M35.5 identifies multifocal fibrosclerosis — a rare systemic connective tissue disorder characterized by fibrotic proliferation at multiple anatomic sites simultaneously, classified under Other Systemic Involvement of Connective Tissue (M35).
Verified May 8, 2026 · 4 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 0
- Region
- Multi-region
Documentation tips
What should appear in the chart to support M35.5.
Source · Editorial brief grounded in 4 cited references ↓
- The provider must explicitly document 'multifocal fibrosclerosis' — do not infer this diagnosis from individual organ fibrosis findings alone.
- Record all affected anatomic sites (e.g., retroperitoneal, mediastinal, orbital) to support medical necessity and the systemic nature of the diagnosis.
- Capture all relevant comorbidities in full — MCC and CC status determines which of DRGs 545, 546, or 547 the claim will land in, directly affecting facility reimbursement.
- If IgG4-related disease is suspected or confirmed, query the provider on preferred diagnostic terminology, as coding follows the documented diagnosis.
- Document prior workup — imaging findings, biopsy results, or specialist evaluation — that establishes the systemic, multifocal character of the fibrosis.
Common coding pitfalls
The recurring mistakes coders make with M35.5 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Assigning M35.5 for a single-site fibrosis (e.g., isolated retroperitoneal fibrosis) — multifocal means two or more anatomic sites must be involved and documented.
- Confusing M35.5 with M35.81 (Multisystem inflammatory syndrome) — these are distinct conditions; MIS has specific COVID-19-related sequencing guidelines that do not apply to M35.5.
- Failing to document comorbidities, which leaves the claim in DRG 547 (without CC/MCC) when a higher-weighted DRG may be supported by the clinical record.
- Using M35.5 when the provider has documented IgG4-related disease without also specifying multifocal fibrosclerosis — always code the documented diagnosis, not the clinician's suspected underlying mechanism.
- Overlooking the M35 category-level Excludes1 note: reactive perforating collagenosis (L87.1) cannot be coded alongside M35.x codes.
Clinical context
Source · Editorial summary grounded in 4 cited references ↓
Multifocal fibrosclerosis encompasses conditions in which dense fibrous tissue replaces normal structures at two or more distinct body sites. Classic manifestations include retroperitoneal fibrosis, mediastinal fibrosis, sclerosing cholangitis, Riedel thyroiditis, and orbital pseudotumor occurring in combination. The condition is now increasingly understood as part of the IgG4-related disease spectrum, though M35.5 remains the designated code when the provider documents the diagnosis as multifocal fibrosclerosis.
Use M35.5 when the physician has established a diagnosis of multifocal fibrosclerosis as a systemic connective tissue disorder. Do not use it for isolated single-site fibrosis (e.g., retroperitoneal fibrosis alone), which maps to its own specific code. Confirm the diagnosis is documented by the treating or consulting provider — rheumatology, internal medicine, or a relevant specialist — before assigning this code.
M35.5 groups into MS-DRG v43.0 DRGs 545–547 (Connective tissue disorders with MCC, CC, or without CC/MCC). The DRG assignment will depend on comorbidities and complications documented in the same encounter, so thorough comorbidity capture directly affects reimbursement. The Excludes1 note at the M35 category level bars reactive perforating collagenosis (L87.1) from this code family.
Sibling codes
Other billable codes under M35 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 4 cited references ↓
01Is M35.5 appropriate when only one site of fibrosis is documented?
02Can M35.5 be used for IgG4-related disease?
03Which DRGs does M35.5 map to under MS-DRG v43.0?
04Does M35.5 require a 7th character?
05What does the Excludes1 note at the M35 category level mean for M35.5?
06Should M35.5 be sequenced as principal or secondary diagnosis?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 — https://www.cdc.gov/nchs/icd/icd-10-cm/index.html
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M30-M36/M35-/M35.5
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M35.5
- 04CMS ICD-10-CM Official Guidelines for Coding and Reporting FY2025 — https://www.cms.gov/files/document/fy-2025-icd-10-cm-coding-guidelines.pdf
Mira AI Scribe
The Mira AI Scribe captures the provider's explicit statement of multifocal fibrosclerosis, all named anatomic sites of fibrotic involvement, relevant imaging or biopsy findings supporting a systemic diagnosis, and any active comorbidities — preventing DRG downgrades from missed CC/MCC capture and protecting against audit exposure from under-documented systemic connective tissue claims.
See how Mira captures M35.5 documentation