Explanation

Acetylcholine receptor antibodies: what is measured?

A nerve passes a signal to a muscle using the substance acetylcholine. This substance fits onto the acetylcholine receptor, a kind of lock on the muscle cell. This test looks at whether there are antibodies against that lock in the blood. If there are, the signal gets through less well and muscles tire more quickly; that is the mechanism behind myasthenia gravis (MG), an autoimmune disease with muscle weakness.

What can it detect?

Support when myasthenia gravis is suspected, an autoimmune disease with muscle weakness that increases with exertion
Help in assessing worsening symptoms in someone already diagnosed with myasthenia gravis

Result

What does the result mean?

Positive result

A positive result means that antibodies against acetylcholine receptors were found. Healthy people do not have them. According to the Dutch Society for Clinical Chemistry (NVKC), the result is positive in about 90 in 100 people with myasthenia gravis, so with matching symptoms this is a strong indication. Symptoms that go with it are a drooping eyelid, double vision, difficulty swallowing or speaking, and muscles that become weaker and weaker over the course of the day. For the diagnosis, testing of nerve-to-muscle transmission is also needed.

Negative result

A negative result means that no antibodies against acetylcholine receptors were found. Myasthenia gravis is then less likely, but not ruled out. In the form that only affects the eye muscles (ocular myasthenia gravis), the result is negative in more than half of people, according to the Dutch Society for Clinical Chemistry (NVKC). In a small proportion of people with the disease, there are instead antibodies against the muscle enzyme MuSK.

After an abnormal result

With a positive result together with matching symptoms, the GP refers you to a neurologist. There, testing of nerve-to-muscle transmission follows (EMG with repetitive stimulation or single-fiber EMG) and usually a CT scan of the chest, because myasthenia gravis can go together with an abnormality of the thymus gland. With symptoms and a negative result, the neurologist can also have antibodies against MuSK measured.

Reliability

How reliable is the test?

The result is mainly meaningful with muscle weakness that fits myasthenia gravis. With matching symptoms, the result is useful, because the antibody hardly ever occurs in healthy people. In people without symptoms, the chance of the disease is so small that even a rare false-positive result is then more common than the disease itself. A negative result does not rule out the disease, certainly not with symptoms that only affect the eyes.

What can affect the result?

In the eye form of myasthenia gravis, the result is negative in more than half of people; a negative result then does not rule out the disease.
In a small proportion of people with the disease, there are antibodies against MuSK instead of against the acetylcholine receptor; these are not measured in this test.
The level of the result says little about how severe the symptoms are and only partly follows the disease in an individual.
Laboratories use different methods and cut-off values; results cannot simply be compared.

Who it is for

Who is the test useful for?

Useful for

Useful with symptoms that fit myasthenia gravis: muscle weakness that increases with exertion and improves with rest, a drooping eyelid, double vision, difficulty chewing, swallowing or speaking, or weakness in the arms and legs that increases over the course of the day. With general tiredness without real muscle weakness, the test adds nothing; tiredness is something different from the muscle weakness this disease is about.

How common is it?

Myasthenia gravis is a rare disease. In the Netherlands, it is estimated to involve a few hundred new patients a year; the disease occurs at any age, in younger people more often in women and at older ages more often in men. Because the disease is rare, a false-positive result is relatively common in people without symptoms.

Thuisarts.nl and NVKC; no exact Dutch incidence figure has been checked

Drawbacks and risks

In people without muscle weakness, the chance of the disease is very small; a positive result then mainly leads to worry and to hospital testing without anything being found.
A negative result does not rule out myasthenia gravis, especially not with symptoms that only affect the eyes, and can therefore give false reassurance.

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