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Rapid COVID tests! What you should know about accuracy

Updated: Jan 19, 2021


The skinny:


There are essentially two broad categories of testing:


1. Antigen test (frequently referred to as a rapid test). This test detects protein fragments specific to the Coronavirus. It can be done in a clinic, doctor’s office, or hospital. Turn around time for results is usually very quick and in some cases, results can be reported within 15 minutes.


2. PCR test. PCR testing is considered the “gold standard” in SARS-CoV-2 detection. This test actually detects RNA (or genetic material) that is specific to the virus and can detect the virus within days of infection, even those who have no symptoms. The test can be done in a clinic, hospital, or even in your car. Turnaround time is longer, generally in the 2-3 day range but results can be in as little as 24 hours. When demand is high, results can take a week or longer.


WHO SHOULD GET A RAPID TEST?

Rapid tests are considered most accurate in a patient who is having symptoms of COVID-19. While the rapid test can get you results very quickly, the results may not always be accurate.




Are rapid COVID-19 tests reliable?


To a certain degree, you’re sacrificing accuracy with speed. By their very nature, the antigen style tests aren’t as sensitive because they require a larger amount virus present to be positive. They can be helpful in rapidly screening symptomatic individuals early in the infection. They can also help to rapidly screen a large group of individuals during an outbreak in a resource-limited setting. Someone with a positive test by this style of test should be treated as infected with COVID-19, but a negative test is less reliable and may need to be confirmed by a more sensitive molecular assay.


WHAT DOES A FALSE NEGATIVE OR FALSE POSITIVE MEAN?


In those who don’t have symptoms, the false negative rate — meaning my test is negative but I actually have the disease — can be as high as 50%. This can have tremendous consequences as we try to contain the spread of COVID-19.


The false positive rate — meaning I test positive but I do not really have the disease — is quite low. So, if you test positive from a rapid test it is more likely you do have the disease.


BOTTOM LINE:


If you’re experiencing symptoms of COVID-19 and a rapid test result indicates you’re positive, you should take that as an indication you have the coronavirus. You should quarantine and contact your doctor.


However, if you have COVID-19 symptoms and a rapid test result is negative, you should still contact your doctor because you may need to receive a PCR test. Don’t be victim of a false sense of security with a false negative result from a rapid COVID-19 test.


WHAT IF I HAVE A NEGATIVE RAPID TEST BUT STILL HAVE SYMPTOMS?


Current recommendation is to have a confirmation test performed (the PCR test) if you still have symptoms and have had a negative rapid test. This is especially true if you’ve had a high-risk exposure. A high risk exposure is considered more than 15 minutes of contact with a COVID-positive individual in a 24-hour period and you have been within 6 feet of this individual.


Keep in mind there are still high false negative rates with the PCR test. Typically I order a respiratory PCR panel which looks for other viruses that could be causing a patient's symptoms if their covid test is negative.


This is a photo of a respiratory PCR panel I recently ordered on a patient's who COVID test came back negative, to look for other viruses:





And for those wanting more detail---keep reading!


Molecular tests (also called PCR tests, viral RNA tests, nucleic acid tests)


What about accuracy? The rate of false negatives — a test that says you don’t have the virus when you actually do have the virus — varies depending on how long infection has been present: in one study, the false-negative rate was 20% when testing was performed five days after symptoms began, but much higher (up to 100%) earlier in infection.


The false positive rate — that is, how often the test says you have the virus when you actually do not — should be close to zero. Most false-positive results are thought to be due to lab contamination or other problems with how the lab has performed the test, not limitations of the test itself.


A molecular test using a nasal swab is usually the best option, because it will have fewer false negative results than other diagnostic tests or samples from throat swabs or saliva. People who are in the hospital, though, may have other types of samples taken.


Antigen tests


How is it done? A nasal or throat swab.


Where can you get these tests? At a hospital, a doctor’s office or at home (the FDA has authorized a home antigen test that requires no prescription).


What does the test look for? This test identifies protein fragments (antigens) from the virus.


How long does it take to get results? The technology involved is similar to a pregnancy test or a rapid strep test, with results available in minutes.