Acetylcholine Receptor Antibody (AChR Antibody) is found in more than 85% to 90% of patients with acquired Myasthenia Gravis (MG), and 63% of patients with only ocular MG have elevated levels. The presence of this antibody is virtually diagnostic of Myasthenia Gravis. The measured titer does not correspond well with the severity of MG. In an individual patient with MG, however, antibody levels are particularly useful in monitoring response to immunosuppressive therapy. As the patient improves, antibody titer decreases. This test is also used in patients who are suspected of having a thymoma, because 59% of these patients have MG. Because congenital MG is not an autoimmune disease, this antibody test is not helpful in the diagnosis of congenital MG.
This antibody blocks neuromuscular transmission by interfering with the binding of ACh to AChR sites on the muscle membrane and preventing muscle contraction. It is this phenomenon that characterizes MG. There are three different AChR antibodies that test for MG. The AChR-binding antibody is most commonly used. If this test is negative and the diagnosis of MG is highly suspected, the AChR-modulating antibody is used and may be more sensitive. Furthermore, a positive modulating antibody test may indicate subclinical MG, contraindicating the use of curare-like drugs during surgery. The AChR-blocking antibody is the least sensitive test (positive in only 61% of patients with MG), but it can be quantified more accurately. The blocking and modulating antibodies are not often positive for about 1 year after onset of MG symptoms. The most commonly used method for the detection of these AChR antibodies is radioimmunoassay.
Causes of Acetylcholine Receptor Antibody False Results
- False-positive results may occur in patients with amyotrophic lateral sclerosis who have been treated with cobra venom.
- False-positive results may be seen in patients with penicillamine-induced or Lambert-Eaton myasthenic syndromes.
- Patients with autoimmune liver disease may have elevated results.
- The use of muscle relaxant drugs (metocurine and succinylcholine) or penicillamine may also cause false-positive results.
- Immunosuppressive drugs may suppress the formation of these antibodies in patients with subclinical Myasthenia Gravis.
Normal Acetylcholine Receptor Antibody Levels
AChR Antibodies are not normally found in blood. Healthy patients should test negative for Acetylcholine Receptor Antibodies or have very low levels less than 0.03 nmol/L.
Causes of finding Acetylcholine Receptor Antibody
- Myasthenia Gravis: 10% of MG patients have a Thymoma.
- OcularMyasthenia Gravis.
- Thymoma: Fifty-nine percent of patients with thymoma have Myasthenia Gravis.