The Muscle-Specific Kinase (MuSK) antibody test is a blood test used to detect antibodies against the MuSK protein, which is important for communication between nerves and muscles at the neuromuscular junction. In some patients with myasthenia gravis (MG), especially those who are negative for acetylcholine receptor (AChR) antibodies, the immune system produces MuSK antibodies instead. These antibodies interfere with normal neuromuscular signaling, leading to muscle weakness.