Diagnosing myasthenia gravis (MG) typically involves a combination of blood tests for specific antibodies, electrical tests of nerve and muscle function, and bedside clinical exams. No single test confirms the diagnosis in every case, so doctors often work through a sequence of steps, starting with the most straightforward and moving to more specialized testing if early results are inconclusive.
Recognizing the Pattern of Symptoms
The diagnostic process begins with your symptom history. MG causes a distinctive type of muscle weakness that worsens with activity and improves with rest. The hallmark is fatigability: muscles work progressively less well the more you use them, then recover after a break. This pattern sets MG apart from most other causes of weakness.
The most common early symptoms involve the eyes. Drooping of one or both eyelids (ptosis) and blurred or double vision are often the first signs. Some people also notice changes in facial expression, difficulty swallowing, slurred speech, shortness of breath, or weakness in the arms, hands, legs, and neck. The degree of weakness varies widely from person to person, and symptoms can fluctuate throughout the day, often worsening in the evening.
Antibody Blood Tests
Blood tests are usually the first diagnostic step after a clinical evaluation raises suspicion. The primary target is the acetylcholine receptor (AChR) antibody. These antibodies interfere with the chemical signals that tell your muscles to contract. In generalized MG, AChR antibody testing has a sensitivity of about 92%, meaning it catches the vast majority of cases. In ocular MG, where symptoms are limited to the eyes, sensitivity drops to roughly 72%, so a negative result does not rule the condition out.
If AChR antibodies come back negative, doctors typically test for a second antibody called MuSK, which targets a different protein at the nerve-muscle junction. MuSK-positive MG accounts for a smaller subset of patients but tends to cause prominent weakness in the face, jaw, and throat muscles. A third antibody, LRP4, is checked in some centers when both AChR and MuSK results are negative.
Roughly 10 to 15 percent of MG patients test negative on standard antibody panels. These “seronegative” patients may still have the same antibodies at levels too low for conventional lab methods to detect. Newer cell-based assays, which display the antibody targets on the surface of living or fixed cells, are significantly more sensitive. One emerging approach is to screen with a fixed cell-based assay first, then reflex seronegative cases to a live cell-based assay for even higher sensitivity. These specialized tests are still largely available at academic centers.
Bedside Clinical Tests
Two simple, noninvasive tests can be done in the exam room and are especially useful when eye symptoms are the main concern.
In the ice pack test, the doctor measures your eyelid drooping, then places an ice pack over the closed eyelid for up to two minutes. Cold temperature slows the breakdown of the chemical messenger acetylcholine, temporarily improving the nerve-muscle signal. If the eyelid opens more than 2 millimeters after the ice is removed, the test is considered positive. It requires no medication and carries essentially no risk.
The sustained upgaze test (sometimes called Simpson’s test) asks you to look upward and hold that gaze for two to three minutes. In MG, the muscles that lift the eyelid fatigue during this sustained effort, causing the lid to droop further or close completely. Progressive drooping during the test supports the diagnosis.
Neither test is definitive on its own, but a clear positive result in someone with the right symptom pattern can strongly point toward MG and guide the next steps.
Electrophysiological Testing
When blood tests are negative or inconclusive, electrical tests of nerve and muscle function become critical. Two main techniques are used, and they differ in sensitivity.
Repetitive Nerve Stimulation
In this test, a nerve is stimulated electrically several times per second while the response of the muscle is recorded. In MG, the muscle’s electrical response progressively decreases with each stimulation. A drop of 10% or more from the first to the fourth or fifth response is considered abnormal. This test is abnormal in about 85% of people with generalized MG but only about 17% of those with purely ocular disease, making it less useful when symptoms are limited to the eyes.
Single-Fiber Electromyography
Single-fiber EMG is the most sensitive electrical test for MG. It uses a fine needle electrode to measure the timing variability (“jitter”) between individual muscle fiber pairs. In a healthy nerve-muscle junction, timing is consistent. In MG, jitter increases because the weakened signal sometimes arrives late or fails entirely. Sensitivity ranges from 82% to 99% depending on the muscles tested, with the highest accuracy when a second muscle is examined if the first appears normal. Because this test requires specialized training and equipment, it is typically reserved for cases where repetitive nerve stimulation is normal or symptoms are mild.
The Edrophonium (Tensilon) Test
This pharmacological test involves injecting a short-acting medication that temporarily blocks the enzyme breaking down acetylcholine at the nerve-muscle junction. If you have MG, muscle strength briefly improves within seconds of the injection, then fades as the drug wears off. The test works best when there is a visible, measurable sign like a drooping eyelid that can be observed before and after.
The test is performed in a monitored setting because the medication can cause slowed heart rate, nausea, or in rare cases serious cardiac effects. Patients with heart disease or reactive airway disease are generally not candidates. Because of these safety concerns and the growing availability of reliable antibody testing, the edrophonium test is used less frequently today than it was in past decades, but it remains a useful tool in select situations.
Thymus Imaging
The thymus gland, a small organ behind the breastbone involved in immune function, is abnormal in a significant proportion of MG patients. In one study of 114 MG patients, about 22% had a thymoma (a tumor of the thymus), and another 30% had thymic hyperplasia (an enlarged, overactive gland). Because thymoma can influence both treatment decisions and prognosis, chest imaging is recommended for all MG patients regardless of antibody results or symptom severity.
CT and MRI of the chest are both effective at identifying thymoma. MRI has shown 100% sensitivity for detecting thymoma in at least one study, with better specificity and overall accuracy than CT or conventional imaging. Some centers now recommend MRI as the initial imaging method after an MG diagnosis.
Ruling Out Conditions That Mimic MG
Several other conditions can look like MG, and part of the diagnostic process is distinguishing them.
- Lambert-Eaton myasthenic syndrome (LEMS) causes weakness that predominantly affects the legs rather than the eyes and face. Unlike MG, strength in LEMS transiently improves after brief exercise rather than worsening. Reflexes are typically absent or reduced (they are normal in MG), and autonomic symptoms like dry mouth and constipation are common. Over 90% of LEMS patients have a specific antibody against voltage-gated calcium channels. On electrical testing, LEMS shows a characteristic pattern: a large increase in muscle response amplitude (100% or more) after brief exercise, the opposite of the decrement seen in MG.
- Congenital myasthenic syndromes are a group of over 30 genetic disorders affecting the nerve-muscle junction. These should be considered in seronegative patients whose electrical tests are abnormal but who have not responded to standard immune-based treatments. Associated features can include muscle wasting, facial differences, scoliosis, or limb deformities. Genetic testing confirms the diagnosis.
A key clinical clue is deep tendon reflexes. They are typically normal in MG but often absent or diminished in both LEMS and certain congenital myasthenic syndromes.
When the First Round of Tests Is Negative
A negative antibody test does not rule out MG. The standard diagnostic pathway for seronegative patients moves from conventional blood tests to cell-based assays, then to electrophysiological testing if those are also negative. Single-fiber EMG, with its sensitivity approaching 99% in generalized disease, is often the deciding test. In some cases, a trial of treatment that targets the nerve-muscle junction can serve as a practical diagnostic step: if symptoms clearly improve, the response itself supports the diagnosis. For patients who remain seronegative and do not respond to immune therapy, genetic testing for congenital myasthenic syndromes becomes important to explore.