So I assume they're using a cheaper ELISA test to look for IgG/M first if the patient is known recovered or asymptomatic, but then how could rtPCR protocols cre

So I assume they're using a cheaper ELISA test to look for IgG/M first if the patient is known recovered or asymptomatic, but then how could rtPCR protocols create false positives transcribing viral RNA unless the primers, probes, or matched regions are too small? Are they matching un-encapsulated RNA fragments? Is there the a type of in vitro protocol for measuring viral infectivity potential of a sample?