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Myasthenia Gravis

How accurate is the blood test for MG?

04/06/2000

Question:

I need some advice. I have many of the symptoms of Myasthenia Gravis. I have not yet been officially diagnosed with it, but I have been on Mestinon since December and it is helping me a lot. In November I had a positive Tensilon test. Since then, I have had two negative Acetycholine Receptor Antibody blood tests. My doctor is unsure of my diagnosis because my blood tests have both been negative. Should we assume that I don`t have Myasthenia Gravis? How accurate is the blood test? Mestinon is the only thing that has helped me in 18 months of being sick. Would it work if I did not have Myasthenia Gravis? What else might it help? What would you recommend I do from here?

Answer:

The blood test for myasthenia gravis (acetylcholine receptor antibody test) is often negative in patients with myasthenia gravis. About half of patients with only eye muscle weakness (ocular myasthenia) will not have antibodies detected. Early in the disease or when it is relatively mild perhaps 1 of every 5 patients may not have antibodies. The next step in confirming the diagnosis a standard electromyogram (EMG) with repetitive stimulations. This test may also be negative and you could have myasthenia. If so, then a single-fiber EMG (performed by an expert) should be done. This test is the most sensitive test for myasthenia. Very few other disorders respond to Mestinon. I have had patients that appear not to have myasthenia but feel better on Mestinon. I think this is a placebo effect.

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Response by:

Henry J Kaminski, MD Henry J Kaminski, MD
Formerly, Professor of Neurology
School of Medicine
Case Western Reserve University