Understanding COVID-19 test results

COVID-19 detection employs three methods: PCR, serology, and rapid tests.

PCR (for polymerase chain reaction) is preferentially performed in a nasopharyngeal sample. If the patient is infected, PCR transforms and amplifies coronavirus genetic material to allow its detection by specific probes. Therefore, a positive test result indicates that the person is infected by the coronavirus and a negative test result shows that the person is not infected. This test must be performed from the third to the tenth day after disease onset; otherwise, there is very little viral genetic material and a false-negative result is likely. PCR is positive while the virus is active in the body of the patient, who must be treated (sometimes even hospitalized) and practice social distancing.

Serology detects antibodies, mainly IgM and IgG. Antibodies are highly specific substances produced by the body in response to an infection. This test is performed using a blood sample, which must be obtained at least 10 days after disease onset. Samples collected before this period may generate false-negative results because the body had no time to produce antibodies. A patient with a negative test result for both IgM and IgG never had contact with the virus. Positive test results indicate that the person had contact with the virus during his/her lifetime. These two antibodies allow us to estimate when infection occurred: IgM indicates a recent infection, whereas IgG indicates an older infection. In addition, since people with no symptoms or those with mild COVID-19 may not have antibodies enough to be detected (i.e., presenting a false-negative result), serological findings must be confirmed at PCR.

Rapid tests can detect both the coronavirus and the antibodies produced in response to it. These tests are similar to over the counter pregnancy tests: a nitrocellulose sheet (similar to a piece of paper) reacts with the sample and, if positive, generates a visual sign. Even though these tests are fast and approved by proper regulatory bodies, a 75% margin of error for negative results is reported. Therefore, negative results at rapid tests create uncertainty and insecurity in determining social distancing requirements, and some experts are against their use.

In this difficult moment, trust your doctor to indicate COVID-19 tests and interpret their results. In addition, follow the proper local guidelines, use masks and gloves, clean your hands with soap and water or specific sanitizers and, if possible, #stayhome.

José Luiz de Oliveira, MD; Renata Scavone, DVM, PhD.