ICD-10-CM · Spine

M46.40

M46.40 identifies discitis — inflammation of an intervertebral disc — when neither the specific spinal region nor the etiology is documented in the medical record.

Verified May 8, 2026 · 5 sources ↓

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

Drawn from CDCICD10DataAAPCIcdcodesCMS

Documentation tips

What should appear in the chart to support M46.40.

Source · Editorial brief grounded in 5 cited references ↓

Record the exact spinal level(s) involved — e.g., L4-L5, C5-C6 — to support a site-specific M46.4x code instead of M46.40.
Document imaging findings (MRI, CT, or X-ray) that confirm disc inflammation and identify the affected vertebral segment; reference the radiology report by date.
If an infectious etiology is identified or suspected, document the organism or causative factor explicitly so M46.5x or a combination code with bacteriology can be evaluated.
Note the patient's symptom onset, duration, and any prior workup to support medical necessity for advanced imaging or intervention.
If multiple disc levels are involved, document each level; M46.49 (multiple sites in spine) may be more appropriate than M46.40.

Related CPT procedures

Procedure codes commonly billed with M46.40. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

72148 $191.72

Non-contrast MRI of the lumbar spine used to evaluate disc pathology, spinal stenosis, nerve root compression, and other structural abnormalities without administration of contrast material.

72158 $318.31

MRI of the lumbar spinal canal and its contents performed first without contrast, then repeated after contrast administration for enhanced visualization.

72141 $190.72

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

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.

63030 $898.15

Single-interspace lumbar laminotomy with nerve root decompression, including partial facetectomy, foraminotomy, and/or herniated disc excision performed via open surgical technique.

72156 View procedure details

22899 View procedure details

72131 View procedure details

62321 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M46.40 and adjacent codes.

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

Using M46.40 when the operative report, MRI, or CT scan clearly identifies the disc level — site-specific codes M46.41–M46.49 are required when the level is documented anywhere in the chart.
Confusing M46.40 (discitis, unspecified etiology and site) with M46.5x (other infective spondylopathies) — when a bacterial or fungal cause is documented, M46.5x is the correct parent category.
Submitting M46.40 on imaging orders without a provider query; payers and CMS LCD policies flag unspecified-site spinal diagnoses as insufficient to establish medical necessity for MRI.
Assuming M46.40 and M46.46 (lumbar discitis) are interchangeable — lumbar is the most common site, but clinical coders must have explicit documentation of lumbar involvement before using M46.46.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

M46.40 is the fallback code within the M46.4 (Discitis, unspecified) family. It applies only when the provider's documentation does not specify which spinal region is affected. The M46.4x subcategory runs from M46.41 (occipito-atlanto-axial region) through M46.49 (multiple sites in spine); if the site can be determined from the record, one of those site-specific codes is required instead. M46.40 is distinct from M46.5x (Other infective spondylopathies), which is used when an infectious organism is identified or strongly implied — if the causative pathogen is known, do not use M46.40.

In practice, M46.40 is appropriate only as a temporary or placeholder code pending provider query. CMS and payer LCD policies consistently require diagnosis codes to be coded to the highest level of specificity, and unspecified-site discitis invites medical necessity denials and compliance flags, especially for imaging orders, antibiotic therapy authorizations, and surgical procedures. MS-DRGs 551 and 552 (Medical back problems with/without MCC) are the inpatient groupings associated with M46.40, but site-specific codes in the same family map to the same DRGs — so there is no DRG trade-off that justifies staying unspecified.

Always query the provider for spinal level documentation before finalizing M46.40. MRI findings, CT reports, and operative notes routinely identify the disc level (e.g., L4-L5, cervical); if any of that information exists in the chart, site-specific coding is required.

Sibling codes

Other billable codes under M46.4 (laterality / anatomic variants).

M46.42

Discitis, unspecified, cervical region

M46.47

Discitis, unspecified, lumbosacral region

M46.41

Discitis, unspecified, occipito-atlanto-axial region

M46.49

Discitis, unspecified, multiple sites in spine

M46.48

Discitis, unspecified, sacral and sacrococcygeal region

M46.44

Discitis, unspecified, thoracic region

M46.43

Discitis, unspecified, cervicothoracic region

M46.46

Discitis, unspecified, lumbar region

Frequently asked questions

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

01When is M46.40 actually appropriate to bill?

M46.40 is appropriate only when the provider's documentation genuinely does not specify the spinal region involved — for example, a preliminary diagnosis pending MRI results. It should not remain on the claim once imaging or operative documentation identifies the disc level.

02What is the difference between M46.40 and M46.5x?

M46.40 covers discitis of unspecified type and site; it does not imply an infectious cause. M46.5x (Other infective spondylopathies) is used when an infectious organism or infectious etiology is identified or clearly documented. If blood cultures or biopsy identify a pathogen, M46.5x is the correct category.

03Which site-specific code should I use for lumbar discitis?

M46.46 identifies discitis of the lumbar region. Use it when the provider or imaging report specifies lumbar involvement. M46.47 covers lumbosacral, so verify the exact level documented before choosing between them.

04Will payers deny M46.40 for MRI authorization?

Many payers and CMS LCD policies require spinal diagnoses to be coded to the highest level of specificity. Submitting M46.40 on an MRI order without documented clinical rationale for why the site is unknown increases the risk of a medical necessity denial. Query the provider and update to a site-specific code whenever possible.

05Does M46.40 require a 7th character extension?

No. M46.40 is an M-code (musculoskeletal chapter) and does not use 7th-character extensions. Those extensions (A, D, S) apply to injury codes in the S- and T-code ranges.

06What MS-DRGs does M46.40 map to for inpatient admissions?

M46.40 groups to MS-DRG 551 (Medical back problems with MCC) or MS-DRG 552 (Medical back problems without MCC) under MS-DRG v43.0. Site-specific M46.4x codes map to the same DRGs, so there is no DRG-level justification for leaving the site unspecified.

07Can M46.40 be used alongside a sepsis code if the discitis is hematogenous?

If the discitis is hematogenous and sepsis is documented, code sepsis first per the sequencing rules in the ICD-10-CM Official Guidelines. The discitis code (and its etiology if identified) would be listed as an additional diagnosis. Also re-evaluate whether M46.5x is more appropriate than M46.40 when an infectious source is established.

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 level identified on imaging (e.g., 'L4-L5 disc space signal change consistent with discitis on MRI'), the suspected or confirmed etiology, and any prior conservative treatment or antibiotic course documented in the encounter note. That specificity drives selection of a site-specific M46.4x code over M46.40, preventing unspecified-code audit flags and medical necessity denials on imaging and procedure authorizations.

See how Mira captures M46.40 documentation