The test detects the presence of antibodies within blood to Mycobacterium bovis (M. bovis), the cause of bTB, by use of individual antigens.
The different antigens used in the test are placed separately as individual spots within a well. The number of antigens used in a test may vary between species. If antibody to bTB is present in the blood sample being tested, then it will bind with the relevant antigen resulting in a chemiluminescent reaction, the light from which can be measured and quantified (see Figure 1 which demonstrates an outcome from a cattle test which uses 11 separate antigens).
Figure 1
Images from three individual testing wells showing individual antigen spots that have bound antibody and are luminescent. This light is measured to produce a quantitative result as displayed in the table (RLU = Relative Light Units).
Thresholds are set for each individual antigen and if the level of Relative Light Units (RLU) is above this threshold, a positive reaction is deemed to have occurred. A positive test result is when two or more antigens are shown to be above the thresholds set for the antigens.
In cattle there are two different thresholds set for each antigen, a high sensitivity setting and a high specificity setting. The high specificity setting requires a higher threshold to be reached for a positive reaction.
The timing of sampling after a tuberculin ‘skin’ test (SICCT test) significantly affects the sensitivity of detection of antibody. Samples taken between 10 and 30 days post–SICCT testing will benefit from an antibody ‘boosting’ effect due to an anamnestic response to the PPD antigens used in the test in animals infected with M. bovis. Samples taken outside this ‘window’ (non-boosted) will have lower levels of antibody and the test will be less sensitive.