Well, there are some pretty lousy tests. But even with very good ones, there are still sampling problems. That long nasal swab is supposed to hurt like a bitch, but very few nurses want to do that. Mine just tickled a bit. No wonder it was negative. It was also about 10 days after first symptoms, so there may not have been any virus present in my throat by then.

About all tests can tell you is if you have a positive PCR test and then another positive test a few days later, you do have it. If you have a top-quality antibody test (like Abbott or Euroimmun) with a positive result, then you do have antibodies.

Negative test results for either PCR or antibody don't tell you anything: Maybe you have it, maybe you don't. Maybe you had it, or maybe not.