ICD-10 Code M45.8: Ankylosing spondylitis of the sacral region

Key takeaways

ICD-10 code M45.8 is the billable code for ankylosing spondylitis of the sacral and sacrococcygeal region, valid for fiscal year 2026.

The M45 family runs M45.0 to M45.9 by spinal region, plus the M45.A subcodes for non-radiographic axial spondyloarthritis.

M45.A8 covers the same sacral site without radiographic sacroiliitis, so it is the sibling code coders most often need.

Documentation must record a physician-confirmed AS diagnosis with the sacral or sacrococcygeal site named, or the claim falls back to M45.9.

Pabau’s claims management software attaches M45.8 to the patient record and treatment plan, so coding errors surface before submission.

ICD-10 Code M45.8 is the billable diagnosis code for ankylosing spondylitis of the sacral and sacrococcygeal region. Assign it when a physician documents confirmed ankylosing spondylitis, known as AS, with the sacral or sacrococcygeal site named in the note.

This reference covers the code’s definition, its place in the M45 family, and the documentation that supports it. It also covers M45.A8, the non-radiographic sibling code that shares the same anatomical site and catches out most coders.

ICD-10 Code M45.8: Definition, billable status, and FY 2026 validity

ICD-10 Code M45.8 is a billable ICD-10-CM diagnosis code. Its official long descriptor is “Ankylosing spondylitis of sacral and sacrococcygeal region.” The code is valid for HIPAA-covered transactions throughout fiscal year 2026. That runs from October 1, 2025 through September 30, 2026, as confirmed by the CMS ICD-10-CM code files.

Ankylosing spondylitis sits in Chapter 13 of ICD-10-CM, which covers diseases of the musculoskeletal system and connective tissue (M00-M99). Within that chapter, M45 covers ankylosing spondylitis across every spinal region. M45.8 is the code for sacral and sacrococcygeal involvement. That site is affected in most cases, because the sacroiliac joint is where the disease starts.

Code details at a glance

The table below provides a quick-reference summary of M45.8’s classification attributes. Use the CDC/NCHS ICD-10-CM web tool to verify the code against the official tabular list for any given fiscal year.

Clinical description: Ankylosing spondylitis of the sacral region

Ankylosing spondylitis is a chronic seronegative spondyloarthropathy characterized by inflammation of the axial skeleton. It centers on the sacroiliac joints and the spine. Untreated, it progresses from sacroiliac inflammation toward the lumbar and thoracic spine, ending in bony fusion known as ankylosis.

Disease activity usually begins in the sacral and sacrococcygeal region. That is why M45.8 often describes early-stage or isolated lower-segment AS rather than late, multi-level disease.

Patients present with inflammatory back pain and morning stiffness lasting more than 30 minutes. The pain improves with movement and worsens with rest. Sacrococcygeal involvement is less common than sacroiliac involvement, but it is well recognized in the AS literature.

Rheumatologists, physiatrists, and musculoskeletal specialists tracking these patients work in sports medicine software and rheumatology EHRs. Coding teams in physical therapy EMR environments meet M45.8 too, when a patient is referred for exercise-based rehabilitation.

Peripheral enthesitis shows up in AS as well, and it is coded separately from M45.8. Plantar fascia involvement confirmed by a positive windlass test needs its own code alongside the spinal diagnosis.

Approximate synonyms for M45.8

The ICD-10-CM alphabetic index accepts several entry points for reaching M45.8. Coders searching by condition name or anatomical site should be familiar with these approved synonyms:

• Ankylosing spondylitis of sacral region
• Ankylosing spondylitis of sacrococcygeal region
• Ankylosing spondylitis, sacral and sacrococcygeal region
• Marie-Strumpell disease (sacral region) — a historical term still appearing in older records
• Rheumatoid spondylitis of sacral region — older clinical wording that maps to M45.8

None of these synonyms has its own separate ICD-10-CM code. They all point to M45.8 as the correct assignment when sacral or sacrococcygeal involvement is the documented site.

M45 subcategory codes: The full ankylosing spondylitis ICD-10 family

The M45 category covers ankylosing spondylitis across every documented spinal site, and each subcategory code names a distinct region. Select the code that matches the physician-documented site, never the most commonly affected site or a default. The AAPC Codify ICD-10-CM lookup confirms the hierarchy and the excludes notes that apply to the category.

The family has two halves. M45.0 through M45.9 cover ankylosing spondylitis by region. The M45.A subcodes, added for fiscal year 2023, cover non-radiographic axial spondyloarthritis by the same regions and are billable and valid for FY 2026.

M45.8 vs M45.9: M45.9 is the unspecified code, and it belongs only on notes that name no spinal region at all. If the physician documents sacral involvement, M45.8 is required. Reporting M45.9 instead is a coding accuracy failure that can affect medical necessity review.

The neighboring regional codes trip coders up in the same way. M45.7 covers the lumbosacral region, which is not the same site as the sacrum on its own. M45.0 is the code once two or more regions are documented.

When imaging shows no radiographic sacroiliitis

If imaging shows no definite radiographic sacroiliitis, the sacral code is M45.A8, not M45.8. M45.A8 is the direct sibling of M45.8, and it carries the same anatomical site with a different radiographic finding.

This is the distinction most code lookups skip. A rheumatologist can confirm axial spondyloarthritis on symptoms, HLA-B27, inflammatory markers, and MRI-only inflammation, with plain films still clear. That patient has documented sacral disease, so M45.8 looks correct until you read the imaging report.

Settle it at the imaging report. Definite structural change on radiograph or CT supports M45.8. Active inflammation on MRI with normal radiographs supports M45.A8. Both are billable for FY 2026, so the choice never blocks the claim, only its accuracy.

Ankylosing spondylitis coding guidelines and documentation requirements

The ICD-10-CM Official Guidelines for Coding and Reporting are maintained by CMS and the National Center for Health Statistics, known as NCHS. Under those guidelines, M45.8 may only be assigned from a physician-documented diagnosis. Coding from signs and symptoms alone is not permitted where an established code exists. The documentation must:

• Confirm the diagnosis of ankylosing spondylitis. Not “possible AS”, “suspected AS”, or “rule out AS”. A qualified diagnosis takes the uncertain condition code, never M45.8.
• Identify the sacral or sacrococcygeal region explicitly. Wording such as “sacral AS”, “ankylosing spondylitis lower spine”, or “sacrococcygeal involvement” satisfies site specificity.
• Use language that reflects an established, active diagnosis. An encounter for AS management, a medication adjustment, or an imaging review all support M45.8.

HLA-B27 and diagnostic workup coding

HLA-B27 is a genetic marker associated with ankylosing spondylitis. The American College of Rheumatology puts it in roughly 90% of AS patients. It is a laboratory result rather than a standalone ICD-10-CM diagnosis code. When the test is ordered and reported, code it using:

• Z13.88 — encounter for screening for disorder due to exposure to contaminants, where a screening context genuinely applies
• R79.89 — other specified abnormal findings of blood chemistry, for a positive HLA-B27 result before AS is confirmed
• Z84.89 — family history of other specified conditions, where a family history of AS is documented

Once AS is confirmed and the sacral site documented, M45.8 becomes the primary code. HLA-B27 positivity is supporting clinical context, not a separately reported chronic condition.

Referred patients often arrive with an HLA-B27 result before any rheumatologist has confirmed AS. Report the presenting symptom code in that window, not M45.8. M54.5 was retired as a billable code in FY 2022, so reach for M54.51 or another specific code from the M54 block. Where only pain is documented, R52 may be the honest answer.

The principle holds either way. Assign the most specific confirmed code, never the closest unconfirmed one.

Excludes notes for M45 and M45.8

The excludes notes on M45 decide whether a second code can sit on the same claim. Two apply, and they work in opposite directions.

• Excludes1: arthropathy in Reiter’s disease (M02.3-). An Excludes1 note means the excluded code can never be reported at the same time as M45.8. The two conditions are mutually exclusive by definition.
• Excludes2: Behçet’s disease (M35.2) — may be coded alongside M45 if both are documented.

Juvenile-onset disease is a separate question. Juvenile ankylosing spondylitis belongs under M08.1, not M45. Confirm the patient’s age and the documented age of onset before assigning M45.8 to a younger patient.

Differential diagnosis codes to consider alongside M45.8

Several conditions present with sacroiliac or sacral spine pain and get confused with ankylosing spondylitis at the coding stage. The table below lists the differential codes that matter most, with guidance on when each one applies instead of M45.8.

M46.1 is the most frequent near-miss for M45.8. It applies when sacroiliac inflammation is documented but AS has not been formally established. Once a rheumatologist confirms AS with sacral involvement, the code moves up to M45.8.

That distinction decides money on biologic claims. Payers check whether the submitted code says confirmed disease or site inflammation only, because coverage criteria differ between the two. A mechanical cause is worth ruling out first, and a positive Kemp’s test points toward facet joint pain rather than inflammatory disease.

For practices tracking these encounters, claims management software with ICD-10 validation lowers the risk of sending the wrong code tier on a high-cost biologic.

How to use M45.8 in practice management software

Accurate ICD-10 assignment is only half the job, because the code still has to reach the claim intact. Here is how practices running practice management software like Pabau handle M45.8 through a rheumatology or musculoskeletal encounter.

1. Attach the diagnosis to the patient record at the encounter level. In Pabau, diagnosis codes are linked to the patient’s clinical record rather than stored as a standalone billing field. This means M45.8 sits alongside the treatment note, supporting medical necessity documentation in one place. The patient record management system stores the full code history for the patient, useful when tracking disease progression across multiple encounters.
2. Link M45.8 to the treatment plan. For patients receiving biologic DMARDs, NSAIDs, or physiotherapy referrals for AS, the diagnosis code should be attached to the relevant treatment plan entry. This creates an auditable link between the confirmed diagnosis and the treatment provided, which payers request during clinical audits.
3. Check for secondary codes before submission. Some encounters need a pain code, an HLA-B27 ancillary code during workup, or a radiology code when imaging was ordered. Pabau flags any encounter that carries a diagnosis code but no linked procedure code, so an incomplete claim package is caught before it goes out.
4. Verify fiscal year validity before claim submission. CMS updates ICD-10-CM codes every October 1, so confirm M45.8 was valid for the fiscal year of service. The ResDAC ICD code reference explains how those updates flow through Medicare claims data.
5. Retain documentation supporting site specificity. For M45.8 specifically, the physician note must explicitly reference the sacral or sacrococcygeal region. A generic “AS follow-up” note without site documentation is insufficient to support M45.8 over M45.9. Audit-ready practices keep the original physician note, any imaging reports referencing sacral involvement, and the coded claim together in the patient file.

Rheumatology and musculoskeletal practices need diagnosis codes, treatment notes, and billing data in one place. Pabau keeps clinical documentation and claim submission in step, which is where most coding errors start. The wider set of practice management software features shows how that carries across specialties.

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Conclusion

Sacral ankylosing spondylitis is a site-specific diagnosis, so the code has to be site-specific too. Falling back to M45.9 when the note names the sacrum costs nothing on the day it is submitted. It weakens the record you will need at the first prior authorization for biologic therapy.

Settle the choice between M45.8 and M45.A8 when you read the imaging report, not when you build the claim. Definite structural change supports M45.8, and MRI-only inflammation supports M45.A8. Once that is decided, the rest of the coding follows without a physician query.

The practices that get this right have the imaging finding, the site, and the code sitting in the same record. Book a demo to see how Pabau ties ICD-10 codes to patient records and treatment plans for rheumatology and musculoskeletal practices.

Continue your research

Documentation names the lumbar region instead? M45.6 covers ankylosing spondylitis of the lumbar spine and the wording that supports it.

Coding AS across the cervicothoracic junction? M45.3 sets out how that region is defined and where coders slip.

Working a thoracolumbar AS claim? M45.5 explains the site boundary and the documentation payers expect.

Disease documented at the top of the spine? M45.1 covers the occipito-atlanto-axial region, the least used code in the family.

Ruling out degenerative spine disease first? M47.9 is the unspecified spondylosis code coders reach for most often.

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