ICD-10-CM · Spine

M45.A4

Axial spondyloarthritis affecting the thoracic spine in which inflammatory disease activity is confirmed clinically and by MRI or laboratory markers but spinal or sacroiliac joint changes are absent on plain radiographs.

Verified May 8, 2026 · 5 sources ↓

Status: Billable
Chapter: 13
Related CPT: 10
Region: Spine

Drawn from CDCICD10DataAAPCCMS

Documentation tips

What should appear in the chart to support M45.A4.

Source · Editorial brief grounded in 5 cited references ↓

Record the specific spinal region by name ('thoracic spine') — not just 'axial spine' — so region-specific codes like M45.A4 are defensible on audit.
Document the absence of radiographic sacroiliitis or structural spinal changes on plain films, and confirm which imaging modality (MRI, X-ray) was used to reach the nr-AxSpA designation.
Capture objective inflammatory markers in the note: MRI findings (bone-marrow edema at thoracic vertebrae or costovertebral joints), CRP/ESR values, or HLA-B27 result that support the nr-AxSpA diagnosis.
If the rheumatologist or referring provider established the nr-AxSpA diagnosis, reference that encounter in the orthopedic note to establish diagnostic basis — particularly important for therapy authorization claims.
Distinguish from ankylosing spondylitis: if the note says 'ankylosing spondylitis,' do not map to M45.A4 — query the provider or use M45.4 per documentation.

Related CPT procedures

Procedure codes commonly billed with M45.A4. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

97165 $100.54

Low-complexity occupational therapy evaluation, typically 30 minutes face-to-face, for patients with no comorbidities affecting occupational performance and a limited set of treatment options.

97167 $100.54

Occupational therapy initial evaluation at high complexity, involving extensive history review, assessment of five or more performance deficits, and high-analytic-complexity clinical decision-making — typically 60 minutes face-to-face.

97168 $68.47

Re-evaluation of an occupational therapy established plan of care when a documented change in functional or medical status requires a revised care plan and updated occupational profile.

72020 $23.71

Single-view radiologic examination of the spine at a specified level.

99213 $95.19

Established patient office or outpatient visit requiring 20–29 minutes of total time or low-complexity medical decision-making.

99214 $135.61

Office visit for an established patient requiring moderate-complexity medical decision making (MDM), or 30–39 minutes of total provider time on the date of service.

97166 View procedure details

97530 View procedure details

72072 View procedure details

72074 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M45.A4 and adjacent codes.

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

Assigning M45.4 (ankylosing spondylitis, thoracic) when the provider documents nr-AxSpA — these are distinct disease entities with separate codes; do not use them interchangeably.
Using M45.A0 (unspecified sites) when thoracic involvement is clearly documented — always code to the highest specificity supported by the record.
Coding M45.A4 alongside M45.4 for the same thoracic region on the same encounter — the M45 parent-level structure does not support simultaneous radiographic and non-radiographic codes at the same region.
Defaulting to M46.8 (other specified inflammatory spondylopathies) for nr-AxSpA — that was a pre-FY2022 workaround; M45.A4 is the correct billable code starting with dates of service on or after October 1, 2021.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

M45.A4 codes non-radiographic axial spondyloarthritis (nr-AxSpA) localized to the thoracic region. It sits under parent code M45.A, which was introduced in FY2022 to give coders a distinct home for nr-AxSpA separate from classic ankylosing spondylitis (M45.4, thoracic). The critical distinction: M45.4 requires radiographic evidence of structural damage (sacroiliitis on plain film, syndesmophytes); M45.A4 applies when the diagnosing clinician has established inflammatory axial disease—typically via MRI bone-marrow edema, elevated CRP/ESR, or HLA-B27 positivity—but conventional X-rays remain normal.

Use M45.A4 only when the treating provider explicitly documents thoracic region involvement. If the inflammatory process spans the thoracic and adjacent regions, evaluate M45.A3 (cervicothoracic), M45.A5 (thoracolumbar), or M45.AB (multiple sites) before defaulting here. Do not assign M45.A4 alongside M45.4 for the same region; the Excludes1 logic at the M45 parent level prevents simultaneous coding of radiographic and non-radiographic forms at the same spinal level.

CMS recognizes M45.A4 as a covered diagnosis for outpatient occupational therapy (LCD A53064). If the patient is pursuing biologic therapy or other advanced treatment, ensure the claim reflects the non-radiographic designation accurately—payers managing prior authorizations for TNF inhibitors or IL-17 inhibitors often differentiate coverage criteria between nr-AxSpA and AS.

Sibling codes

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

M45.A1

Non-radiographic axial spondyloarthritis occipt-atlan-ax rgn

M45.A7

Non-radiographic axial spondyloarthritis of lumbosacr region

M45.A5

Non-radiographic axial spondyloarthritis of thrclm region

M45.A8

Non-radiographic axial spondyloarthritis sacr/sacrocygl rgn

M45.A0

Non-radiographic axial spondyloarthritis unsp site in spin

M45.A2

Non-radiographic axial spondyloarthritis of cervical region

M45.AB

Non-radiographic axial spondyloarthritis mult site in spine

M45.A6

Non-radiographic axial spondyloarthritis of lumbar region

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 5 cited references ↓

01What is the difference between M45.A4 and M45.4?

M45.4 is ankylosing spondylitis of the thoracic region — a radiographic diagnosis requiring structural changes visible on plain film (sacroiliitis, syndesmophytes). M45.A4 is nr-AxSpA of the thoracic region — inflammatory axial disease confirmed by MRI or labs but without those plain-film findings. They are mutually exclusive for the same region and the same encounter.

02Can I use M45.A4 if the provider documents 'axial spondyloarthritis' without specifying radiographic status?

No — query the provider. 'Axial spondyloarthritis' without a radiographic/non-radiographic qualifier is ambiguous. The answer determines whether you code M45.4 or M45.A4. Using M45.A4 without provider confirmation of non-radiographic status is an unsupported assumption.

03When should I use M45.AB instead of M45.A4?

Use M45.AB when the provider documents nr-AxSpA at multiple spinal regions. If only the thoracic region is documented as involved, M45.A4 is correct. If thoracic plus lumbar (or other regions) are explicitly documented, M45.AB captures the multi-site involvement.

04Is M45.A4 covered for outpatient occupational therapy claims?

Yes. CMS LCD article A53064 (Outpatient Occupational Therapy) lists M45.A4 as an ICD-10-CM code that supports medical necessity. Verify that the therapy plan of care links functional deficits to thoracic nr-AxSpA specifically.

05Was M45.A4 available before FY2022?

No. The entire M45.A subcategory, including M45.A4, was introduced as new codes in FY2022 (effective October 1, 2021). Claims with dates of service before that date should not carry M45.A4; coders historically used M46.8 as a workaround for nr-AxSpA prior to FY2022.

06Does M45.A4 require a 7th-character extension?

No. M45.A4 is an M-code in Chapter 13 (musculoskeletal). 7th-character extensions (A/D/S) apply to injury S-codes, not to inflammatory spondylopathy M-codes. M45.A4 is complete as a 6-character code.

07Should I code HLA-B27 positivity separately alongside M45.A4?

HLA-B27 positivity is a supporting finding, not a separately reportable diagnosis in most orthopedic or rheumatology claim contexts. Document it in the record to support medical necessity, but there is no standard ICD-10-CM code for HLA-B27 status that is routinely appended to M45.A4 on the claim.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

Mira AI Scribe

Mira AI Scribe captures the spinal region name, imaging modality and key findings (MRI bone-marrow edema, absence of plain-film sacroiliitis or syndesmophytes), inflammatory lab values (CRP, ESR, HLA-B27), and the provider's explicit nr-AxSpA designation from the encounter note. This prevents downcoding to M45.A0 (unspecified) or mismapping to M45.4 (ankylosing spondylitis), both of which can trigger claim edits or prior-authorization mismatches for biologic therapy.

See how Mira captures M45.A4 documentation