The test for antibodies is not a good test.
I try to explain how it works on how I've understood it, this is not a medical point of view but a normal person point of view.
You have a 30% chance to have a false positive from other Coronavirus (flu or other seasonal viruses).
IF they say "you are ok cause you to have antibodies on your body it will be a massacre.
If they find you positive and THAN they will do to you the normal COVID test (the one 7 hours long) than it can be a good thing.
I knew about the problems with false test results, both false positive and false negative. Antibody test is worse than RT-PCR test in both sensitivity and specificity of the test but the cost is lower and for massive testing, antibody test is feasible and applicable.
Besides the sensitivity, specificity, false negative and false positive, the current prevalence has its impact on predictive values (both positive predicted value (PPV) and negative predicted value (NPV). I meant when one person receives a positive test, it does not mean he was actually infected. The probability of his infection will depend on sensitivity, specificity of the test; and the prevalence of the population in which he lives.
Expenses of massive testing are huge and governments have to consider about this aspect. They have to choose between antibobdy test and RT-PCR or other tests to maximize number of people they can test. Massive testing play its role as
screening. If one has positive test, he will go through quarrantine and further test later (to confirm that positive result). If one has negative test, it does not he is safe and he has to maintain social distancing and other heath hygiene practice too and maybe future re-tests.
Prevalence thus impacts the positive predictive value (PPV) and negative predictive value (NPV) of tests. As the prevalence increases, the PPV also increases but the NPV decreases. Similarly, as the prevalence decreases the PPV decreases while the NPV increases.
PPV = (sensitivity x prevalence) / [ (sensitivity x prevalence) + ((1 – specificity) x (1 – prevalence)) ]
NPV = (specificity x (1 – prevalence)) / [ (specificity x (1 – prevalence)) + ((1 – sensitivity) x prevalence) ]
