Rapid Tests Get Scrutiny

Just how sensitive are those rapid tests and what are the odds that a 'positive' result will be false. Little is known for sure. Compiled by staff

Published on: Jun 29, 2004

USDA's announcement last Friday that a head of cattle had thrown a positive test for the presence of bovine spongiform encephalopathy (BSE) was considered an "inconclusive" result. The animal, which had been evaluated using a rapid test from BioRad, a California company, gave a positive result, which triggered the ag agency to send tissues to its National Veterinary Services Laboratory in Ames, Iowa for further testing.

While USDA officials expressed confidence in a press conference Friday night that the in-depth test would show a negative result, some question how sensitive those rapid tests are and whether USDA should be so confident. In a story reported on the Web by The New Scientist the writers point out that the testing program targets animals showing BSE-like symptoms and the rapid tests are very sensitive aiming to catch any possible infected animal.

USDA does not provide information on the odds of the rapid test being positive and the European Commission, which tested the BioRad product before its use in 1999, never published the data. But industry sources told The New Scientist that BioRad throws a false positive at a rate of about one in a thousand in initial tests. BioRad is used in Germany and Belgium to test cattle.

In Europe and Japan, if a cow's brain initially tests positive using the BioRad product, the animal is tested twise more. Only if one of those repeated tests is also positive are the tissues sent for further testing. The false positive rate after that repeated testing is even lower, about one in 100,000. The Japanese report that in tests of 1.5 million cattle, they've seen more than 100 false positives.

The cattle industry, like USDA, will be waiting with great anticipation for the Ames test results later this week. Those could arrive as early as Wednesday. We'll keep you posted.