Is it true that the BJD screening test is only 20% accurate?
There are two measures of accuracy related to how good a test is at correctly identifying infected and non-infected animals – test sensitivity and specificity. Test sensitivity for BJD – that is, the ability to detect infected animals – is low because the growth of the bacteria and development of JD in infected animals is slow. The lower the sensitivity of the test, the greater the number of infected animals that test negative (i.e. a higher false negative rate). Tests for antibodies in blood and for bacteria in faeces are usually negative in the early stages of infection. A review of the ELISA antibody test in 2000 concluded that, in early infection, the test detected about 20% of infections, and in advanced disease it detected 80%. On average, this review recommended a figure of 30%. A subsequent analysis of the Victorian program indicated that the sensitivity in subclinically infected cows, mostly in seasonal dairy herds, is probably less than 20%. Test specificity refers to how goo
