Dr. Damien Barrett:

It depends on the context. From the point of view of specificity and where testing has thrown up false positives, the skin test is superior. It has a specificity of about 99.95%, which means in reality that about one in every 5,000 is a false positive. The difficulty with the skin test is that the sensitivity is no better than 80%, which means that out of every ten infected cattle, it is not detecting two even when it is performed to a high standard. What we do in practice is that where we know there is cattle-to-cattle transmission, and where we know there is likely to be undetected infection, we then come in with the blood test to augment the sensitivity.