Acetylcholine receptor antibody (AChR)

Acetylcholine receptor antibodies are misdirected immune proteins that target the acetylcholine receptor — the docking site on muscle where a nerve delivers its 'contract' signal. By blocking and destroying these receptors, the antibodies interrupt the nerve-to-muscle message, so muscles tire and weaken with use. Measuring them is the key blood test for myasthenia gravis, an autoimmune condition causing muscle weakness that gets worse with activity and better with rest. The classic pattern is drooping eyelids, double vision, and weakness of the face, throat, limbs or breathing that fluctuates through the day and worsens as muscles are used. A clearly positive AChR antibody in someone with these symptoms strongly confirms the diagnosis — the test is highly specific. About 85% of people with generalised myasthenia gravis are AChR-positive; those who are negative may instead carry MuSK antibodies or be 'seronegative', so a negative result does not rule the condition out. Because myasthenia gravis is linked to the thymus gland and to other autoimmune conditions (especially thyroid disease), a positive result usually prompts further assessment.

• Myasthenia gravis — the hallmark finding; a positive AChR antibody with fatigable muscle weakness strongly confirms the diagnosis.
• A higher titre — supports the diagnosis, though the level does not reliably track how severe the disease is day to day.
• Associated thymus abnormality — a positive result often prompts imaging of the thymus (thymoma or hyperplasia).
• Overlap with other autoimmune disease — thyroid disease and others are more common in AChR-positive people.

• Negative — the normal result; makes myasthenia gravis less likely but does not exclude it.
• Seronegative or MuSK-antibody myasthenia — a negative AChR result in someone with typical symptoms may reflect these subtypes, needing further testing.