Every test has what is called a false positive (eg-you get positive results, even if you do not have the illness) and a false negative (eg-you get negative results, even if you do have the illness).

The limits of the test are chosen so that there is an optimal true positive and true negative rate. Those can be slightly different for each lab, based on the equipment and reagents that are being used.

Many times there is a also a gray zone, in which the chance of a true positive is too high to be ignored, yet too low to be conclusive. Some labs will just cut it into one of the categories, and others will report it as equivocal.

We all have a very low level of various antibodies (some which are auto-reactive) that mostly have no clinical significance. This is why there is a certain cut off that has to be used to differentiate between such a non-specific response and a true positive test.

Those are again different according to the specific test and the specific lab performing it.

In deciding if a test is truly positive (or truly negative) one has to incorporate the clinical picture with the results of the test.