What is ICD-10 code M45.9?

M45.9 is the ICD-10-CM diagnosis code for Ankylosing spondylitis of unspecified sites in spine. Ankylosing spondylitis affecting the spine when the specific location or extent of involvement is not documented or specified. M45.9 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. M45.9 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. M45.9 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. M45.9 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for M45.9?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For M45.9, V28 maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders at RAF 0.617 while V24 mapped to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease at RAF 0.421. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting M45.9?

Use only when the specific spinal region cannot be determined from documentation Query the provider for more specific anatomical location when possible to assign a more precise code Because M45.9 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM M45.9: Ankylosing spondylitis

ICD-10-CM Code View

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Code lookupM45.9Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the musculoskeletal system and connective tissue (M00-M99) / Spondylopathies (M45-M49)

M45.9

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Ankylosing spondylitis of unspecified sites in spine

Ankylosing spondylitis affecting the spine when the specific location or extent of involvement is not documented or specified.

Buddy Insight

Unspecified site ankylosing spondylitis indicates confirmed diagnosis without documentation of specific spinal regions involved.

CMS-HCC V28

Mapped

HCC 93

Rheumatoid Arthritis and Othe...

RAF 0.617

CMS-HCC V24

Mapped

HCC 40

Rheumatoid Arthritis and Infl...

RAF 0.421

ACA/HHS

Mapped

HCC 56

HHS-HCC 56

Varies by metal level

ESRD/PACE

Mapped

HCC 40

Rheumatoid Arthritis and Infl...

RAF 0.058

RXHCC

Mapped

HCC 83

Rheumatoid Arthritis and Infl...

RAF 0.222

Code Book Path

Official

M45Ankylosing spondylitis

M45.9Ankylosing spondylitis of unspecified sites in spine

Inclusion Terms

Official

Rheumatoid arthritis of spine

Excludes 2

Official

Behçet's disease (M35.2)

Related Child Codes

Official

M45.0Ankylosing spondylitis of multiple sites in spine

M45.1Ankylosing spondylitis of occipito-atlanto-axial region

M45.2Ankylosing spondylitis of cervical region

M45.3Ankylosing spondylitis of cervicothoracic region

M45.4Ankylosing spondylitis of thoracic region

Includes

Official

ICD-10-CM does not list Includes notes for M45.9 in this effective period.

Excludes 1

Official

arthropathy in Reiter's disease (M02.3-)
juvenile (ankylosing) spondylitis (M08.1)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for M45.9 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for M45.9 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for M45.9 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Confirmed diagnosis of ankylosing spondylitis

Clinical criteria satisfaction per rheumatology guidelines

Imaging findings consistent with spondyloarthropathy

Inflammatory back pain characteristics documented

MEAT Support

HCC Buddy guidance

Confirmed diagnosis of ankylosing spondylitis

Clinical criteria satisfaction per rheumatology guidelines

Imaging findings consistent with spondyloarthropathy

Inflammatory back pain characteristics documented

Audit Caution

HCC Buddy guidance

Using when more specific anatomic codes are supported by documentation

Missing opportunities to code specific spinal regions involved

Confusing with other spondylopathies that aren't ankylosing spondylitis

Not querying provider for anatomic specificity when possible

Common Mistakes

HCC Buddy guidance

M45.6 — Lumbar region when specific location is documented

M45.7 — Lumbosacral when both regions are mentioned

M46.9 — Unspecified inflammatory spondylopathy without ankylosing features

M48.10 — Ankylosing hyperostosis which is non-inflammatory

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is M45.9 an HCC code?

Yes. M45.9 (Ankylosing spondylitis of unspecified sites in spine) maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under V24), with a community non-dual aged RAF of 0.617. It is billable for payment year 2026.

Coder answer: M45.9 is billable and maps to V28 HCC 93, Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: M45.9
Description: Ankylosing spondylitis of unspecified sites in spine
HCC (V28): HCC 93 — Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders
RAF: 0.617
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 93, Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders

0.617

V24HCC 40, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.421

ESRDHCC 40, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.058

RxHCCHCC 83, Rheumatoid Arthritis and Inflammatory Connective Tissue Disease

0.222

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

Work M45.9 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for M45.9

For M45.9 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed M45.9 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

M45.9 is the ICD-10-CM diagnosis code for ankylosing spondylitis of unspecified sites in spine. Ankylosing spondylitis affecting the spine when the specific location or extent of involvement is not documented or specified. M45.9 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering spondylopathies (m45-m49).

Under the CMS-HCC V28 risk adjustment model, M45.9 maps to Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders (HCC 93) with a community, non-dual, aged base RAF weight of 0.617. Under the older CMS-HCC V24 model, M45.9 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 40) with a community, non-dual, aged base RAF weight of 0.421. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

Use only when the specific spinal region cannot be determined from documentation. Because M45.9 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for M45.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use only when the specific spinal region cannot be determined from documentation
•Query the provider for more specific anatomical location when possible to assign a more precise code

Clinical Significance

Unspecified site ankylosing spondylitis indicates confirmed diagnosis without documentation of specific spinal regions involved. This chronic inflammatory condition requires lifelong monitoring and treatment with biologics or targeted synthetic DMARDs to prevent spinal fusion and disability.