The discovery of a Canadian-born cow with bovine spongiform encephalopathy (BSE) in the state of Washington in December 2003 raised awareness of the need for increased screening of animal feed to ensure the absence of prohibited ruminant proteins in ruminant feed.
Since the FDA does not have pre-market approval over veterinary diagnostic devices such as feed test kits for detection of prohibited ruminant protein, CVM’s Office of Research initiated a study to evaluate the performance characteristics of several commercially available test kits. The study included two tests that use lateral flow, or “dip-stick” diagnostic devices designed for general use, and two that were designed for use by laboratory personnel. CVM recently presented the results of the FDA’s completed evaluation of the two lateral flow test kits: Neogen Corporation’s Reveal for Ruminant in Feed test and Strategic Diagnostics, Inc.’s, (SDI) Feedchek test.
CVM researchers found that Neogen’s test was 100 percent selective when conducted by multiple analysts. Therefore, this test never gave a false positive result. The test was able to detect ruminant protein down to only one percent, which was the level stated in the label guarantee.
CVM researchers evaluated Strategic Diagnostics’ test and observed variable selectivity that seemed to be related to difficulty in reading the test. The test exhibited 62 percent selectivity when conducted by one analyst and 97 percent selectivity when conducted by another analyst. Therefore, the test reported false positives in three percent and up to 38 percent of the samples. The test was able to detect ruminant protein to the level of 0.1 percent.
False positive samples are true negative samples that were incorrect-ly identified as being positive, while false negative samples are truly positive samples that were incorrectly identified as negative. Selectivity is the capacity to detect true negative samples, and sensitivity is the capa-city to detect true positive samples.
FDA also identified critical issues with reading the results for both test kits. The color development begins when the test strips are placed in solutions extracted from the feed sample. Neogen’s test strips were accurate only when they were read 15 minutes after color development had begun. SDI’s test strips were accurate between three and five minutes after color development had begun. Reading the SDI test longer than five minutes after color development has been initiated can potentially result in false positive reactions, as test strips turn positive after five minutes. Therefore, when using these test kits, it is important to take the readings at the appropriate time intervals.
These critical pieces of information were not contained in the package inserts of the test kits FDA evaluated, and could potentially lead to incorrect interpretation of the test strips, resulting in a false negative determination if read too soon (both Neogen and SDI), or a false positive determination with the SDI strips if read after five minutes. The Neogen Corporation has already incorporated this change (reading the strip 15 minutes after initiation of color development) into their package insert.
These test kits can be an impor-tant tool for surveillance and quality assurance although they appear to be less sensitive than feed microscopy and polymerase chain reaction, known as PCR, techniques that are capable of detecting at least 0.1 per-cent bovine meat and bone meal.
December 2004 Render