An acetylcholine receptor (AChR) binding antibody test is a blood test that measures the level of antibodies that mistakenly attack acetylcholine receptors, which are essential for muscle contraction. It is primarily used to help diagnose myasthenia gravis (MG), an autoimmune disease causing muscle weakness and fatigue. A positive result indicates the presence of these antibodies, strongly supporting an MG diagnosis, although a normal result doesn’t rule out the disease completely