M13.10

Single-joint arthritis that does not fit any more specific classification category, with the affected joint undocumented or unspecified.

Verified May 8, 2026 · 5 sources ↓

Status: Billable
Chapter: 13
Related CPT: 9
Region: General

Drawn from CDCICD10DataAAPC

Documentation tips

What should appear in the chart to support M13.10.

Source · Editorial brief grounded in 5 cited references ↓

Name the specific joint affected — if it's documented, M13.10 is incorrect and a site-specific M13.1x code applies.
Document the basis for 'not elsewhere classified': note what diagnoses were ruled out (OA, gout, septic arthritis, RA) or are pending workup.
Record whether this is an acute presentation or ongoing condition, and whether laboratory or imaging studies have been ordered, to support the unspecified coding at this visit.
If laterality is documented even without a specific arthritis type, drop to the unspecified-laterality site code (e.g., M13.169 for unspecified knee) rather than M13.10.
If a definitive diagnosis is established at a follow-up encounter, update the diagnosis code — payers may flag persistent use of unspecified codes across multiple visits.

Related CPT procedures

Procedure codes commonly billed with M13.10. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

20600 $56.11

Needle aspiration and/or injection of a small joint or bursa — such as a finger or toe joint — performed without ultrasound guidance.

20605 $57.12

Aspiration and/or injection of an intermediate joint or bursa — such as the wrist, elbow, ankle, acromioclavicular joint, or olecranon bursa — performed without ultrasound guidance.

20610 $68.81

Aspiration and/or injection of a major joint or bursa (shoulder, hip, knee, or subacromial bursa) performed without ultrasound guidance.

27310 $689.06

Open arthrotomy of the knee for exploration, drainage of infection, or removal of a foreign body or loose material from the joint space.

29870 $602.89

Diagnostic arthroscopy of the knee, with or without synovial biopsy — a separate procedure designation meaning it bundles into any same-session surgical knee arthroscopy.

73562 $42.42

Three-view radiographic examination of the knee joint, capturing anteroposterior, lateral, and a third angle such as a sunrise or oblique view.

73564 $49.43

Radiologic examination of the knee consisting of four or more views, including oblique and tunnel projections, for a complete diagnostic workup.

73721 $204.41

MRI of a lower extremity joint (hip, knee, or ankle) performed without contrast material.

77080 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M13.10 and adjacent codes.

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

Using M13.10 when the joint is documented: any named joint requires a site-specific M13.1x code, not the unspecified-site fallback.
Applying M13.10 to osteoarthritis or arthrosis: M13 has an Excludes1 note for M15–M19, making M13.10 invalid whenever OA is established.
Coding M13.10 for septic arthritis, gout, or pseudogout: those diagnoses have specific codes (M00–M10 range) that must be used when documented.
Confusing M13.10 (monoarthritis, unspecified site) with M12.9 (arthropathy, unspecified) or M19.90 (unspecified osteoarthritis, unspecified site) — each represents a different diagnostic category.
Leaving M13.10 on the claim at subsequent visits without a diagnostic update: payers expect specificity to increase as workup results return.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

M13.10 is the fallback code for monoarthritis when the clinical picture confirms arthritis in a single joint but the type cannot be classified under a more specific category — and when the provider has not documented which joint is involved. It sits under M13.1 (Monoarthritis, not elsewhere classified), itself a child of M13 (Other arthritis). The M13 category carries an Excludes1 note for arthrosis and osteoarthritis (M15–M19), so if degenerative joint disease is established, M13.10 is wrong — use the appropriate M15–M19 code instead.

In orthopedic practice, M13.10 functions as a temporary holding code — acceptable when a patient presents with acute single-joint inflammation or pain of unclear etiology at an initial or urgent visit, pending workup. Once imaging, lab results, or clinical criteria establish a specific diagnosis (crystal arthropathy, septic arthritis, reactive arthritis, OA, etc.), the code must be updated to the appropriate specific code. Prolonged use of M13.10 without a documented diagnostic workup is a payer audit risk.

Site-specific codes within M13.1 exist for shoulder, elbow, wrist, hand, hip, knee, and ankle/foot — with laterality granularity (right, left, unspecified) at those sites. M13.10 is only appropriate when the joint site is genuinely undocumented. If the joint is known, use the corresponding site-specific code (e.g., M13.161 for right knee monoarthritis NEC).

Frequently asked questions

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

01When is M13.10 appropriate versus a site-specific M13.1x code?

M13.10 is only appropriate when the provider has not documented which joint is affected. If any joint is named — even without laterality — use the corresponding unspecified-laterality site code (e.g., M13.169 for unspecified knee). M13.10 is a true last resort.

02Can M13.10 be used for osteoarthritis of an unknown joint?

No. The M13 category has an Excludes1 note for arthrosis and osteoarthritis (M15–M19). If OA is the diagnosis, use the appropriate M15–M19 code — M19.90 for unspecified OA at an unspecified site.

03Is M13.10 valid for a first orthopedic visit when workup is pending?

Yes, provided the joint is truly undocumented at that encounter. It is a reasonable placeholder at an initial visit when single-joint arthritis is the working diagnosis and the specific type or joint cannot yet be confirmed. Update the code once results are available.

04What DRGs does M13.10 map to?

M13.10 maps to MS-DRG 553 (Bone diseases and arthropathies with MCC) and MS-DRG 554 (Bone diseases and arthropathies without MCC) under MS-DRG v43.0, per ICD10data.com.

05Can M13.10 be used for septic arthritis or crystal-induced arthritis when the joint is unknown?

No. Septic arthritis and crystal arthropathies (gout, pseudogout) have specific codes in the M00–M10 range that take precedence whenever the type is established, regardless of whether the site is documented.

06What is the difference between M13.10 and M12.9?

M12.9 is 'Arthropathy, unspecified' — a broader catch-all for any joint disease of unspecified type. M13.10 specifically implies monoarthritis (one joint) that doesn't fit a more defined category. Use M13.10 only when single-joint involvement is documented but the site is not.

07Does M13.10 require a 7th character extension?

No. M-codes in ICD-10-CM do not use 7th-character encounter extensions. The A/D/S extension convention applies to injury codes (S-codes), not musculoskeletal disease codes.

Sources & references

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

01CDC ICD-10-CM Tabular List 2026 (effective October 1, 2025)
02
icd10data.com
https://www.icd10data.com/ICD10CM/Codes/M00-M99/M05-M14/M13-/M13.10
03
icd10data.com
https://www.icd10data.com/ICD10CM/Codes/M00-M99/M05-M14/M13-
04
aapc.com
https://www.aapc.com/codes/icd-10-codes/M13.10
05
aapc.com
https://www.aapc.com/codes/icd-10-codes/M13

Mira AI Scribe

Mira captures the joint name, side, acute vs. chronic onset, any imaging ordered or resulted, and diagnoses actively excluded at this encounter. That documentation drives the upgrade from M13.10 to a site- and laterality-specific code — preventing payer downcoding and audit flags tied to repeated use of unspecified-site diagnosis codes.

See how Mira captures M13.10 documentation