evidence: Health care providers typically rely on molecular tests, particularly when people have COVID-19 symptoms, whereas antigen testing is often used when quick results are needed or for general screening and surveillance. [...] The first test created to detect COVID—and still the most widely used—is a molecular test called PCR (polymerase chain reaction), Dr. Campbell says. "PCR and similar tests look for the COVID virus’s RNA," meaning genetic material that comes only from the virus, he explains. "They tend to be quite sensitive, but even among these, they are on a continuum of sensitivity and vary a whole lot." [...] Since the symptoms of COVID-19 and the flu are similar, it’s helpful to know there are molecular tests you can have that detect each virus using a single sample. In fact, there are even tests that diagnose COVID-19, flu, and RSV (respiratory syncytial virus), a virus that causes common cold symptoms, at once.