| ||Many bovine spongiform encephalopathy (BSE) testing laboratories throughout the world have adopted the use of "rapid tests" to quickly screen cattle for BSE. Rapid testing allows for reduced turn-around-time of reporting test results, which in turn allows for reduced holding time of carcasses at slaughtering plants or rendering facilities. By using automated equipment, a large number of samples (hundreds to thousands) can be tested rapidly.
The Animal Health and Assurance Division (AHAD) of Alberta Agriculture and Rural Development (ARD) is screenning for BSE prions in the O.S. Longman Laboratory in Edmonton in the highly bio-containment level 3 transmissible spongiform encephalopathy (TSE) laboratory. The Bio-Rad TeSeE ELISA rapid test was selected as the diagnostic test for rapid BSE screening. Bio-Rad TeSeE is an enzyme-linked immunosorbant assay (ELISA) test. It has been validated by the European Union (EU), Canada and the United States (US) and has demonstrated both high sensitivity and specificity for detecting BSE prions in cattle.
Many European countries, the United Kingdom (UK), Japan and the US also currently use the Bio-Rad TeSeE ELISA for BSE screening. Due to the test’s widespread usage, the international community accepts results generated by Alberta’s FSD laboratory.
Sensitivity is a measure of a test’s ability to detect truly infected subjects. A test with a high sensitivity will have a high probability of detecting truly infected subjects. For example, if a screening test is used to detect BSE in 100 samples known to have BSE and identifies a positive result in 99 of them, its sensitivity would be 99/100 x 100 = 99%. A test with 99% sensitivity would have a 99% chance of detecting one infected sample, even if there was only one positive case in one million negative cases.
Specificity is a measure of as a test’s ability to correctly classify subjects as being uninfected in the absence of disease. A test with a high specificity generates few false positive results. For example, if a screening test is used to detect BSE in 100 samples known to be uninfected with BSE and produced a negative result in 99 of them, its specificity would be 99/100 x 100 = 99%. A test with 99% specificity would be expected to produce false positive results 1% of the time.
Research shows that the probability of Bio-Rad TeSeE ELISA failing to detect a single case of BSE in a large group of samples is in the range of 1.1 – 3.6 per 1000 tests (p< 0.05) (sensitivity). Reports from Japan, where every single animal is tested, show that the false positive rate for the Bio-Rad TeSeE ELISA test is 1 in 30,000 (specificity). When either of these situations happens, the test is known to be “inconclusive” and will require further testing to truly identify whether the sample is positive or negative.
Reasons, besides test sensitivity and specificity, for why “inconclusive” or “positive reactor” tests results might occur include:
- Technical error—The Bio-Rad TeSeE ELISA has the capability of detecting cattle with both clinical signs of BSE (clinical BSE infections) as well as apparently healthy cattle that are in the late stages of the incubation period of BSE (pre-clinical infections). However, for the test to be able to detect pre-clinically infected cattle, a specific area of the brainstem, called the obex, must be sampled and tested. Research has shown that BSE prion proteins accumulate primarily in the brainstem of cattle and the obex is the site where they begin to accumulate first. The obex is only 2-3 millimetres wide, thus laboratory technicians who prepare and process samples for BSE testing must be very skilled and precise. Inaccurate sampling and missing the obex is one factor that can affect the result of the test.
The initial screening for BSE prions is conducted in the ARD AHAD TSE lab in Edmonton. If the initial Bio-Rad TeSeE ELISA screening test generates either an "inconclusive" or " Initial Reaction ", duplicate sections of the obex are prepared and both are tested again. If both tissues are negative, the brain sample is considered negative for BSE. Tissues are then sent to National and OIE BSE Reference Laboratory, Canadian Food Inspection Agency, Lethbridge for quality assurance purposes. If however, both of the repeat tests yield an "inconclusive" or "Initial Reaction" test result, further confirmatory tests, using immunohistochemistry staining (the international "gold standard" test for detecting prions) and a western blot technique, are conducted to determine true positivity. These confirmatory tests are performed National and OIE BSE Reference Laboratory, Canadian Food Inspection Agency, Lethbridge.
- Condition of sample—The more intact and well preserved the obex is, the more accurate testing results will be. Sometimes, due to decomposition or physical trauma (e.g. during stunning or humane killing of the animal; or during removal of the head or brainstem), the obex may not arrive at the laboratory intact. If this happens, it is more difficult for a technician to accurately identify and sample the obex. The sample may also be contaminated with other material, which may interfere with test accuracy. Severe decomposition of a sample may result in an inaccurate test result.