Due to the risk of false negative results that can occur with commonly used tests to diagnose COVID-19, the CDC recommends anyone who has tested negative after coming in contact with a confirmed case should remain in self-quarantine for 14 days since their last exposure. This is because it can take up to 14 days for a person to develop signs and symptoms of illness, and a shorter self-quarantine could potentially allow a person who is actually infected, but does not have symptoms, to continue to spread the virus to others. Self-quarantine is especially important for contacts who frequent congregate settings, like nursing homes, schools, correctional facilities, etc., where the virus that causes COVID-19 can spread easily and consequences of additional infections can be high (e.g. spread of infection to individuals who are elderly or have underlying health conditions).
Right now, there are two types of diagnostic tests available in the US to diagnose COVID-19 infection: PCR tests, which detect the virus’s genetic material; and antigen tests, which detect specific proteins that are part of the virus. A few PCR tests and most antigen tests are rapid diagnostic tests, meaning results may be available in as little as 15 to 45 minutes. In-laboratory PCR tests are considered more accurate and take 24-48 hours to get a result, but depending on demand, this kind of test can take several days for the test to be completed and results to be reported. A third type of test, antibody testing, cannot diagnose COVID-19 and only tells you if you had a COVID-19 infection in the past.
The accuracy of any diagnostic test for COVID-19 (PCR and antigen) depends on many factors, including if the sample was collected properly, and if it was properly shipped to the lab. Also, regardless of the type of test, all results are affected by when the test sample was collected after exposure or onset of symptoms. For example, if a person is tested on the day they were exposed, the test result has a higher likelihood of being negative, because there may not be enough COVID-19 virus in the nose or saliva yet to be detected. The chance of getting a false negative result decreases if a person is tested a few days after they have been exposed or have developed symptoms. That said, the risk of false negative results again can be possible after about 7 days from initial exposure and/or onset of symptoms, as virus levels can drop quickly after about a week of being infected. Remember, even with lower virus levels, an individual can still be infectious and able to pass the virus to others.
Generally speaking, if a test result comes back positive, it is almost certain that the person tested is infected with COVID-19. A negative test result is less definite, especially when done with a rapid diagnostic test as these kinds of tests are considered less accurate than in-laboratory tests. Because of this, if a person has a negative result with a rapid diagnostic test, their doctor may order an in-laboratory PCR test to confirm the result. In-laboratory PCR testing is currently considered the gold standard for diagnosing COVID-19, but remember that getting a result, depending on demand, can take several days and the timing of when an in-laboratory test is performed also can affect the result.
While waiting for any COVID-19 test result, CDC recommends that you should self-quarantine at home. Quarantine keeps a person who was in close contact with someone who has COVID-19 away from others. Someone who is self-quarantining is taking proactive steps to prevent the spread of disease to loved ones, co-workers, neighbors and others. Staying home in quarantine until 14 days after last contact includes checking your temperature twice daily, watching for symptoms of COVID-19 and, if possible, avoiding close contact with people who are at higher-risk for getting very sick from COVID-19.
If an individual is a contact of a confirmed COVID-19 case, CDC recommends they get tested and states the following:
A positive test, whether or not you have symptoms:
o Considered confirmatory for COVID-19 infection
o If you are positive you need to isolate yourself, either at home or in a healthcare facility. Isolation keeps someone who is sick or tested positive for COVID-19 without symptoms away from others, even in their own home.
o Stay home until after at least 10 days since symptoms first appeared and at least 24 hours with no fever without fever-reducing medication and symptoms have (except symptoms of loss of taste or smell, which can persist for weeks).
o If you live with others, stay in a specific “sick room” or area, if available, and away from other people or animals, including pets. Use a separate bathroom, if available.
o Refer to What you can expect to happen if you are diagnosed with COVID-19
· A negative test and you do not have symptoms:
o If your test is negative and you do not have symptoms, self-quarantine for 14 days after your last exposure to COVID-19 and follow all recommendations from the health department.
o A negative result before the end of your quarantine period does not rule out possible infection.
o You do not need a repeat test unless you develop symptoms.
· A negative test and you have symptoms:
o If your test is negative and you have symptoms you should self-quarantine for 14 days after your last exposure to COVID-19 and follow all recommendations from the health department. A second test and additional medical consultation may be needed if your symptoms do not improve. o If your symptoms worsen or become severe, you should seek emergency medical care.
Care must be taken in interpreting PCR and antigen tests for SARS-CoV-2 infection, the virus that causes COVID-19. If there is a high suspicion that an individual may have COVID-19, with or without symptoms, infection should not be ruled out on the basis of a single negative test result. Your doctor and local health department can advise on assessing exposure risk, quarantine recommendations, and determining when retesting may be indicated.
Clay Roscoe, MD, MsC (public health) is Medical Director for Southwest District Health. Nikole Zogg, PhD, MPH is District Director for Southwest District Health.