Because the ELISA-based test, which uses antibodies to rapidly detect the prions that cause the brain-wasting disease, was known to provide some false-positives, the USDA had called the initial test result last week "inconclusive."
According to a statement issued by the USDA late yesterday, test results were reactive in both the first and second screens of the animal suspected of having the disease. The department's National Veterinary Services Laboratories then ran the standard immunohistochemistry test a second time on the sample, and, for the second time on the IHC test, the result was negative.
"Negative results from both IHC tests makes us confident that the animal in question is indeed negative for BSE," the USDA said in a statement.
The department noted that this is the third time the Bio-Rad rapid test had generated a false positive result.
In a statement issued after the USDA's announcement, Bio-Rad said, "The test is designed to be extremely sensitive so that it will detect any sample with the potential of being positive."
Shares in the Hercules, Calif.-based firm dropped $2.78, or 4.8 percent, in mid-afternoon trade on the American Stock Exchange.
The pull back comes after a recent run-up in Bio-Rad's share price, which started the day after the USDA reported that results from the two rapid screening tests for BSE were inconclusive. The news led to concern that, almost a year to the day, the nation's beef supply may have been contaminated from a cow with the disease.
As reported in yesterday's issue of BioCommerce Week, a GenomeWeb News publication, Bio-Rad closed Monday at $56.55, up nearly 6 percent since the markets opened on Thursday.