Single-fiber electromyography (SFEMG) is the most sensitive diagnostic test for myasthenia gravis, detecting the disease in 82% to 99% of cases depending on which muscles are tested and how many. No blood test or other electrodiagnostic study matches that range. But sensitivity alone doesn’t determine which test gets ordered first, and understanding the full diagnostic toolkit helps explain why.
How SFEMG Works and Why It Leads in Sensitivity
SFEMG measures the timing variability between nerve signals and muscle fiber responses. In myasthenia gravis, the communication between nerves and muscles is disrupted by antibodies that attack the neuromuscular junction. This disruption causes tiny, measurable delays called “jitter.” SFEMG picks up this jitter even when other tests come back normal.
Across multiple studies reviewed by the American Association of Neuromuscular and Electrodiagnostic Medicine, SFEMG sensitivity ranged from 82% to 99%. The highest sensitivity occurred when a second muscle was tested if the first appeared normal. In a study published in Neurology, SFEMG reached 94% sensitivity in a mixed population of myasthenia gravis patients, and still hit 90% in the subset of patients whose blood antibody tests and other nerve studies had already come back negative.
The choice of muscle matters. In ocular myasthenia gravis (where weakness is limited to the eye muscles), testing the orbicularis oculi, the muscle around the eye, caught only 62% of cases. Testing the superior rectus or levator palpebrae, the muscles that actually move and lift the eyelid, pushed sensitivity to 100% in one study of 17 patients.
Why SFEMG Isn’t Always Ordered First
Despite its superior sensitivity, SFEMG requires specialized equipment, a highly trained examiner, and a cooperative patient who can keep the tested muscle still during recording. Not every neurology clinic has the setup or expertise. The test also has a specificity of about 79%, meaning it can flag abnormal jitter in conditions other than myasthenia gravis, such as motor neuron disease or inflammatory neuropathies. A positive result confirms a problem at the neuromuscular junction but doesn’t pinpoint the exact cause without clinical context.
For these reasons, most diagnostic workups start with simpler, faster tests and reserve SFEMG for cases where results remain inconclusive.
Antibody Blood Tests: The Usual Starting Point
The acetylcholine receptor (AChR) antibody test is the most commonly ordered first-line test. It is highly specific, meaning a positive result strongly confirms the diagnosis. According to Mayo Clinic Laboratories, its sensitivity is approximately 90% in patients with generalized myasthenia gravis who are not on immunosuppressive treatment. That drops to about 70% to 71% when weakness is confined to the eye muscles.
For the roughly 10% to 15% of patients who test negative for AChR antibodies, two additional antibody targets can be checked. About 5% to 10% of myasthenia gravis patients carry antibodies against a protein called MuSK, and another 1% to 5% have antibodies targeting LRP4. Testing for these narrows the pool of truly “seronegative” patients, those with no detectable antibodies at all, to a small minority.
Blood tests are quick, widely available, and require nothing more than a standard blood draw. When AChR antibodies come back positive, the diagnosis is essentially confirmed without needing electrodiagnostic testing at all.
Repetitive Nerve Stimulation: A Middle-Ground Option
Repetitive nerve stimulation (RNS) is the other major electrodiagnostic test. It delivers a series of electrical impulses to a nerve and measures whether the muscle response fades with repeated stimulation. A reproducible drop (decrement) of 10% or more between the first and fourth or fifth stimulus counts as abnormal.
RNS is more widely available than SFEMG and easier to perform, but considerably less sensitive. In generalized myasthenia gravis, sensitivity can reach as high as 100% in some studies, though it is often lower depending on which muscle is tested. One study found only 53% sensitivity when limited to the anconeus muscle near the elbow. In ocular myasthenia gravis, sensitivity plummets to 10% to 17%, making RNS a poor standalone test for patients whose symptoms are confined to the eyes. In one cohort of ocular MG patients, only 27.5% had an abnormal RNS result.
Bedside and Clinical Tests
The ice pack test is a simple bedside option for patients with drooping eyelids (ptosis). A bag of ice is placed over the closed eyelid for two minutes. If the ptosis improves, it suggests myasthenia gravis, because cooling slows the enzyme that breaks down the nerve signaling chemical at the junction. A conventional version of this test showed 43.3% sensitivity with 100% specificity, while a modified version combining sustained upgaze with the ice pack reached 73.3% sensitivity with 96.7% specificity. The test is useful as a quick screening tool but cannot replace laboratory or electrodiagnostic confirmation.
The edrophonium (Tensilon) test, which involves injecting a short-acting drug that temporarily improves muscle strength, was historically a hallmark of MG diagnosis. It is still used in some settings but has become less common due to the risk of cardiac side effects and the availability of safer, more precise alternatives.
How These Tests Fit Together in Practice
The typical diagnostic sequence reflects a tradeoff between convenience and sensitivity. Blood antibody tests come first because they are widely available, inexpensive, and highly specific. If AChR, MuSK, and LRP4 antibodies all come back negative, RNS is often the next step because most neurology practices can perform it. If RNS is also normal but clinical suspicion remains high, SFEMG becomes the decisive test, capable of catching cases that every other test missed.
This layered approach means that many patients never need SFEMG at all. A positive AChR antibody test resolves the question for roughly 85% to 90% of generalized MG cases. SFEMG plays its most critical role in the harder-to-diagnose patients: those with purely ocular symptoms, those who are seronegative, and those early in the disease course when antibody levels may be too low to detect. In these patients, SFEMG’s ability to detect subtle neuromuscular junction dysfunction makes it the test most likely to provide a definitive answer.