Rapid Diagnostics to the Rescue

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Good news for the fight against drug resistance came with two, short items in the business sections of last week's newspapers. Back-to-back FDA approvals gave U.S. doctors rapid tests for both MRSA (methicillin resistant staph aureus) and a panel of a dozen common respiratory viruses.

Both diagnostic tests use gene amplification, or polymerase chain reaction (PCR), to identify their targets. Most importantly they do so fast --- in a matter of hours rather than days, the latter being the standard with traditional laboratory cultures.

| Staph. aureus, photo credit unknown

This is such great news because without such rapid diagnostics, doctors routinely resort to powerful antibiotics when treating infections--as a hedge against the worst possible culprits. This is a particular Catch 22 in hospitals: Withhold the antibiotic and you risk losing a patient to a multidrug-resistant superbug such as MRSA. Routinely put patients on heavy-duty antibiotics and you're sure to breed bacteria that can shrug off still more antibiotics.

In addition, wiping out a patient's "good," or protective, bacteria with antibiotics leaves the body vulnerable to invasion by dangerous bacteria in their environment --- hospitals being the most dangerous places in this regard.

The first of the new rapid tests is BD Diagnostics' GenOhm StaphSR Assay for detecting MRSA. The assay makes clever use of a pair of linked gene-probes. One end lights up when it finds a gene unique to Staphylococcus aureus. The other does so when it finds the cassette of genes for methicillin resistance. Earlier versions of the test produced false positives when patients harbored both drug-susceptible Staph. aureus and one or more harmless bugs that happened to carry the methicillin-resistance cassette. By linking the two probes, the newly approved test lights up only when it finds both genes in the same bacterium.

Luminex's xTag Respiratory Viral Panel simultaneously probes for the 12 viruses responsible for 85 percent of respiratory infections, including common colds and flu. This can help physicians identify the majority of respiratory infections that DON'T call for antibiotics (which target bacteria, not viruses). Unfortunately, the panel does not rule out the possibility that a patient with a viral infection is simultaneously infected with a disease-causing bacterium.

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