Diagnostic Testing for COVID-19: A Quick Summary for PCPs

Laurie Scudder, DNP, NP


August 06, 2020

Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Information about COVID has evolved so quickly that it can be difficult for clinicians to feel confident that they are staying current. These summaries include links to our reference article on diagnosis of COVID-19, which is constantly updated to make sure you have the latest information.

Diagnostic testing for COVID-19 is critical. No one disputes that. But what is in dispute is whom to test, when to test, how to test, what to do while waiting for results, and how accurate those results are when you finally get them.

Here are the answers to those questions, based on the current information.

Whom to test. This is the (relatively) easy part. The ideal answer is that everyone should be tested. The Infectious Diseases Society of America issued tier-based recommendations way back in March, and they still apply. First priority continues to be patients who are ill, healthcare workers, and those with known exposure. But to truly figure out the amount of community spread in a given area, we need to test people who do not have a clear indication for testing. That is particularly true as more people return to work and the Centers for Disease Control and Prevention (CDC) has issued guidelines for workplaces to establish testing programs. Universal testing is recommended for some high-risk settings, such as nursing homes.

One key change: CDC no longer recommends testing to determine whether someone with a known infection is still infectious.

When to test. People with any symptoms suggestive of COVID should be tested, ideally as soon as feasible. But given the ongoing shortages of tests, that may not be possible, particularly for those requiring only symptomatic care. Rather, these patients should be treated as probable cases, with appropriate instructions regarding quarantine. Testing of those with known exposures ideally should be done about 5 days after exposure, which is the mean incubation period; virtually all persons who will develop symptoms will do so within 11.5 days of exposure.

How to test. Only viral nucleic acid or antigen tests should be used to diagnose acute illness. CDC does not currently recommend using serologic assays, now broadly available, for diagnosis of acute infection, though they obviously play an important role in understanding the transmission dynamic of the virus in the general population.

Testing strategies vary from state to state and even within communities in a single state. It is recommended that clinicians check with their own local or state health department for specifics on tests available, indications for testing, and processing details. While often forgotten, it is worth emphasizing that no diagnostic tests have been approved by the US Food and Drug Administration (FDA). Rather, they are available under emergency use authorization (EUA), meaning that they have not been fully vetted by the FDA.

In late July, the FDA expanded authorization for real-time reverse transcription–polymerase chain reaction (rRT-PCR) molecular assays, utilizing nasal or nasopharyngeal swabs, to permit testing of all persons, regardless of exposure history or symptoms. The FDA maintains a list of all approved diagnostic tests and corresponding labs. Current evidence suggests that no one test is better than any other — and most clinicians won't have a choice anyway. Patients will have to get what is available via their health department or insurance plan.

Two point-of-care antigen tests using nasopharyngeal or nasal samples have been issued an EUA. These tests can be used only in settings with a valid CLIA certificate.

Several commercial laboratories have received approval to process diagnostic tests using patients' self-collected saliva rather than swabs. One lab has now received authorization for in-home testing without any input from a clinician. These testing options can be a boon for patients who have symptoms or exposure and for whatever reason are unable to get to a diagnostic site. These samples are collected at home and mailed to a lab. Note that these tests are not yet widely available.

Waiting for results. If waiting for results meant a day or even a couple of days, the answer to this one would be easier. But if the wait extends to 1 and even sometimes 2 weeks, then the test is not able to meaningfully guide clinical decisions. The latest guidance from the CDC is that individuals with symptoms suggestive of COVID who do not require hospitalization should remain at home in self-quarantine for at least 10 days from symptom onset. Asymptomatic individuals with a known exposure to someone else with COVID, or participation in a high-risk event like an indoor gathering involving more than 10 persons, should self-quarantine either until they receive a negative test result or 14 days after the exposure.

Accuracy of results. A positive rRT-PCR antigen test is highly accurate, indicating presence of SARS-CoV-2 RNA. There appears to be no significant cross-reactivity with other respiratory viruses or even other coronaviruses. A small study conducted in Korea suggests that patients with persistent positive tests who are beyond 10 days from the initial positive test and are now symptom free are no longer infectious.

For patients with a high suspicion of COVID-19, a negative test should not rule out the infection. The number of false-negative results is not well known, though the resultant risk is "substantial." A number of factors affect the likelihood of a false-negative test, including when the sample was collected relative to the timing of illness and the type of specimen collected; for example, nasopharyngeal swabs are more likely to be accurate vs nasal or throat specimens. Repeat or serial testing increases the sensitivity but may not always be available. Although rRT-PCR is the current criterion standard, more inclusive consensus-based criteria are likely to emerge because of the concern about these false-negative results.

Laurie Scudder, DNP, NP, is a nurse practitioner and an editorial director at Medscape. Follow her on Twitter

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