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AChR autoantibody subtype testing may improve accuracy of myasthenia gravis evaluations



– When testing for acetylcholine receptor (AChR) autoantibodies in patients with suspected myasthenia gravis, testing for binding antibodies and for modulating antibodies is more accurate than testing for either subtype alone, researchers reported at the annual meeting of the American Association of Neuromuscular and Electrodiagnostic Medicine. Testing for both subtypes may be the most accurate approach, regardless of whether patients have coexisting neuromuscular disorders, the researchers said.

“The advent of improved methods of detecting AChR autoantibodies has greatly facilitated the diagnosis of myasthenia gravis,” said Pritikanta Paul, MBBS, a neuromuscular fellow at Mayo Clinic in Rochester, Minn., and his colleagues. AChR antibody assays frequently are part of evaluations for myasthenia gravis, but clinicians lack consensus as to which antibody subtypes – binding, blocking, or modulating – should be tested. Clinicians test for binding antibodies most commonly, while studies have found blocking antibodies to be “least useful as an initial diagnostic test,” Dr. Paul and his colleagues said.

To assess how combinatorial antibody testing and the presence of coexisting neuromuscular disorders affect testing’s sensitivity and specificity, the researchers reviewed clinical and electrophysiologic testing data from 360 patients with suspected myasthenia gravis who underwent serologic autoantibody testing between 2012 and 2015.

Titers of AChR binding antibodies greater than 0.02 nmol/L were considered positive, as were AChR modulating antibodies more than 20%. The researchers used a greater than 10% decrement of the compound muscle action potential to repetitive nerve stimulation at 2 Hz or positive response on single-fiber EMG as electrophysiologic confirmation of myasthenia gravis.

In all, 123 of the 360 patients had a final clinical and electrophysiologic diagnosis of myasthenia gravis, including 23 with ocular myasthenia gravis and 100 with generalized myasthenia gravis.

The sensitivity of testing for AChR binding autoantibodies was 92%, and the sensitivity of testing for modulating autoantibodies was 90%. In comparison, the sensitivity of testing for either antibody subtype or both was 94%.

Among 45 patients with myasthenia gravis and coexisting neuromuscular disorders, including peripheral neuropathy, mononeuropathies, radiculopathy, and motor neuron disease, the sensitivities of testing for binding antibodies, modulating antibodies, and either or both were 96%, 91%, and 96%, respectively.

Of the 237 patients who did not have myasthenia gravis, 89 had electrophysiologic confirmation of alternative diagnoses. Among these 89 patients, AChR autoantibody testing yielded 11 false positives. Three patients tested positive for both binding and modulating antibodies, six for binding antibodies only, and two for modulating antibodies only. Those with false-positive results had diagnoses that were “diverse and clinically distinguishable from myasthenia gravis,” including myalgia, neuropathy, blurred vision, epilepsy, encephalopathy, and hemifacial spasm, the researchers said.

The researchers had no relevant disclosures.

SOURCE: Paul P et al. AANEM 2019, Abstract 236.

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