Acetylcholine receptor antibody test for myasthenia gravis

Normally, a nerve releases acetylcholine, a chemical messenger that crosses a tiny gap and docks onto acetylcholine receptors on the muscle, triggering it to contract. Lab Tests Online UK explains that in myasthenia gravis the immune system produces autoantibodies that interfere with this process, either destroying the receptors, blocking them or making them disappear from the muscle surface. The muscle then does not receive proper signals and becomes weak.

The AChR antibody test measures these antibodies in a blood sample from a vein in the arm. Most commonly, doctors test for binding antibodies, which attach to the receptors. Sometimes, if the test is negative but myasthenia gravis is strongly suspected, other antibody types are tested, such as anti-MuSK antibodies.

The NHS describes myasthenia gravis as a rare long-term condition causing muscle weakness, most commonly affecting the muscles that control the eyes and eyelids, facial expressions, chewing, swallowing and speaking. Symptoms tend to get worse when you are tired, often towards the end of the day. Lab Tests Online UK lists the symptoms that typically lead to the test.

• Drooping eyelids that worsen as the day goes on.

• Double vision.

• Difficulty swallowing or chewing, or slurred speech after talking for a while.

• A weak neck, trouble breathing or difficulty walking.

• Weakness that gets worse with activity and improves with rest.

If you have AChR antibodies and symptoms of myasthenia gravis, Lab Tests Online UK says you very likely have the condition, since these antibodies are not normally present in healthy people. The higher the level, the more likely you are to have myasthenia gravis, but the level does not track symptom severity. Very rarely, very low levels may be found in people who never develop the condition.

A negative result does not completely rule it out. The NHS says not everyone with the condition will have a high level of antibodies, particularly if it is only affecting the eye muscles (ocular myasthenia), and that the blood test may be repeated later if the result is normal but symptoms continue or get worse. If still suspected, other antibody tests such as anti-MuSK may be requested.

Lab Tests Online UK notes that AChR antibodies can occasionally appear in other conditions, including thymomas (tumours of the thymus gland), small cell lung cancer, autoimmune liver disease, Lambert-Eaton syndrome and treatment with certain drugs such as penicillamine. The test may also be requested if imaging scans find a thymoma, because these can be associated with myasthenia gravis.

The NHS adds that problems with the thymus gland are closely associated with myasthenia gravis, which is why a CT or MRI scan of the chest may be arranged to check whether the thymus is bigger than usual or has grown abnormally.

The NHS says myasthenia gravis can be difficult to diagnose and several tests may be needed. These include electromyography, in which very small needles are inserted into muscles to measure the electrical activity, scans of the chest, and sometimes an MRI brain scan to make sure symptoms are not caused by a problem in the brain. An edrophonium test, an injection that may produce a temporary improvement in muscle strength, is rarely done because of a risk of serious side effects.

Referral is to a specialist, usually a neurologist. Treatments described by the NHS include avoiding triggers such as tiredness and stress, medicine to improve muscle weakness and, in some people, surgery to remove the thymus gland. Lab Tests Online UK adds that immunosuppressants and plasma exchange or immunoglobulin may be used, and that repeat AChR antibody tests may be used to monitor disease activity.

No preparation is needed; you can eat and drink normally. Tell the person arranging the test about all medicines. Lab Tests Online UK says drugs such as penicillamine can cause antibody production, and that antibody levels may vary over time with disease stage.

If symptoms suddenly get worse, for example severe breathing or swallowing difficulty, the NHS says urgent treatment in hospital may be needed, and this should not wait for test results.