2026 ICD-10-CM Diagnosis Code M33.21Polymyositis with respiratory involvement
ICD-10-CM Codes›M00–M99›M30-M36›M33
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 93
- Chronic Condition
M33.21 is a billable ICD-10-CM diagnosis code for polymyositis with respiratory involvement. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 545 through 547. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 4 closely related codes. Coders also document this condition as interstitial lung disease due to juvenile polymyositis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Systemic lupus erythematosus and connective tissue disorders.
For Medicare Advantage risk adjustment, M33.21 maps to CMS-HCC Category 93 (Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders) under the V28 model, adding a risk factor of about 0.617 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
M33.21 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Interstitial lung disease due to juvenile polymyositis
- Lung disease with polymyositis
- Polymyositis
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Dermatomyositis
a subacute or chronic inflammatory disease of muscle and skin, marked by proximal muscle weakness and a characteristic skin rash. the illness occurs with approximately equal frequency in children and adults. the skin lesions usually take the form of a purplish rash (or less often an exfoliative dermatitis) involving the nose, cheeks, forehead, upper trunk, and arms. the disease is associated with a complement mediated intramuscular microangiopathy, leading to loss of capillaries, muscle ischemia, muscle-fiber necrosis, and perifascicular atrophy. the childhood form of this disease tends to evolve into a systemic vasculitis. dermatomyositis may occur in association with malignant neoplasms. (from adams et al., principles of neurology, 6th ed, pp1405-6)Polymyositis
diseases characterized by inflammation involving multiple muscles. this may occur as an acute or chronic condition associated with medication toxicity (drug toxicity); connective tissue diseases; infections; malignant neoplasms; and other disorders. the term polymyositis is frequently used to refer to a specific clinical entity characterized by subacute or slowly progressing symmetrical weakness primarily affecting the proximal limb and trunk muscles. the illness may occur at any age, but is most frequent in the fourth to sixth decade of life. weakness of pharyngeal and laryngeal muscles, interstitial lung disease, and inflammation of the myocardium may also occur. muscle biopsy reveals widespread destruction of segments of muscle fibers and an inflammatory cellular response. (adams et al., principles of neurology, 6th ed, pp1404-9)
Patient EducationClinical
Myositis
Myositis means inflammation of the muscles that you use to move your body. An injury, infection, or autoimmune disease can cause it. Two specific kinds are polymyositis and dermatomyositis. Polymyositis causes muscle weakness, usually in the muscles closest to the trunk of your body.
Read the full article at MedlinePlus
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Convert M33.21 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About M33.21Overview
What is the ICD-10 code for polymyositis with respiratory involvement?
The ICD-10-CM code for polymyositis with respiratory involvement is M33.21 (sometimes written as M3321). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
Is M33.21 (Polymyositis) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report polymyositis with respiratory involvement on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does M33.21 group to?
When polymyositis with respiratory involvement is the principal diagnosis on an inpatient stay, it groups to MS-DRG 545, 546, 547, with relative weights from 0.8362 to 2.4817 depending on complications. Higher weights mean higher Medicare reimbursement.
Is M33.21 a CC or MCC?
CMS lists M33.21 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 4 closely related codes in its exclusion list.
What is the ICD-9 equivalent of M33.21?
Under the General Equivalence Mappings, polymyositis with respiratory involvement converts to ICD-9-CM 710.4 (polymyositis) and 517.8 (lung involv in oth dis). The mapping is approximate, so confirm the match fits the documentation.
What HCC is M33.21?
M33.21 (polymyositis with respiratory involvement) maps to CMS-HCC Category 93 (Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders), commonly written as HCC 93, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 40 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 227.
Does M33.21 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, M33.21 adds a risk adjustment factor of about 0.617 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.288 to 0.617 depending on the payment segment). HCC 93 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 93 category page.
