In addition to a complete medical and neurological evaluation, several tests may be used to establish an MG diagnosis, starting with the clinical exam. To examine for weakness with activity, a clinician may have a patient do a sustained task, like looking upwards to see if the eyelids begin to droop. To test for muscle weakness, a clinician may ask you to do exercises with certain muscles repeatedly and observe your ability to recover and recuperate strength to see if it improves after rest.
A diagnosis of MG is confirmed via several methods, including the following:
- Acetylcholine receptor (AChR) antibody testing—Blood test to detect abnormal antibodies. Approximately 85% of people diagnosed with MG test positive for this group of antibodies.
- Anti-MuSK antibody testing - Blood test for MG patients who have tested negative for the acetylcholine antibody (AChR). Approximately 6% of MG patients without AChR antibodies test positive for the anti-MUSK antibody.
- LPR4 antibody testing - Blood test that may be given to patients who test negative for AChR and MuSK antibodies. This is a newer antibody found to cause MG.
- Cell-based assay testing - a more sensitive type of blood test which may be able to detect AChR antibodies in patients who were previously thought to be seronegative. In a 2022 study published in the Journal of Neuroimmunology, 18.2% of seronegative patients tested positive for AChR antibodies using cell-based assay testing.
- Ice Pack tests - Ice Pack tests are examinations performed by specialists to evaluate strength and recovery responses that could be consistent with an MG diagnosis. This test involves measuring the eyelid opening, placing an ice pack over the eye for 2–5 minutes, then remeasuring the eyelid opening
- CT scan - A scan of the chest is commonly performed for those with confirmed MG or MG symptoms to look for thymoma, a tumor of the thymus gland that can cause MG.
- Electromyography (EMG) - Repetitive nerve stimulations (RNS) applying electrical shocks are often used to gauge muscle response and action potential and whether results are consistent with an MG diagnosis.
- Single-fiber electromyography (SFEMG) - Detects neuromuscular transmission defects by recording action potentials from individual muscle fibers using a small needle electrode.
- Medication trial - Patients who test negative for antibodies known to cause MG might be asked to try some of the medications commonly used to treat the disease to see if they improve symptoms. These include Mestinon (pyridostigmine bromide), corticosteroids (i.e., prednisone, etc.) or intravenous immunoglobulin (IVIg).
- Edrophonium test - Edrophonium is a drug that prevents the breakdown of acetylcholine, allowing the clinician to understand your muscle response and whether the results are consistent with an MG diagnosis. This test, once common, is now rarely used because of the risk of serious side effects.
Results from some of these tests can be negative or inconclusive, even for someone who exhibits symptoms of MG. A clinician skilled in recognizing and distinguishing MG from other conditions is important in determining a proper diagnosis.