Myositis immunostains on paraffin muscle: three that work, two that do not

Design

Method development and paired comparison with frozen sections

Population

20 muscle biopsies including dermatomyositis, IBM, IMNM and mimics

Primary outcome

Concordance of FFPE versus frozen immunohistochemistry

Effect

MHC-II 90%, MxA 90%, p62 80%; MHC-I and MAC unsuccessful on FFPE

Myositis classification relies on immunohistochemistry usually done on snap-frozen muscle, which many laboratories cannot process. A Histopathology study, published in June 2026, developed a protocol for formalin-fixed, paraffin-embedded muscle using heat-induced antigen retrieval and overnight antibody incubation, and compared it with frozen sections in 20 biopsies covering dermatomyositis, inclusion body myositis, immune-mediated necrotising myopathy and mimics.

MHC-II, MxA and p62 stained well on paraffin, with concordance against frozen sections of 90%, 90% and 80%, and no statistically significant differences. Every discrepancy involved a single marker and may reflect the patchy nature of the disease. MHC-I and the membrane attack complex could not be reliably stained on paraffin.

For laboratories without a frozen-section muscle service, which many smaller laboratories lack,this offers a usable partial panel: MxA for dermatomyositis, p62 for inclusion body myositis, MHC-II for antisynthetase and inclusion body myositis patterns. The study is small, and losing MHC-I and MAC means some diagnoses, particularly necrotising myopathy, will still need frozen tissue or serology.

• Where frozen muscle is unavailable, consider MHC-II, MxA and p62 immunohistochemistry on paraffin sections.
• Do not rely on MHC-I or membrane attack complex staining on paraffin; they failed in this protocol.
• Validate the protocol locally against frozen sections or referral cases before reporting.
• Correlate with myositis-specific antibodies, especially when necrotising myopathy is suspected.
• Send a frozen sample to a reference centre where feasible for full classification.

Why it matters

Myositis classification need not wait for a referral centre when frozen tissue is not available.

Don't overread it

Twenty cases with a few per diagnosis cannot establish diagnostic accuracy for each myositis subtype.

The statistics, in plain English

Concordance of 90% means 18 of 20 cases matched between paraffin and frozen sections for that marker. Non-significant p-values in 20 cases show no large difference, not equivalence.

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