TB continues to cause widespread problems throughout our practice area with many of our herds now on sixty-day short interval tests.
Currently TB testing of cattle is the main method of control as illustrated above. We are often asked about the accuracy of
the TB test and the complaint that a lot of reactors “didn’t have it “when culled.
Here are some facts to try and explain this.
Sensitivity – is a measure of how many positive animals can be missed by the test.
Specificitity – is a measure of how many animals classed as reactors are not.
A TB skin test read at standard interpretation is 81% sensitive with 99.98% specificity.
A TB skin test read at severe interpretation is 85% sensitive with 99.9% specificity.
A Gamma Interferon blood test which can detect TB much earlier is 90% sensitive with 96.5% specificity.
What does this mean?
If a herd had 100 positive animals a standard TB test will find 81 of them leaving 19 behind with a 0.02% or 1 in 5000 chance that any of the reactors will be negative.
This means that a positive skin test is a positive animal nearly every time!
A Gamma Interferon test will find 90 of these theoretical 100 positive animals, so a higher rate, but still leaves 10 behind.
The Gamma Interferon test has a lower specificity and so has a higher chance of 3.5%, or 1 in 28, of needlessly culling uninfected cattle.