Published in the October 2011 issue of Today’s Hospitalist
Hospitals and labs have a new weapon in the fight against a constantly changing enemy: MRSA.
This summer, the FDA approved a new screening test that detects seven types of MRSA and covers the most frequently found resistant strains of S. aureus. But the test also helps eliminate the possibility of false positives.
The NucliSens EasyQ MRSA test, which uses nasal swab samples, was approved with sensitivity and specificity levels in the high 90th percentiles. But what makes the technology unique is the fact that it identifies the presence of MRSA by detecting two different targets.
Other screening assays look only for the presence of the "cassette" that carries the mecA gene, which infers resistance to methicillin. But that’s not always an absolute indicator of MRSA. (The cassette is a DNA sequence.)
"It was found over time that mutations can occur that cause mecA to drop out of the cassette, rendering that strain of Staphylococcus aureus susceptible to methicillin," says Steve Shumoski, marketing manager for molecular diagnostics for bioMerieux, which produces the test. "But the genomic portion of the cassette is still there, so some assays end up producing a false positive."
The NucliSens test goes one step farther by looking not only for the cassette that carries the gene, but for the mecA gene itself. "We’ve built a backup target into our screening assay," Mr. Shumoski explains.
New techniques are essential to detect an organism that can change with lightning speed. "Because these organisms replicate so quickly, they have ample opportunity to alter their genetic material," explains Cynthia L. Fowler, MD, senior medical director. "We’re constantly monitoring for changes in the prevalent strains and making changes to our tests to keep up. It’s an ongoing process."
The NucliSens test is part of a larger system sold by bioMerieux, so MRSA tests can be batched with nearly 50 other tests at once. Because the system conducts batch testing so efficiently, Dr. Fowler says, it gives clinicians greater access to MRSA test results.
"Labs that want to offer broader rapid molecular screening will be able to offer greater access to rapid MRSA screening through this system," she explains.