Necrotizing autoimmune myopathy

Earlier identification and treatment initiation

Immune-mediated necrotizing myopathy (IMNM), also called necrotizing autoimmune myopathy (NAM), is a disease that causes severe muscle weakness. Timely diagnosis and treatment is important to improving outcomes, but differentiating IMNM from other myopathies on initial clinical assessment may be difficult because:

  • A clinical presentation of proximal limb weakness in association with an elevated serum creatinine kinase may not be specific to IMNM.
  • A muscle biopsy may be confounded by the presence of overlapping histopathological features with other myopathies.

Therefore, a positive HMGCR-IgG or SRP-IgG autoantibody result will aid in achieving a shorter time to diagnosis and assist with sorting out these diagnostic challenges.


of IMNM patients will be seropositive for HMGCR-IgG or SRP-IgG

Assess probability of necrotizing myopathy for your patients

Early initiation of immunotherapy results in better outcomes for patients with IMNM, but diagnosis is often challenging. Mayo Clinic has created an online calculator to help physicians assess the probability of IMNM based on their patients’ symptoms. Please note: A calculator based on a statistical model cannot replace sound clinical judgement; thorough understanding of a patient’s history, along with clinical examination and laboratory testing, are still needed for a clinic-sero-pathological diagnosis of IMNM. A study validating the effectiveness of this tool is available here.

An advanced approach to diagnosis

We offer improved clinical assay performance through:

  • A single evaluation with the two most pertinent antibodies associated with IMNM: HMGCR-IgG and SRP-IgG. The combination of both antibodies maximizes clinical sensitivity for diagnosing patients with this disease.
  • Detection of SRP-IgG using a sensitive immunofluorescence-based assay and then confirming by a second immunoblot technology.
  • Antibodies are tested in parallel, providing results within a week.

Key testing

Necrotizing Autoimmune Myopathy

Christopher Klein, M.D., gives an overview of the necrotizing myopathy evaluation. He discusses when this testing should be ordered, how this testing compares to previous testing approaches, and what clinical action can be taken due to the results of this testing.

Learn more about how to order this evaluation at your institution.

Articles and resources