Remember what you learnt in Module A - a gold standard is a perfect test:

• It always gives a positive result when the animal is infected.
• It always gives a negative result when the animal is non-infected.

However, existing reference (gold standard) tests can be invasive, expensive or take a long time. Consequently, we will sometimes prefer or are forced to use an imperfect test. In this case, two parameters are used: the diagnostic sensitivity and diagnostic specificity. They describe the performance of that test in comparison to the true disease status as assessed by the gold standard.