Cell-based assay better at detecting MG-driving antibodies

CBA showed greater diagnostic accuracy than two other antibody tests

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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A cell-based assay for detecting myasthenia gravis (MG)-causing antibodies in the blood results in greater diagnostic accuracy than two other antibody tests, according to data from a study in China.

The cell-based assay (CBA) was superior to enzyme-linked immunoabsorbent assay (ELISA) and radioactivity radioimmunoprecipitation assay (RIPA) at detecting antibodies against the acetylcholine receptor (AChR), which are present in about 85% of MG patients.

It also was superior, but to a less extent, at detecting antibodies against the muscle-specific kinase (MuSK), which are found in about 6% of patients.

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The study, “A multicenter, prospective, double-blind study comparing the accuracy of autoantibody diagnostic assays in myasthenia gravis: the SCREAM study,” was published in The Lancet Regional Health.

An autoimmune disease, MG is characterized by the production of self-reactive antibodies that mistakenly attack proteins important for the neuromuscular junction — the point of near contact between nerves and muscles where they communicate. AChR and MuSK are the most common targets of MG-driving self-reactive antibodies.

Blood tests to measure levels of these antibodies are integral to the diagnosis and monitoring of MG.

Various types of assays have been developed over the years with capability to measure AChR- and MuSK-targeted antibodies, but high-quality evidence is lacking as to which are most accurate and effective.

“This can impact the accuracy in clinical decision making across centers, resulting in a challenge for neurologists managing MG patients and conducting clinical trials,” the researchers wrote.

Comparing three tests for MG-causing antibodies

As such, a team of scientists in China conducted a clinical trial (NCT05219097) to compare the sensitivity and specificity of three commercially available tests for detecting MG-causing antibodies.

Here, a test’s sensitivity referred to the percentage of MG patients being correctly identified (true-positive rate), while its sensibility concerned the percentage of unaffected cases being correctly ruled out (true-negative rate).

A total of 2,272 people with suspected MG were recruited from nine centers across China between January 2021 and September 2022 and evaluated for the condition via a range of clinical assessments by two consulting neurologists.

Ultimately, 2,043 people (89.9%) were diagnosed with MG: 1,100 with generalized disease and 993 with the ocular form. Of the 229 people without MG, nearly three-quarters (73.4%) were diagnosed with other conditions, while 61 were considered healthy.

Blood samples were collected from each person and underwent antibody testing via CBA, RIPA, and ELISA.

CBA involves looking directly at interactions between antibodies in blood samples and their target proteins (bound to a fluorescent molecule) at the surface of lab-grown cells in a petri dish. The other approaches typically look at antibody-target interactions without the use of cells and via labeling with enzymes (ELISA) or radioactive molecules (RIPA).

CBA shows the highest detection numbers

Overall, anti-AChR antibodies were detected in about 1,280-1,478 of the 2,043 MG patients across the tests and anti-MuSK antibodies were found in 50-59 patients, with CBA showing the highest detection numbers.

Results showed that the CBA posted the highest sensitivity for anti-AChR antibodies (72.3%), followed by RIPA (64.1%) and ELISA (62.7%). Among the 1,478 samples found to be positive for these antibodies by CBA, only about 80% were classified as positive by RIPA (86.3%) and ELISA (84.6%).

CBA also was superior to the other two tests at detecting anti-MuSK antibodies among MG patients (2.9% vs. 2.6% with ELISA and 2.4% with RIPA).

All three methods resulted in low numbers of false-positives among people without MG, being associated with a specificity of more than 94% for anti-AChR antibodies and greater than 99% for anti-MuSK antibodies. Here, both CBA and RIPA showed the highest specificity for both antibodies (97.8%-100%).

Overall, the diagnostic accuracy of CBA based on anti-AChR antibodies was 85.8%, which was significantly higher than that of RIPA (84.3%) or ELISA (80.9%). The diagnostic accuracy of CBA for MuSK antibodies was 51.4%, which was similar to those of RIPA (51.3%) and ELISA (50.5%).

“This study provides high-level evidence supporting CBA as a first-line assay in the diagnosis and management of MG as well as evaluating patients with suspected MG for whom AChR or MuSK antibodies are undetectable by RIPA or ELISA assays,” the researchers wrote.

Although anti-MuSK antibodies are the second most common type, they are still quite rare relative to those against AChR. That can make it difficult to determine a test’s true sensitivity for these antibodies, the researchers noted.

Increase the sample size

“Increasing sample size for this particular group of MG patients is warranted for a definite conclusion on the superiority between the three methods tested,” the team wrote.

A few patients without MG, but with other autoimmune diseases, tested positive for MG-associated antibodies across multiple tests. The scientists believe these are unlikely to be false-positives, but rather reflect the real-world rates of these antibodies in non-MG cases.

That finding reinforces “the notion that definite diagnosis of MG requires clinical manifestations in conjunction with a battery of laboratory tests,” the team wrote.