Why are there sometimes false readings of htlv i&ii in blood donations?

2 types of test. We screen by looking for antibodies; this picks up all the infected individuals but also folks whose antibodies may be due to some past infection. The more specific confirmatory tests are much more expensive but only if they are positive should the blood / person be considered infected.
Sensitivity and. Specificity. Screening tests are designed to have high sensitivity so that no cases are missed, in the bargain, it produces false positives. A positive screening test should be confirmed with a more specific test.