Risk of False Acetylcholine Receptor Autoantibody Positivity by Radioimmunoprecipitation Assay in Clinical Practice.
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BACKGROUND AND OBJECTIVES: Radioimmunoprecipitation assay (RIPA) is the gold standard for acetylcholine receptor (AChR)-immunoglobulin G (IgG) detection in patients with myasthenia gravis (MG), with a reported specificity of ≈99%. The risk of "false" AChR-IgG positivity in clinical practice is often considered negligible, although data on large, real-life populations are scarce. The objective of this study was to determine the positive predictive value (PPV) and risk of false AChR-IgG positivity with RIPA in a large cohort of patients with suspected MG. METHODS: We retrospectively identified patients consecutively tested for AChR-IgG by RIPA at the University-Hospital of Sassari over 20 years (2003-2022) (n = 4,795). Medical records of AChR-IgG-positive patients (titer ≥0.5 nmol/L) were reviewed by 2 investigators to identify nonmyasthenic cases with false antibody positivity, defined as follows: (1) clinical phenotypes not consistent with MG and/or (2) symptoms better explained by alternative diagnoses. The characteristics of myasthenic and nonmyasthenic patients with AChR-IgG positivity were compared. A sample of nonmyasthenic patients was retested by fixed cell-based assay (CBA). RESULTS: Among 445 of 4,795 patients testing positive for AChR-IgG during the study period, 83 were excluded (insufficient information). Of 362 AChR-IgG-positive patients included, 50 (13.8%) were designated as nonmyasthenic. The PPV and specificity were 86.2% (95% CI 82.2-89.6) and 98.9% (95% CI 98.5-99.2), respectively. Alternative diagnoses in nonmyasthenic patients included ophthalmologic diseases (n = 8), rheumatic diseases (n = 7), pseudoptosis (n = 5), myopathy (n = 4), functional disorders (n = 3), cranial nerve palsy (n = 2), parkinsonism (n = 2), demyelinating diseases (n = 2), and others (n = 17). Compared with patients with MG, nonmyasthenic patients were younger (median age 65 [range 7-91] vs 38 [range 5-80] years), more frequently female (155/312 [49.8%] vs 37/50 [74%]), had lower AChR-IgG titers (median 6 [range 0.5-28] vs 0.7 [range 0.5-5.5] nmol/L), and were more likely to become seronegative on subsequent tests (9/120 [8%] vs 6/11 [55%]). After stratification by titer ≥1 nmol/L, the PPV increased to 96.6% (95% CI 94-98.3). Serum of 7 nonmyasthenic patients was retested by CBA, giving negative results (n = 6) or selective positivity against the fetal AChR isoform (n = 1). DISCUSSION: False AChR-IgG positivity may occur in clinical practice with RIPA and associates with low antibody titer. Caution is needed when titers between 0.5 and 0.9 nmol/L are detected in low-probability situations because failure to recognize false antibody positivity may lead to misdiagnosis and inappropriate treatments.