ICD-10-CM · Spine

M46.21

Bacterial or fungal infection of the vertebral bone at the occipito-atlanto-axial region — the anatomical junction encompassing the occiput (C0), atlas (C1), and axis (C2) — producing active osteomyelitis of the uppermost cervical spine.

Verified May 8, 2026 · 5 sources ↓

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

Drawn from CDCICD10DataCMSAAPC

Documentation tips

What should appear in the chart to support M46.21.

Source · Editorial brief grounded in 5 cited references ↓

Specify the exact vertebral level(s) involved (C0/occiput, C1/atlas, C2/axis) — 'upper cervical osteomyelitis' alone is insufficient for M46.21.
Record the causative organism and source: hematogenous spread vs. contiguous infection vs. post-procedural, and add the appropriate B95-B97 or B35-B49 organism code.
Document MRI or CT findings (endplate erosion, marrow signal change, paravertebral soft-tissue involvement) and biopsy/culture results to support medical necessity for both the diagnosis code and any surgical CPT.
If an epidural abscess is present at this level, document it separately so G06.1 can be added as an additional code — it changes DRG severity.
Note neurological status at each encounter: myelopathy, instability, or cranial nerve findings elevate the clinical picture and support higher-acuity coding.

Related CPT procedures

Procedure codes commonly billed with M46.21. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

22548 $1,943.60

Arthrodesis of the clivus-C1-C2 complex via anterior transoral or extraoral approach, with or without odontoid process excision.

22554 $1,215.79

Anterior interbody arthrodesis of the cervical spine below C2, performed via anterior approach with minimal diskectomy to prepare the interspace for fusion — not performed for decompression purposes.

22590 $1,559.15

Posterior arthrodesis of the craniocervical junction, spanning from the occiput through C2, performed to eliminate pathologic motion at the skull-cervical interface.

22595 $1,499.03

Posterior arthrodesis of the atlas and axis (C1-C2), surgically fusing the first and second cervical vertebrae through a posterior approach to stabilize the upper cervical spine.

63001 $1,193.75

Posterior cervical laminectomy covering 1 or 2 vertebral segments, performed to decompress the spinal cord or cauda equina, without facetectomy, foraminotomy, or discectomy.

63015 $1,444.59

Cervical laminectomy spanning more than two vertebral segments for spinal cord or cauda equina exploration and/or decompression, performed without facetectomy, foraminotomy, or discectomy.

72141 $190.72

MRI of the cervical spinal canal and its contents performed without contrast material.

20240 $126.59

Open surgical biopsy of a superficial bone, such as the ilium, sternum, spinous process, rib, or femoral trochanter, performed through a skin incision to obtain tissue for diagnosis.

20245 $303.28

Surgical removal of a bone tissue sample from a deep anatomical site — such as the humeral shaft, ischium, or femoral shaft — through an open incision for pathological analysis.

72156 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M46.21 and adjacent codes.

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

Defaulting to M46.20 (site unspecified) when the record clearly states C1-C2 or occipitoatlantal involvement — the site-specific code M46.21 is required when location is documented.
Confusing M46.21 with M46.22 (cervical region): the occipito-atlanto-axial region covers C0-C2 only; C3-C7 disease maps to M46.22.
Omitting the organism code — vertebral osteomyelitis requires a secondary code for the infectious agent (e.g., B95.61 for MSSA, B95.62 for MRSA) when documented.
Coding M86.x (general osteomyelitis) instead of M46.21 — the Alphabetic Index directs vertebral osteomyelitis to M46.2x, not the M86 category.
Missing epidural abscess (G06.1) or instability as additional codes when documented — these are separately reportable and impact DRG assignment.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

M46.21 is the site-specific code for vertebral osteomyelitis confined to the occipito-atlanto-axial region (occiput through C2). Use it when the provider explicitly documents osteomyelitis — confirmed by MRI, CT, bone biopsy, or culture — at C0-C2. This region is anatomically distinct from the mid-cervical spine; do not collapse it into M46.22 (cervical region, C3-C7) simply because the process extends toward the neck.

Vertebral osteomyelitis at this level is a surgical emergency consideration: proximity to the brainstem and vertebral arteries means instability or epidural extension can cause rapid neurological deterioration. Documentation should capture the causative organism when identified (add a code from B95-B97 for bacterial agents, or B35-B49 for fungal), and any associated epidural abscess (G06.1) coded additionally.

On the inpatient side, M46.21 groups to MS-DRG 539-541 (Osteomyelitis with/without MCC/CC) or DRG 456-458 when surgical spinal fusion accompanies the admission. Confirm CC/MCC capture from comorbidities — the DRG split is entirely comorbidity-driven, making thorough problem-list coding financially material.

Sibling codes

Other billable codes under M46.2 (laterality / anatomic variants).

M46.27

Osteomyelitis of vertebra, lumbosacral region

M46.23

Osteomyelitis of vertebra, cervicothoracic region

M46.25

Osteomyelitis of vertebra, thoracolumbar region

M46.28

Osteomyelitis of vertebra, sacral and sacrococcygeal region

M46.26

Osteomyelitis of vertebra, lumbar region

M46.20

Osteomyelitis of vertebra, site unspecified

M46.24

Osteomyelitis of vertebra, thoracic region

M46.22

Osteomyelitis of vertebra, cervical region

Frequently asked questions

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

01What vertebral levels does M46.21 cover?

M46.21 covers the occipito-atlanto-axial region: the occiput (C0), atlas (C1), and axis (C2). Any osteomyelitis documented at C3 or below in the cervical spine maps to M46.22 instead.

02Do I need a secondary code for the infectious organism?

Yes. The ICD-10-CM Tabular List instructs coders to use an additional code (B95-B97 for bacteria, B35-B49 for fungi) to identify the causative organism when it is documented. Omitting this is a common audit finding.

03When should M46.21 be the principal diagnosis versus a secondary code?

M46.21 is the principal diagnosis when the osteomyelitis is the condition chiefly responsible for the admission. If the patient is admitted for a procedure (e.g., anterior cervical fusion for instability caused by the infection), sequencing follows the reason for the encounter; M46.21 may still be principal if the infection drove the surgical decision.

04Can I use M46.21 if the diagnosis is still presumptive pending culture?

For inpatient encounters, ICD-10-CM guidelines permit coding a confirmed diagnosis even when test results are pending at discharge, if the provider documents the condition as established. For outpatient encounters, code only confirmed diagnoses — use signs/symptoms codes until confirmation.

05How does M46.21 differ from M86.x (osteomyelitis)?

M86.x covers osteomyelitis of non-vertebral sites. The ICD-10-CM Alphabetic Index routes 'osteomyelitis, vertebra' to M46.2x, making M46.21 the correct code for this site — M86 codes for the spine are excluded.

06Which DRGs does M46.21 map to?

M46.21 groups to MS-DRG 539 (Osteomyelitis with MCC), 540 (with CC), or 541 (without CC/MCC) for medical admissions. If cervical fusion is performed in the same admission, it can map to DRG 456-458 (Spinal fusion with infection). Accurate comorbidity coding determines which tier applies.

07Is M46.21 valid for outpatient orthopedic office visits?

Yes, M46.21 is billable in any setting. In the outpatient orthopedic context, it supports imaging orders (72141, 72156), bone biopsy (20240, 20245), and specialist referrals. Medical necessity documentation must reflect confirmed or suspected vertebral osteomyelitis at C0-C2.

Sources & references

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

01CDC ICD-10-CM Tabular List 2026 (effective Oct 1, 2025)
02
icd10data.com
https://www.icd10data.com/ICD10CM/Codes/M00-M99/M45-M49/M46-/M46.21
03
cms.gov
https://www.cms.gov/medicare-coverage-database/view/article.aspx?articleId=59674&ver=18
04
aapc.com
https://www.aapc.com/codes/icd-10-codes/M46.21
05
aapc.com
https://www.aapc.com/codes/icd-10-codes/M46.2

Mira AI Scribe

Mira's AI scribe captures the documented vertebral level (C0/C1/C2), imaging findings (MRI marrow edema, endplate destruction, paravertebral phlegmon), culture or biopsy organism, and any neurological deficits at this encounter. That specificity locks in M46.21 over the unspecified M46.20 and ensures organism and complication codes (epidural abscess, instability) are surfaced before billing — preventing a DRG downgrade from missing CC/MCC documentation.

See how Mira captures M46.21 documentation