A good screening test is very sensitive. This means it picks up most cases of the disease so a negative result tells you with good level of certainty that the disease is absent. Thus a good screening test "rules out". It therefore also has high number of false positives.
It is usually not very specific, i.e if it is positive, it may be due to something other than the disease but it must be very sensitive i.e if it is negative, the disease is most likely absent.
Expl is the Overnight Dexamethasone suppression test for Cushing's syndrome
If cortisol drops : Cushing's is ruled out
However, if cortisol remains high, it may be Cushing's or Stress or something else i.e it's not specific
Also, ESR for temporal arteritis
If low, TA is ruled out
If high, there are several inflammatory diseases that could be responsible so it's not specific
Sensitivity rules out with its negative results, SnOut, specificity rules in with its positive results, SpIn
A confirmatory test is therefore highly specific for the particular disease or event.