M35.1 classifies other overlap syndromes, including mixed connective tissue disease (MCTD) — an autoimmune condition combining clinical features of systemic lupus erythematosus, systemic sclerosis, polymyositis, and Raynaud's phenomenon, typically confirmed by positive anti-U1 RNP antibodies.

Documentation tips

What should appear in the chart to support M35.1.

Source · Editorial brief grounded in 5 cited references ↓

• Document the specific overlap syndrome by name — 'mixed connective tissue disease' or the component conditions overlapping (e.g., SLE + scleroderma features) — not just 'overlap syndrome NOS'.
• Record serologic findings explicitly: positive anti-U1 RNP antibody titer supports MCTD and justifies M35.1 over the unspecified M35.9.
• List each active clinical overlap feature present at the encounter (Raynaud's phenomenon, synovitis, esophageal dysmotility, myopathy) to support medical necessity and distinguish from a single-disease diagnosis.
• If a rheumatologist's outside note establishes the MCTD diagnosis, reference it explicitly in the encounter note so the coder can confirm M35.1 is appropriate rather than defaulting to M35.9.
• Note any Excludes1 conditions being managed separately (e.g., polyangiitis overlap syndrome under M30.8) to ensure they are coded independently and not under M35.1.

Related CPT procedures

Procedure codes commonly billed with M35.1. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

Common coding pitfalls

The recurring mistakes coders make with M35.1 and adjacent codes.

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

• Coding M35.1 when the provider documented only 'connective tissue disease' without specifying an overlap syndrome or MCTD — that defaults to M35.9, not M35.1.
• Assigning M35.1 alongside M30.8 for polyangiitis overlap syndrome: the Excludes1 note at M35 prohibits using both codes for the same encounter.
• Using M35.1 for a single defined autoimmune diagnosis (e.g., pure SLE or pure systemic sclerosis) — overlap coding requires documented features of two or more distinct connective tissue diseases.
• Omitting additional codes for significant manifestations (e.g., Raynaud's phenomenon, interstitial lung disease, inflammatory myopathy) that are separately reportable and affect DRG severity and reimbursement.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Inclusion & exclusion notes

Per the official ICD-10-CM Tabular List.

Source · CDC ICD-10-CM Official Tabular List · 2026

Sibling codes

Other billable codes under M35 (laterality / anatomic variants).