Which Test When


ByRichard Davis

People are flocking to testing sites to find out if they have COVID. That is a good thing and it means that a good number of people are taking responsibility for their safety and the safety of others. But when it comes to testing, there are options and it can be difficult to decide when to get tested and which test is best under certain circumstances.

The state of Vermont made over 300,000 rapid diagnostic testing (RDT’s) kits available a few days ago and they were grabbed up within a short period of time. Before people use them it would be a good idea to do some research and find out how to use the kits and when to use them. I will be getting some and I hope the instructions in the kits are useful.

A recent article in the prestigious New England Journal of Medicine (NEJM), RAPID Diagnostic Testing for SARS-COVID-2, is one of the best sources of information about RDT’s that I have come across and it is the source I am using for much of this piece.

It is helpful to remember that symptoms of COVID 19 appear 2 to 14 days after exposure and that the average onset is 5-6 days after infection. Most people infected with COVID have mild-to-moderate symptoms and many people who are infected have no symptoms and can transmit the virus.

Most of the studies I have read indicate that vaccine immunity wanes after a second vaccine shot but that a booster can increase the level of protection significantly. The majority of boosted people who get COVID get a mild case.

Here is the official testing guidance from the CDC as stated in the NEJM article. “Rapid diagnostic tests (RDTs) that are authorized by the Food and Drug Administration to diagnose severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection are either nucleic acid amplification tests to detect genes or antigen-based immunoassays to detect proteins of SARS-CoV-2. RDTs are approved for use in persons with symptoms of coronavirus disease 2019 (Covid-19) and in asymptomatic persons who are close contacts of a person with Covid-19 or who have been in a potential high-risk transmission setting. Symptomatic persons should undergo testing as soon as possible, quarantine while awaiting test results, and consider retesting if they have a negative RDT, particularly if they have a high pretest probability of infection. Asymptomatic persons with a known exposure to SARS-CoV-2 should undergo testing 5 to 7 days after exposure, and if the RDT is negative, they should undergo testing again 2 days later. Persons with a known exposure to SARS-CoV-2 who are not fully vaccinated should quarantine while awaiting test results, and persons who test positive should isolate, contact a health care provider or public health department, and inform close contacts about the infection.”

There are two kinds of RDT’s, the molecular RDT and the antigen-based RDT. The antigen test has a lower sensitivity than the molecular RDT. Antigen tests can detect infection early in the disease course when viral loads are high, within 5 to 7 days after symptom onset.

Keep in mind that the laboratory PCR test is the gold standard And that means that if you are not confident in the results of an RDT you should try to get a PCR test.

Because RDT’s are not foolproof the NEJM authors warn, “Among persons with a moderate-to-high pretest probability, which includes symptomatic persons and asymptomatic persons who have had close contact with a person with Covid-19, a positive RDT indicates a confirmed SARS-CoV-2 infection. However, RDTs may have false negative results, and repeat testing should be considered in cases of high clinical suspicion or worsening symptoms and in the serial screening of asymptomatic persons. A second negative RDT 2 days after the initial test or a negative laboratory-based NAAT would help to rule out SARS-CoV-2 infection.”

They go on, “In persons with exposure to SARS-CoV-2, testing is generally not useful in the first 48 hours after exposure, since the virus will not have achieved a sufficient viral load. The most appropriate window for testing is generally considered to be 5 to 7 days after exposure, which is the average peak of symptoms and viral load. Therefore, for a single-test strategy, asymptomatic, exposed persons could use an RDT 5 to 7 days after exposure. For a two-test strategy, which is the FDA-approved indication for most RDTs for asymptomatic screening, a second RDT should be performed 2 days after a negative test. All symptomatic persons should be tested at the onset of symptoms and, if test results are negative, repeat testing should be considered if clinical suspicion remains high or symptoms worsen. In persons with low pretest probability of infection who have a positive RDT, a confirmatory test should be performed promptly.”

Hopefully, this information will be helpful. If you want to read the entire NEJM article it can be downloaded for free from their web site.

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