Polymerase chain reaction (PCR) testing for SARS-CoV-2 is currently the most effective frontline test to detect exposure and infection. A positive test, taken from a nasal sample, is presumptive evidence of disease. But a negative test does not rule out infection for a variety of reasons. As many as 25% of the nasal swab tests are likely false negatives. Causes of false-negative tests include improper collection, improper handling or storage, the prolonged time between collection and testing, or laboratory error.
Since the PCR swab test gives us no information about a person’s immunity to infection, we have been anxiously waiting for a method to test for antibodies to the virus. From experience with other infectious agents, we know that the presence of IgG antibodies indicates that immunity exists, and a presumption can often be made that the patient cannot be re-infected. If this were, in fact, the case with SARS-CoV-2, we could send people with antibodies back to work, expecting that they will be protected. Unfortunately, not all enzyme-linked immunoassay (ELISA) tests are alike in their ability to reliably predict immunity.