"This inconclusive result does not mean we have found a new case of BSE. Inconclusive results are a normal component of most screening tests, which are designed to be extremely sensitive so they will detect any sample that could possibly be positive. In addition, this animal did not enter the human food chain nor the animal feed chain."
The statement above by John Clifford, Chief Veterinary Medical Officer for the USDA speaks volumes on the "Sensitivity" of screening tests.
Screening tests are in fact designed to be extremely oversensitive so that no positive animal can pass through it without detection.
Read it again, and again. Let it soak in..............................
Link: http://www.usda.gov/wps/portal/usdahome?contentidonly=true&contentid=2006/03/0080.xml