Reliability of the skin and blood tests for bovine TB

In the extract below quoted specificities imply that in Great Britain the blood test is on average about 40 times more likely than the skin test to indicate that a healthy animal is infected. Average specificity of the skin test is quoted to be 99.9% (1 in 1000 wrongly classified as a reactor) whereas that of the blood test is quoted to be 96% (4 in 100 wrongly classified as a reactor).

The extract below is taken from a reply received on 27th March 2009 sent by DEFRA in response to the FOI request shown in Reference 1.

" There are two immunologically based diagnostic tests used in Great Britain (GB) for bTB screening of cattle herds. The primary screening test is the single intradermal comparative cervical tuberculin (SICCT) test, which is commonly known as the comparative tuberculin skin test. Additionally, the ancillary gamma-interferon (γ-IFN) diagnostic blood test has been used since 2002 alongside the skin test in certain prescribed circumstances.

The diagnostic sensitivity and specificity ranges for these tests have been evaluated in several articles and reports published in the veterinary scientific literature over the years. These are in the public domain and for more information you are kindly referred to those on our website at


http://www.defra.gov.uk/animalh/tb/pdf/gifn_trialfinalreport.pdf
http://www.defra.gov.uk/animalh/tb/pdf/gifn_specificityreport.pdf

To summarise, the published estimates for the animal-level sensitivity of the SICCT test in various countries, if correctly performed, range from 52 to 100%, with a median value of 80% at standard interpretation. In other words, this test can be expected to miss about 2 in every 10 infected cattle on a single round of testing (a 20% false negative probability).

Other studies carried out in TB-free cattle populations have found the animal-level specificity of this test to lie between 78.8% and 100%, with a median value of 99.5%. In particular, the SICCT test applied to cattle in bTB-free herds in GB is believed to have a specificity of 99.9%, which is equivalent to a 0.1% probability of false positives, or a one in 1,000 chance that a non-infected animal will be wrongly classified as a reactor.

With regard to the γ-IFN blood test, performance evaluation carried out in a number of countries shows that at the laboratory cut offs used in GB, it has a sensitivity of between 73 and 100%, with a median value of about 87% (i.e. a false negative probability of 10 to 15%, which is slightly lower than that of the SICCT test). Because the two tests detect slightly different sub-groups of infected cattle, by combining the two tests a higher overall sensitivity can be achieved.

A trial conducted in GB to evaluate the specificity of the γ-IFN blood test confirmed the findings of previous studies in other countries, in that it estimated its specificity to be about 96% (i.e. a 4% probability of false positive reactors). This is higher than the false positive probability for the SICCT test and it is one of the reasons why the γ-IFN blood test cannot be used for routine bTB surveillance.

Nevertheless, there is no consensus with regard to the most appropriate values of these classical test characteristics that currently apply to GB. It is important to appreciate that the sensitivity and specificity estimates cited above are indicative averages. The actual performance of a screening test in a particular herd under field conditions is, of course, dependent on a range of variables, such as the diligence of the tester, in adhering to the correct testing procedure, the within-herd prevalence of cattle sensitised to other non-TB environmental mycobacteria, and factors that may alter the immune response to tuberculin of individual animals (e.g. nutritional status, pregnancy, stress levels, concurrent infections, etc). Additionally, for all diagnostic tests there is a trade-off between sensitivity and specificity, so that different interpretations of the test can be used under different disease situations. Sensitivity is enhanced in herds with post-mortem or cultural evidence of TB infection by application of the so-called severe interpretation. "

References

1. Information request: False negatives and positives associated with bovine TB tests
2. Review of the gamma interferon testing policy