M45.8.
Clinical context
Source · Editorial summary grounded in 7 cited references ↓
M45.A8 applies when a provider has established a diagnosis of non-radiographic axial spondyloarthritis (nr-axSpA) with documented involvement at the sacral and sacrococcygeal level — and crucially, when conventional radiographs do not show the sacroiliac joint changes required to meet modified New York criteria for ankylosing spondylitis. The defining distinction is imaging: MRI evidence of active sacroiliitis (bone marrow edema) or positive HLA-B27 with inflammatory back pain history supports nr-axSpA, while bilateral grade ≥2 sacroiliitis on X-ray drives you to M45.8 (ankylosing spondylitis of the sacral and sacrococcygeal region) instead.
The M45.A subcategory was introduced as a new code effective October 1, 2021, giving nr-axSpA its own dedicated classification separate from ankylosing spondylitis and other inflammatory spondylopathies. Before using M45.A8, confirm the region is specifically the sacral or sacrococcygeal segment. If involvement spans multiple spinal regions, use M45.AB. If the exact region is unspecified, fall back to M45.A0 — but only when the record genuinely lacks regional specificity, not as a default.
This code is commonly used in rheumatology-adjacent orthopedic or spine practices managing patients with inflammatory low back pain who have been worked up with MRI and serologic testing. Supporting documentation should include the imaging modality and findings, HLA-B27 status if tested, CRP/ESR results, and a clinician statement that radiographic sacroiliitis criteria are not met. Biologic therapy authorizations (e.g., IL-17 or TNF inhibitors approved for nr-axSpA) frequently require M45.A-range codes on prior authorization submissions.
Sibling codes
Other billable codes under M45.A (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 7 cited references ↓
01What is the key clinical difference between M45.A8 and M45.8?
02Can I use M45.A8 if the provider only documents 'sacroiliitis' without specifying radiographic vs. non-radiographic?
03Is M45.A8 appropriate for a patient whose MRI shows bone marrow edema at the SI joint but who has not yet been formally diagnosed with nr-axSpA?
04When should I use M45.AB instead of M45.A8?
05Does M45.A8 support prior authorization for biologics approved for nr-axSpA?
06When was M45.A8 added to ICD-10-CM and is it still valid in FY2026?
07Can M45.A8 be used for a patient who later develops radiographic sacroiliitis?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 (effective October 1, 2025)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M45-M49/M45-/M45.A8
- 03icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M45-M49/M45-/M45.A
- 04ucb-usa.comhttps://www.ucb-usa.com/sites/default/files/2023-11/nr-axspa-icd-10.pdf
- 05aapc.comhttps://www.aapc.com/codes/icd-10-codes/M45.A8
- 06icdcodes.aihttps://icdcodes.ai/diagnosis/spondyloarthritis/documentation
- 07icd10cmtool.cdc.govhttps://icd10cmtool.cdc.gov/?fy=FY2024&query=M45
Mira AI Scribe
Mira captures the imaging modality (MRI vs. X-ray), specific sacroiliac region involved, HLA-B27 status, inflammatory markers, and the clinician's explicit statement that radiographic sacroiliitis criteria are not met — preventing a downcode to M45.A0 (unspecified) or an erroneous upcode to M45.8 (ankylosing spondylitis) that would trigger a payer audit or biologic PA denial.
See how Mira captures M45.A8 documentation