Laboratory Test
Anti–acetylcholine receptor antibody The anti–acetylcholine receptor (AChR) antibody (Ab) test is reliable for diagnosing autoimmune myasthenia gravis (MG). It is highly specific (as high as 100%, according to Padua et al). [ 4 ] Results are positive in as many as 90% of patients who have generalized MG but in only 50-70% of those who have only ocular MG; thus false negatives are common in cases of purely ocular MG. False-positive anti-AChR Ab test results have been reported in cases of thymoma without MG and in patients with Lambert-Eaton myasthenic syndrome , small cell lung cancer, or rheumatoid arthritis treated with penicillamine, as well as in 1-3% of the population older than 70 years. Tindall reported AChR Ab results and mean Ab titers in a group of patients with MG (see the Table below). [ 25 ] Table. Prevalence and Titers of Antibody to Acetylcholine Receptor in Patients with Myasthenia Gravis (Open Table in a new window) Osserman MG Class* Mean Anti-AC
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