November 4, 2004
FDA Evaluates Test Kits to Detect Animal Proteins in Animal Feed
Scientists in FDA's Center for Veterinary Medicine have evaluated two commercial test kits that are designed to detect animal proteins in animal feed. A January 26, 2004, HHS Press Release entitled “Expanded "Mad Cow" Safeguards Announced to Strengthen Existing Firewalls Against BSE Transmission”, announced that FDA would continue to support the development and evaluation of feed tests for detection of materials prohibited for use in ruminant feeds.
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 animal proteins in ruminant feed. Scientific evidence has demonstrated a clear link between the practice of feeding ruminants, such as cattle, the rendered remains of other ruminants with the spread and dissemination of BSE.
Since the Food and Drug Administration does not have pre-market approval over veterinary diagnostic devices such as feed test kits for detection of prohibited animal protein, the Office of Research in FDA's Center for Veterinary Medicine (CVM) initiated a study to evaluate the performance characteristics of several commercially available test kits. The study included two tests that used lateral flow, or “dip-stick” diagnostic devices designed for general use, and two that were designed for use by laboratory personnel. This CVM UPDATE presents 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% selective when conducted by multiple analysts. Therefore, this test never gave a false positive result. The test was able to detect animal protein down to only 1%, which was the level stated in the label guarantee.
CVM researchers evaluated Strategic Diagnostics Inc.'s test and observed variable selectivity that seemed to be related to difficulty in reading the test. The test exhibited 62% selectivity when conducted by one analyst and 97% selectivity when conducted by another analyst. Therefore, the test reported false positives in 3% and up to 38% of the samples. The test was able to detect animal protein to the level of 0.1%.
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 3 and 5 minutes after color development had begun. Reading the SDI test longer than 5 minutes after color development has been initiated can potentially result in false positive reactions, as test strips turn positive after 5 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 5 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 important tool for surveillance and quality assurance although they appear to be less sensitive than feed microscopy and polymerase chain reaction (PCR) techniques that are capable of detecting at least 0.1% bovine meat and bone meal.
Definition of terms:
False positive samples are true negative samples that were incorrectly identified as being positive.
False negative samples are truly positive samples that were incorrectly identified as negative.
Selectivity is the capacity to detect true negative samples.
Sensitivity is the capacity to detect true positive samples.
Issued by:
FDA, Center for Veterinary Medicine,
Communications Staff, HFV-12
7519 Standish Place, Rockville, MD 20855
Telephone: (240) 276-9300 FAX: (240) 276-9115
Internet Web Site: http://www.fda.gov/cvm
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