Ten things to know about viral testing for SARS-CoV-2
Written by Leann Poston M.D.
Diagnostic testing for COVID is one of the most powerful tools we have to control the spread of the disease, but what if the tests are not as accurate as you might hope?
According to a recent article by Steven Woloshin, M.D., Neeraj Patel, B.A., and Aaron S. Kesselheim, M.D., J.D., M.P.H. in the New England Journal of Medicine. These are the important points you need to know about the accuracy of your COVID test:
What are false positives and false negatives?
A false positive identifies a person as having COVID when they do not. A false positive can result in quarantines and contact tracing when it is not needed. A false negative is when a person has COVID and is identified as negative. A false negative can result in further spread of the disease.
What do specificity and sensitivity mean?
The terms sensitivity and specificity are used when evaluating the accuracy of medical testing. Sensitivity is the ability to identify someone as having a disease who truly has the disease. In other words, you have COVID and the test comes back positive.
Whereas, specificity is the ability to identify those who do not have the disease when they truly do not have the disease. In other words, you do not have COVID and the test says you do not have COVID.
What is the Emergency Use Authorization?
Under section 564 of the Federal Food, Drug, and Cosmetic Act, The F.D.A. commissioner may allow unapproved medical products to be used in an emergency to diagnose, treat, or prevent serious or life-threatening conditions.
Are the current COVID tests evaluated for sensitivity and specificity?
According to the authors, this critical question is unclear. They state that it is unknown whether the test results are calibrated against a known positive such as a person who is defined as ill with COVID by an independent panel. The Emergency Use Authorization allows the test to be tested against a reference standard that has been authorized, such as a reference reverse-transcriptase-polymerase-chain (RT-PCR) reaction test on a known positive.
Is the RT-PCR a reliable test for a reference standard?
If the test comes back positive, it is reliable because the actual gene sequence of the virus was detected. However, there are two studies from China that raise concerns that the RT-PCR test may have a significant number of false negatives.
How likely is it that the COVID test is generating false negatives?
To evaluate this, you could use Bayes theorem. Bayes theorem considers the probability that the person is infected and the test sensitivity. The probability that you are infected depends on the prevalence of the disease in your area, your exposure history, and your symptoms.
To determine sensitivity and specificity, the test should be run under a multitude of conditions such as changing the location of the test sample population, testing areas of high and low infection rates, testing people who have had the illness for different lengths of time and who have different levels of illness severity.
The testing sensitivity depends on how good the sample is, how high the viral load is, how long the person has been ill, the skill of the sample collector, and how the sample is processed.
How can you use this information to determine whether you are safe to visit with people?
Take the probability that you are infected based on whether you live in a hotspot for the virus or not and then multiple it by the sensitivity of the test. Ideally, you would want to be less than a 5% chance of being positive.
If you live in a hotspot, your probability of being infected is approximately 50%, and the sensitivity of current tests is estimated to be at around 70%, which means that you have a 35% chance of being infected.
What factors determine the accuracy of the test?
The accuracy of the tests depends on many personal factors such as the infection rate in your area, how the test is acquired and processed, how far you are into the disease, and your viral load.
What can cause a false negative?
The SAR-COV-2 tests are taken from samples in the nasopharynx. This space is difficult to reach. As this study explains, taking samples from the blood or the mouth can result in false negatives because the viral load may be lower. The FDA has authorized samples to be taken from the nose or mouth.
Where can you find out more about your test?
Under the Emergency Use Authorization Act, many commercial and healthcare systems have notified the F.D.A. that they have validated their own COVID -19 tests and have started patient testing. On the F.D.A. InVitro Diagnostic E.U.A.s page, you can review the laboratories which have applied for and received authorization to test.
Tests are only useful to help make decisions about what we can safely do if the tests are accurate. The F.D.A. needs to enforce accurate sensitivity and specificity testing on all lab tests. These tests must be done in a variety of circumstances, including testing asymptomatic people in low-risk areas to evaluate how high the false negative testing rate really is.
The FDA has authorized at-home testing. Look into how your test should be done and what testing has been done to verify the accuracy of the test before you pay for it. Tests with high false negative rates may give symptomatic people a false sense of security that results in further spread of disease.
While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice.