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M86.9
M86.9

M86.9: Osteomyelitis, unspecified

The ICD-10-CM code for osteomyelitis when the record does not state whether the bone infection is acute, subacute or chronic.

Billable
FY2027

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TABLE OF CONTENTS

At a glance

Code Family

M86: Osteomyelitis

DRG Grouping

MS-DRG 539, 540 or 541 in MDC 08 under the FY2027 MS-DRG v44 grouper. MS-DRG 456, 457 or 458 applies where a qualifying spinal fusion is performed.

HCC Mapping (v28)

HCC 92: Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis.

Condition

Osteomyelitis

Family Specificity

Category M86 holds 195 billable codes across nine subcategories in the FY2027 code set. Acuity sits at the fourth character. Site and laterality sit at the fifth and sixth characters, or at the sixth and seventh in the M86.8X codes. M86.9 records none of the three.

Effective Date

Valid in the FY2026 code set and unchanged in the FY2027 update.

Which M86 code do I assign?

Category M86 splits first on acuity and then on whether the infection reached the bone through the bloodstream. Site occupies the fifth character and laterality the sixth, except in the M86.8X codes, where they sit at the sixth and seventh. An unstated acuity closes off both.

When documentation says…
Code
Acute osteomyelitis of the right ankle, contiguous spread from a foot ulcer
M86.171
Other acute osteomyelitis, right ankle and foot. Other acute covers acute bone infection that is not documented as hematogenous. That includes spread from an adjacent ulcer, from surgery and from trauma. The note must state acute, name the region and name the side.
Acute hematogenous osteomyelitis of the left femur with positive blood cultures
M86.052
Acute hematogenous osteomyelitis, left femur. Assign this branch only where the record ties the bone infection to bloodstream seeding. Without that statement an acute infection belongs in the M86.1- codes.
Subacute osteomyelitis of the right shoulder
M86.211
Subacute osteomyelitis, right shoulder. Subacute has its own subcategory in the M86.2- codes. It is not a synonym for chronic. Assign it only where the provider writes subacute.
Chronic osteomyelitis of the right tibia with a draining sinus
M86.461
Chronic osteomyelitis with draining sinus, right tibia and fibula. A documented draining sinus or sinus tract moves chronic disease out of other chronic osteomyelitis and into the M86.4- codes.
Chronic osteomyelitis of the left calcaneus, no sinus tract, not documented as hematogenous
M86.672
Other chronic osteomyelitis, left ankle and foot. Other chronic is the residual chronic branch. It is reached once multifocal disease, a draining sinus and hematogenous spread have all been ruled out of the documentation.
Osteomyelitis of the right calcaneus in a patient with type 2 diabetes
E11.69
Type 2 diabetes mellitus with other specified complication. The Alphabetic Index path Diabetes, diabetic, type 2, with, osteomyelitis leads to E11.69. Guideline I.A.15 presumes the relationship unless the documentation clearly states that the conditions are unrelated. Sequence E11.69 first, then add the specific M86 code under the Use additional code instruction carried at E11.69.
Osteomyelitis of the right foot, no acuity stated in the record
M86.9
Osteomyelitis, unspecified. The site axis sits below the acuity axis. A note that names the bone but not the acuity still codes here. The site and the side are both lost. Query the provider for acute, subacute or chronic before settling on this code.

Tabular notes

EXCLUDES1

Category M86 excludes osteomyelitis due to echinococcus (B67.2), osteomyelitis due to gonococcus (A54.43) and osteomyelitis due to salmonella (A02.24). Excludes1 means the two codes are not reported together for the same condition. Where one of these organisms is documented as the cause, the organism specific code replaces M86.9 rather than joining it.

EXCLUDES2

Category M86 also carries an Excludes2 note for osteomyelitis of the orbit (H05.0-), osteomyelitis of the petrous bone (H70.2-) and osteomyelitis of a vertebra (the M46.2 codes). Excludes2 means the condition is classified elsewhere. A patient may carry both codes where both sites are documented. Spinal osteomyelitis belongs in the M46.2- codes, which run from the occipito-atlanto-axial region at M46.21 through the sacral and sacrococcygeal region at M46.28.

USE ADDITIONAL

Category M86 carries two Use Additional instructions. Use an additional code from B95-B97 to identify the infectious agent, such as B95.62 for methicillin resistant Staphylococcus aureus or B95.61 for the methicillin susceptible organism. Use an additional code from M89.7- to identify a major osseous defect where one is documented.

FY2027: EFFECTIVE 1 OCTOBER 2026

M86.9 was valid in the FY2026 code set and is unchanged in the FY2027 update. The FY2027 update does change subcategory M86.8X. It replaces the six character codes M86.8X1 through M86.8X8 with seven character codes. The six character codes stay in the tabular as headers and are no longer billable. Each limb site gains seven character codes for right, left and unspecified side, such as M86.8X71, M86.8X72 and M86.8X79 for the ankle and foot. Other site splits into skull (M86.8X80), face and sinuses (M86.8X81) and other site (M86.8X89). The Excludes and Use Additional notes at category M86 are unchanged.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

HCC 92: Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis.

M86.9 and the site specific M86 codes in the v28 crosswalk map to the same category. The M86.8X codes added in FY2027 postdate the crosswalk. The HCC does not change with the level of detail carried in the osteomyelitis code.

Vertebral osteomyelitis coded to M46.2- also maps to HCC 92. Redirecting a spinal case out of category M86 therefore does not cost the HCC.

MS-DRG ASSIGNMENT: FY2027 (V44)

MS-DRG 539: osteomyelitis with MCC.

MS-DRG 540: osteomyelitis with CC.

MS-DRG 541: osteomyelitis without CC/MCC.

Those three sit in MDC 08, diseases and disorders of the musculoskeletal system and connective tissue.

M86.9 also groups to MS-DRG 456, 457 and 458, spinal fusion except cervical with spinal curvature, malignancy, or infection, where the qualifying procedure is performed.

DIABETIC FOOT OSTEOMYELITIS: SEQUENCING

In the FY2027 Alphabetic Index, Diabetes, diabetic, type 2, with, osteomyelitis leads to E11.69. The separate path Diabetes, diabetic, type 1, with, osteomyelitis leads to E10.69. Guideline I.A.15 therefore presumes a causal relationship between the diabetes and the bone infection unless the documentation clearly states that the conditions are unrelated.

Assign E11.69, type 2 diabetes mellitus with other specified complication. Add the specific M86 code under the Use additional code to identify complication instruction carried at E11.69. Use E10.69 for type 1. Use E08.69, E09.69 or E13.69 for the remaining diabetes categories.

Guideline I.B.7 sequences the code carrying the Use additional code note first. E11.69 therefore precedes the osteomyelitis code.

On an inpatient claim that sequencing moves the case out of MDC 08 and into MDC 10. The DRG family shifts from MS-DRG 539, 540 or 541 to MS-DRG 637, 638 or 639: diabetes with MCC, diabetes with CC and diabetes without CC/MCC.

The osteomyelitis code in that pair still needs acuity, site and side. Pairing E11.69 with M86.9 records the diabetes link and discards everything else.

PRESSURE ULCER AND PERIPHERAL VASCULAR DISEASE

No instructional note links L89 to M86. No causal relationship is presumed between a pressure ulcer and osteomyelitis. Code the ulcer site and stage from L89 and the bone infection from M86 as separate diagnoses.

A stage 4 pressure ulcer with exposed bone does not by itself support an osteomyelitis code. The provider must document the bone infection.

Peripheral vascular disease is coded separately, from I70.2- for atherosclerosis of native arteries of the extremities or I73.9 for peripheral vascular disease, unspecified. Where diabetes is documented, the index presumes the angiopathy is diabetic and directs E11.51 without gangrene or E11.52 with gangrene.

WHERE SPECIFICITY IS LOST

Category M86 holds 195 billable codes in the FY2027 code set. All but M86.9 and M86.8X9 record acuity, site or both.

Because the site axis sits below the acuity axis, a note that names the bone but not the acuity still codes to M86.9. Documenting acute, subacute or chronic is what unlocks the fifth and sixth characters.

A bone debridement procedure code names the site and the side. An M86.9 diagnosis on the same claim records neither.

Medicare and commercial payers

The CMS-HCC v28 mapping governs Medicare Advantage risk adjustment. The MS-DRG assignment governs Medicare Fee-for-Service inpatient claims under the FY2027 IPPS.

SEVERITY DESIGNATION (MS-DRG V44.0)

M86.9 is a CC. It appears in Appendix G, diagnoses defined as complications or comorbidities, in the ICD-10 MS-DRG Definitions Manual Version 44.0. As a secondary diagnosis it can move a case to a with-CC tier where the DRG pair offers one.

Documentation checklist to support M86.9

Chart elements that support code selection and reduce specificity-driven denials.

FAQ

Is M86.9 a billable code?

Yes. M86.9 is a valid billable code at four characters and takes no fifth or sixth character. Category M86 holds 195 billable codes in the FY2027 code set. All but M86.9 and M86.8X9, other osteomyelitis, unspecified sites, record acuity, site or both.

What is the nearest wrong code to M86.9?

The nearest wrong codes are M86.10, other acute osteomyelitis, unspecified site, and M86.60, other chronic osteomyelitis, unspecified site. Both record an acuity that M86.9 does not. Assigning one of them when the note never states acuity is an assumption the record does not support. Assigning M86.9 when the note does state acuity discards detail the record already holds.

How do I code osteomyelitis in a patient with diabetes?

In the FY2027 Alphabetic Index, Diabetes, diabetic, type 2, with, osteomyelitis leads to E11.69. The separate path Diabetes, diabetic, type 1, with, osteomyelitis leads to E10.69. Guideline I.A.15 therefore presumes the diabetes and the osteomyelitis are related unless the documentation clearly states that they are unrelated. Assign E11.69 for type 2 diabetes, then add the specific M86 code under the Use additional code to identify complication instruction at E11.69. E11.69 is sequenced first. Use E10.69 for type 1, and E08.69, E09.69 or E13.69 for the remaining diabetes categories.

Does M86.9 map to an HCC?

Yes. M86.9 maps to HCC 92, Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis, under CMS-HCC v28. The site specific M86 codes in the v28 crosswalk map to the same category. The M86.8X codes added in FY2027 postdate the crosswalk. The HCC does not change with the level of detail in the osteomyelitis code.

Where does vertebral osteomyelitis go?

Vertebral osteomyelitis goes to the M46.2- codes, osteomyelitis of vertebra, which carry their own site axis running from the occipito-atlanto-axial region through the cervical, cervicothoracic, thoracic, thoracolumbar, lumbar, lumbosacral and sacral regions. Category M86 carries an Excludes2 note directing it there. Osteomyelitis of the jaw is classified to M27.2. Osteomyelitis of the petrous bone goes to the H70.2- codes.

How do I code osteomyelitis under a pressure ulcer?

Osteomyelitis under a pressure ulcer is coded as two codes with no linking instruction. Assign the L89 code for the pressure ulcer site and stage and the M86 code for the bone infection. No causal relationship is presumed between the two. A stage 4 ulcer with exposed bone does not by itself support an osteomyelitis code. The provider must document the bone infection.

Does M86.9 behave differently on an inpatient claim?

The code is the same. The grouping is not. M86.9 as a principal diagnosis groups to MS-DRG 539, 540 or 541 in MDC 08 under the FY2027 MS-DRG v44 grouper. Where the diabetes relationship applies and E11.69 is sequenced first, the case moves to MDC 10 and to MS-DRG 637, 638 or 639. On the outpatient side an uncertain diagnosis is not coded at all. An inpatient discharge diagnosis documented as probable or suspected may be coded as though it existed.

Did anything change for M86.9 in FY2027?

M86.9 itself is unchanged in the FY2027 update, effective 1 October 2026. It was valid in the FY2026 code set. The FY2027 update does change subcategory M86.8X. It replaces the six character codes M86.8X1 through M86.8X8 with seven character codes that add laterality or a finer site, such as M86.8X71, M86.8X72 and M86.8X79 for the ankle and foot. The notes at category M86 are unchanged.

Related codes

Codes in the same family as M86.9. Only codes with a published page render as links.

Sibling codes in the M86 family

  • M86.0-: Acute hematogenous osteomyelitis.
  • M86.1-: Other acute osteomyelitis.
  • M86.2-: Subacute osteomyelitis.
  • M86.3-: Chronic multifocal osteomyelitis.
  • M86.4-: Chronic osteomyelitis with draining sinus.
  • M86.5-: Other chronic hematogenous osteomyelitis.
  • M86.6-: Other chronic osteomyelitis.
  • M86.8-: Other osteomyelitis.

Additional codes to consider

Codes outside the M86 family considered alongside M86.9.

Companion: code alongside when documented

  • B95.61: Methicillin susceptible Staphylococcus aureus infection as the cause of diseases classified elsewhere.
  • B95.62: Methicillin resistant Staphylococcus aureus infection as the cause of diseases classified elsewhere.
  • B96.20: Unspecified Escherichia coli [E. coli] as the cause of diseases classified elsewhere.
  • B96.89: Other specified bacterial agents as the cause of diseases classified elsewhere.
  • M89.7-: Major osseous defect.
  • E11.69: Type 2 diabetes mellitus with other specified complication.
  • E10.69: Type 1 diabetes mellitus with other specified complication.

Consider instead

  • M86.10: Other acute osteomyelitis, unspecified site.
  • M86.60: Other chronic osteomyelitis, unspecified site.
  • M46.2-: Osteomyelitis of vertebra.
  • M27.2: Inflammatory conditions of jaws.
  • H70.2-: Petrositis.
  • H05.0-: Acute inflammation of orbit.
  • A54.43: Gonococcal osteomyelitis.
  • A02.24: Salmonella osteomyelitis.
  • A01.05: Typhoid osteomyelitis.
  • B67.2: Echinococcus granulosus infection of bone.

Sources

All claims on this page derive from government and professional-standards sources.

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