The U.S. Centers for Disease Control has stated that testing for the presence of the coronavirus is essential to containing the spread of the COVID-19. Recent FDA guidance states that test manufacturers should pass along certain information to persons being tested, so that that benefits and risks associated with tests are understood. While the FDA guidance only applies to the manufacturers of tests and their distributors, the FDA guidance, and the concerns that it addresses, should be taken into account by anyone who performs or arranges for COVID-19 testing.
Two kinds of tests are currently available. One is a test for the presence of the SARS-CoV-2 virus itself in the nose, throat or bodily secretions (sometimes referred to as a “PCR test” or a “molecular test”). The other is a blood test that detects antibodies that are indicative of current or prior infection by the virus (sometimes called a “serum test”). The CDC initially limited PCR testing to persons who had symptoms of COVID-19 and who had been in contact with other infected persons or traveled from other countries. Now both PCR testing and antibody testing are becoming more widely available for symptomatic persons, healthcare workers, and at-risk populations, such as nursing home residents and, under some health department standing orders, PCR testing is available for asymptomatic members of the general population in certain circumstances. Some communities have arranged for testing on demand, or even community-wide universal testing at no cost to residents. Similarly, some employers are considering arranging for testing members of their workforce as part of their re-opening plans.
This transition from testing persons already displaying symptoms to widespread testing and population screening puts a premium on the accuracy of the tests currently in use. It also puts a premium on the understanding of the test results, and the risks of misinterpretation of the results.
There are two measures of the accuracy of the clinical test: sensitivity and specificity. Sensitivity is the likelihood that the test will reliably identify a person who actually is infected or has been infected. Another way to think of this is that a test with high sensitivity will rarely produce a false negative result. Specificity, on the other hand, is the likelihood that the test will not identify a person who has not been infected as being infected. A test with high specificity will rarely produce a false positive result.
False negative and false positive results each present their own risks. A false negative test might delay administration of appropriate treatment or give an individual a false assurance that precautions to prevent the spread of the illness are not necessary. A false positive could lead to unneeded self-quarantine, or lead to the provision of unnecessary diagnostic or public health precautions.
Significantly, a test that functions well in normal clinical practice to confirm a diagnosis for a person showing symptoms will not necessarily be useful for screening purposes (that is, for testing large numbers of persons who show no symptoms). If a disease is rare in the population being studied (this is called “low prevalence”) a test that is insufficiently specific may identify more false positives than it correctly identifies true positive results. This is not necessarily because the test is fundamentally flawed, but because this incidence of disease within the test population is less than the margin of error of test. Screening large numbers of people under such circumstances could be highly misleading, causing the public to incur unnecessary costs or causing individuals to submit to overly restrictive protective measures. The FDA has stated:
We do not currently know the prevalence of SARS-CoV-2 antibody positive individuals in the U.S. population, and prevalence may change based on the duration the virus is in the country and the effectiveness of mitigations. Moreover, prevalence may vary widely between locations and between different groups of people, such as health care workers, due to different rates of infection. In low prevalence populations, which will be much of the asymptomatic general population, the result of a single antibody test is not likely to be sufficiently accurate to make an informed decision regarding whether or not an individual has had a prior infection or truly has antibodies to the virus.
As of this date, no test for SARS-CoV-2 has been approved by the FDA. However, a number of PCR tests and antibody tests have been made available through Emergency Use Authorizations (EUAs) issued by the FDA. The FDA also has released a “Fact Sheet for Recipients” to accompany each EUA. The FDA requires that the Fact Sheet be made available on the manufacturer’s web site, and that it be made available to patients. However, the FDA does not regulate health care providers that administer these tests. The Fact Sheets are typically a page and a half long, and describe the purpose of the test, the known and potential benefits and risks of the test, and the significance of the test’s results, including the possibility of false positive results and false negative results.
Understanding that there are limits to the accuracy of all tests, especially when used for screening purposes, several safeguards should be observed.
First, anyone administering a test that is available pursuant to an EUA should provide the Fact Sheet for Patients to the Individual being tested, along with a consent form that alerts the individuals to the meaning of these tests and their limitations.
Second, to the greatest extent practicable, the delivery of test results should be accompanied by the opportunity to obtain follow-up counseling and interpretation of the results or guidance on how the subject of the test can obtain such counseling.
Third, if these tests are being used to make decisions in the workplace, such as fitness to return to work, the limitations of the tests noted by the FDA should be taken into account. PCR tests can only detect the presence of the virus at a particular moment in time, and cannot replace other safeguards for the protection of others. Antibody tests can suggest the presence of the virus in the past, but a negative result cannot be used to rule out the possibility of early infection. All consumers of this information should be aware of the limitations of both kinds of tests, and act accordingly.
