ICD-10 Code M45.7: Ankylosing spondylitis of lumbosacral region

Key Takeaways

ICD-10 Code M45.7 describes ankylosing spondylitis of the lumbosacral region, a billable ICD-10-CM diagnosis code valid for FY2026.

M45.7 became effective October 1, 2025 and belongs to the M45 parent category covering ankylosing spondylitis by anatomical site.

Coders must specify the lumbosacral region explicitly in clinical documentation to justify M45.7 over unspecified code M45.9.

Pabau’s claims management software supports accurate diagnostic code entry, reducing claim errors for rheumatology and musculoskeletal practices.

ICD-10 Code M45.7 is a billable, site-specific ICD-10-CM diagnosis code for ankylosing spondylitis of the lumbosacral region, valid for FY2026 with an effective date of October 1, 2025. Physical therapy practices use it when a clinician has confirmed the lumbosacral spine as the affected site, rather than defaulting to the unspecified code M45.9.

M45.7 sits within the M45 parent category, which covers ankylosing spondylitis across all spinal regions, and within the broader M40-M54 dorsopathies block. The CDC/NCHS ICD-10-CM tool confirms its validity for FY2026.

Code details at a glance

The table below captures the key reference data for M45.7. Coders and billing staff can use this as a quick-check before claim submission.

Ankylosing spondylitis of the lumbosacral region: clinical overview

Ankylosing spondylitis (AS) is a chronic inflammatory arthritis that primarily affects the spine and sacroiliac joints, causing progressive stiffness and, over time, vertebral fusion. The lumbosacral region encompasses the junction between the lumbar spine (L1-L5) and the sacrum, including the sacroiliac joints.

This is typically where AS begins clinically, making M45.7 one of the most commonly used AS subcodes in rheumatology and musculoskeletal billing.

AS is strongly associated with the HLA-B27 antigen, according to clinical guidance from the American College of Rheumatology (ACR) and the WHO ICD-10 classification. The condition typically presents in adults under 45 with insidious inflammatory back pain that improves with exercise and worsens with rest.

For coders, the critical point is that lumbosacral-region involvement must be explicitly stated in the clinical record.

• Anatomical site: Lumbar spine (L1-L5) and sacrum, including SI joints
• Onset pattern: Typically before age 45; insidious, inflammatory in character
• HLA-B27 association: Present in approximately 90% of AS cases per ACR clinical information
• Radiological findings: Sacroiliitis on imaging is a core diagnostic criterion
• Why site specificity matters: Payers expect anatomical precision; M45.7 is not interchangeable with M45.9

Approximate synonyms and related terms for ankylosing spondylitis ICD-10 coding

The following terms appear in clinical notes and documentation and map to ICD-10 Code M45.7. Coders should recognize these synonyms to ensure correct code assignment when physician language varies.

• Ankylosing spondylitis of lumbosacral region
• Lumbosacral ankylosing spondylitis
• Rheumatoid arthritis of the lumbosacral spine (included under M45 per WHO)
• Inflammatory spondylitis, lumbosacral
• Spondylitis ankylopoietica, lumbosacral region
• Bechterew disease, lumbosacral region
• Marie-Strümpell disease affecting lumbosacral spine

Note that “axial spondyloarthritis” (axSpA) is the broader clinical umbrella that includes both radiographic AS (coded as M45) and non-radiographic axSpA. When the clinical record specifically confirms radiographic AS affecting the lumbosacral region, M45.7 is the appropriate code. Good clinical documentation practices should capture these distinctions at the point of care.

Includes and excludes notes for the M45 category

The includes and excludes notes at the M45 parent level apply to all subcodes, including M45.7. Misreading these notes is a common source of coding errors and claim denials. The CMS ICD-10 coding guidelines require coders to apply parent-level notes when assigning any subcode.

M45.7 within the M45 ankylosing spondylitis subcodes

M45 is not itself a billable code. Payers require the site-specific subcode. The table below maps the complete M45 subcode set so coders can confirm the correct anatomical region before submission. Practices using claims management software can validate subcode specificity during the coding workflow rather than at the denial stage.

Documentation requirements for ankylosing spondylitis diagnosis code M45.7

Auditors reviewing AS claims look for three things: a confirmed diagnosis, a specified anatomical site, and supporting clinical evidence. Missing any one of these gives payers grounds to deny or downcode the claim.

Rheumatology practices apply this same specificity standard to other rheumatic diagnoses, including M35.3 for polymyalgia rheumatica. Specificity at the point of care is always easier than querying physicians after the visit.

The checklist below reflects what must appear in the clinical record to support M45.7 during an audit or payer review.

• Confirmed AS diagnosis: Physician documentation explicitly states ankylosing spondylitis, not “possible AS” or “rule out AS”
• Anatomical site: Lumbosacral region specifically identified (not just “lower back” or “spine”)
• Imaging reference: Sacroiliitis on X-ray or MRI documented, or prior imaging cited
• HLA-B27 status: Test result referenced where available. Not required for coding but strengthens clinical support
• Symptom duration: Chronic, inflammatory nature noted (not acute mechanical back pain)
• Treatment context: NSAIDs, biologic therapy, or physical therapy referenced, indicating confirmed diagnosis management

Physical therapy compliance requirements also intersect with AS coding when physical therapists contribute to the clinical record. Their documentation of functional limitations in the lumbosacral region can support the physician’s diagnosis code. Use digital intake and forms to standardize the capture of anatomical site and symptom history at every visit, reducing the back-and-forth of physician queries.

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Commonly associated and comorbid ICD-10 codes with M45.7

AS rarely presents in isolation. Coders working on rheumatology claims regularly encounter the diagnoses below alongside M45.7. The AAPC ICD-10-CM code reference notes comorbidity patterns for inflammatory spondylopathies that billing staff should recognize. Practices supporting chiropractic and spinal management encounter several of these secondary codes routinely.

Differential diagnosis coding: M45.7 vs similar codes

This is where the most preventable coding errors occur. The lumbosacral spine is coded under several overlapping categories depending on the nature of inflammation, patient age, and underlying etiology. The comparison below addresses the cases coders find most ambiguous. A chiropractic billing cheat sheet covering CPT and ICD-10 codes can help coders build the same pattern recognition across other musculoskeletal condition families.

The most frequent error is using M45.9 when the physician has documented the lumbosacral region but the coder defaults to unspecified. The same specificity rule applies well beyond musculoskeletal codes: O70.4 requires the same site-specific documentation standard in obstetric coding.

If the site is documented, it must be coded. Equip your team with patient records and notes tools that surface this information at the point of coding.

ICD-10 code history and annual updates for M45.7

M45.7 has been a stable, valid code across recent ICD-10-CM fiscal years. No description changes or retirements have been recorded for this subcode in the annual CMS update files. Its current status is confirmed for FY2026 by the CDC/NCHS ICD-10-CM web tool.

Practices coding AS claims should verify the annual CMS ICD-10-CM update files each October. While M45.7 has remained stable, adjacent codes in the M46 and M47 blocks have seen periodic revisions. Clinical exam references like the Noble compression test guide can help support the physical findings documented for related musculoskeletal conditions.

Conclusion

Ankylosing spondylitis claims fail when the lumbosacral site is documented but not coded. ICD-10 Code M45.7 is the correct, billable code for confirmed AS affecting the lumbosacral region, valid through FY2026 and unchanged from prior years. The M45.7 vs M45.9 choice comes down to a single question: did the physician specify the anatomical region?

Pabau, practice management software for rheumatology and musculoskeletal teams, helps capture anatomical site data at the point of care, closing the missing documentation details that cause avoidable denials. Book a demo to see how Pabau supports accurate diagnostic coding workflows.

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