The primary blood test for myasthenia gravis is the acetylcholine receptor (AChR) antibody test, which detects the antibodies responsible for the disease in roughly 80-85% of people with generalized MG. If that test comes back negative, additional antibody tests can identify other subtypes. A standard MG blood panel typically takes 6 to 8 days to return results.
AChR Antibody Test
This is the first blood test ordered when myasthenia gravis is suspected. It looks for antibodies that attack acetylcholine receptors, the docking sites where nerve signals tell muscles to contract. When these receptors are blocked or destroyed, muscles weaken and fatigue easily.
A normal result shows no detectable antibodies, or less than 0.02 nmol/L, though exact reference ranges vary slightly between labs. Any level above normal is considered positive and strongly supports an MG diagnosis. The test’s specificity is high, around 95-98% depending on the method used, meaning false positives are rare.
Sensitivity depends on the testing method. The most widely used approach (called a radioimmunoassay) detects antibodies in about 64% of all MG patients. A newer technique called a cell-based assay pushes detection up to around 72%. These numbers reflect all MG patients, including those with mild or eye-only symptoms. In generalized MG specifically, detection rates are higher. Ocular MG, which only affects the eye muscles, has lower detection rates of roughly 50%.
MuSK Antibody Test
If the AChR antibody test is negative, the next step is testing for antibodies against a protein called MuSK (muscle-specific kinase). About 37% of people with generalized MG who test negative for AChR antibodies will test positive for MuSK antibodies instead.
MuSK-positive MG looks different clinically. It affects women more often, tends to hit facial and throat muscles harder, and causes more frequent breathing crises. People with this subtype also respond differently to treatment. More than 70% don’t improve with standard first-line medications that work well for AChR-positive patients. Recognizing this distinction matters because it changes the treatment approach.
LRP4 Antibody Test
When both AChR and MuSK antibodies come back negative, a third antibody test targets a protein called LRP4. About 19% of people who are negative for both AChR and MuSK antibodies will test positive for LRP4 antibodies, though rates range from 7% to 33% depending on the population studied.
LRP4-positive MG tends to be milder. Around 81% of these patients have only mild symptoms at diagnosis, and about 27% have purely ocular symptoms. The condition is more common in women, with an average onset around age 33 for women and 42 for men. The good news is that LRP4-positive patients generally respond well to the same treatments used for AChR-positive MG.
When All Antibody Tests Are Negative
About 15% of people with MG test negative for all three antibodies using standard clinical lab tests. This is called seronegative myasthenia gravis, and it’s the second largest patient subgroup after AChR-positive MG. Being seronegative doesn’t mean the disease isn’t real. It means the antibodies are either present at levels too low for current tests to detect, or they target something not yet routinely tested for.
Specialized research labs can sometimes pick up “clustered” antibodies using live cell-based assays, a more sensitive technique not widely available in clinical practice. In one large study, this method identified AChR antibodies in about 4.5% of patients who had tested negative on standard assays. That’s a small but meaningful number for people stuck without a diagnosis.
For seronegative patients, diagnosis relies on other tools: nerve conduction studies that measure how quickly muscles fatigue with repeated stimulation, single-fiber EMG testing, and sometimes a trial of medication to see if symptoms improve. In rare cases, a muscle biopsy looking for immune deposits at the nerve-muscle junction can help confirm the diagnosis.
Striational Antibodies and Thymoma Screening
Some MG blood panels include striational antibodies, which target proteins found in skeletal muscle fibers like titin, myosin, and actin. These were originally added to help predict whether an MG patient might have a thymoma, a tumor of the thymus gland that’s closely linked to the disease.
In practice, striational antibodies are more common in MG patients who have cancer (46%) than in those who don’t (26%), but their accuracy for actually predicting a thymoma is poor. The sensitivity is only about 56% and the specificity 71%, which means they miss nearly half of thymomas and flag many patients who don’t have one. A negative result is especially unreliable: in one study, 45% of MG patients with confirmed thymomas had negative striational antibody results. A CT scan of the chest remains far more reliable for thymoma detection than any blood test.
What to Expect From Testing
MG antibody tests are standard blood draws, no different from routine lab work. A typical myasthenia gravis panel from a major lab takes 6 to 8 business days for results, though confirmatory or reflex testing can add time. Your doctor may order the AChR test alone first, then move to MuSK and LRP4 only if it’s negative, or order a comprehensive panel upfront.
A positive antibody test in someone with compatible symptoms is usually enough to confirm the diagnosis. A negative test, however, does not rule MG out. If your symptoms strongly suggest myasthenia gravis but your blood work is negative, nerve and muscle testing is the logical next step. The combination of clinical symptoms, antibody results, and electrical testing together gives the most reliable picture.