There seems to be some confusion over what Acetylcholine Receptor Antibody test results mean. The three AChR tests are binding, blocking, and modulating. The blocking test is hardly done any more, and is seen as not necessary for diagnosis (according to the Mayo Clinic, who invented the test). The majority of MG patients test positive to the AChR binding test. However, in about 3 - 4% of patients (I've seen a higher number listed), only the modulating antibody is positive.

In the first article, it states very simply what each antibody does.

"Binding" antibodies attach to the acetylcholine receptors on nerve cells and may initiate an inflammatory reaction that destroys them.

"Blocking" antibodies may sit on the receptors, preventing acetylcholine from binding.

"Modulating" antibodies may cross-link the receptors, causing them to be taken up into the muscle cell and removed from the neuromuscular junction.

Every lab has different ranges for their tests. Some labs list an absolute number as positive, while other labs list a percentage as "negative, intermediate, or positive." Some labs use the terms unequivocal (leaving no doubt) or equivocal (ambiguous).

A false negative result can be due to being on a steroid or immunosuppressant therapy, having a low IgG level, or being in the early stages of MG. Some patients with ocular MG may test negative in the beginning of the disease.

There are, of course, other antibodies in MG, such as MuSK. More causes of MG are being found such as LRP4 and the SYT2 mutation.

If a patient has a question about a particular lab, they may want to contact that lab directly. Every lab has different classifications and guidelines. I should also add that these classifications of what is positive have changed over time, which seems contradictory to earlier findings. For example, the modulating antibody was previously shown as an absolute range of negative results, and anything greater was seen as positive. Now they are using higher ranges of that antibody as a positive result. I'm afraid that some patients might not receive a diagnosis due to this change. The best neuros take the clinical correlations into consideration when diagnosing MG, not only test results.

I hope this helps to clarify what each result means and how they are classified.

Annie

Acetylcholine Receptor (AChR) Antibody: The Test

Muscle autoantibodies in myasthenia gravis: beyond diagnosis?

ARBI - Clinical: Acetylcholine Receptor (Muscle AChR) Binding Antibody, Serum