An antibody test is not the same thing as a PCR test.

Antibodies are cells that your own immune system generated in response to an infection and purposed to eliminate or neutralize the infection.

Antibodies do not generally contain in their structures components of the virus or infection (unless you are looking at a macrophage that has absorbed a bacteria cell or viral infected cells or virus proteins).

So to test for antibodies, you would get a blood draw and they would likely be looking for an unusual antibody that is generally detected in those recovered from a particular illness vs those who have not had the illness.

For covid19, the antibody test is likely looking for the type of antibodies that the body produces when exposed to either the spike protiens, or to a typical coronavirus / sars-cov1 infection antibody type.

The PCR test is literally looking for evidence of the viral infection (dna/ rna) by basically running multiplying cycles against all DNA/RNA that exists in the biological material retrieved from a nasal swab or a spit specimen. The multiplying factor for PCR is determined by how many "cycles" the test sample is put through.

After about 30 cycles, the test is generally useless as it will amplify things so much that you can likely find a positive result for any thing you are checking for. This really has to do with a 30 plus cycle run causing the noise values in the analysis to overwhelm any actual signal value which thus gives many false positives.

It is thus important to understand the significance of the CDC specifying that the non vaccinated be tested by PCR at 40 cycles. Now you can understand why the PCR test in its current use --whether they isolated the virus to calibrate it or not-- is completely useless given their test parameters.

Bottom line, antibody tests when carried out in a scientifically and medically useful way (because in clown world today, I can't be sure they haven't monkeyed with the antibody testing too) are a good diagnostic to show you had something and possibly to identify what that something is. PCR tests are being used currently to tell if people have an active Covid-19 infection. The actual usefulness of the PCR diagnostic is dubious in the best of times, but especially for the situation we are in, the PCR testing is intentionally broken and useless to push high infection rates as a narrative.