The Answer Key to COVID Testing
By Katie E. Golden, MD
May 14, 2020
The success of reopening our state relies partly on increased testing, helping us to understand who has the infection, who is at risk, and how we can keep everyone safe. There has been a rapid evolution of testing availability and modalities over the past month, however, and it can be confusing to understand the details of these emerging changes. So we want to take the time to explain the tests that are currently available, and how patients can get tested.
There are two categories of tests for COVID: the traditional PCR test, as well as the newer antibody test.
The ‘PCR’ test was the first, and remains the most common, way we test for the virus. It is performed with a swab from the nose, throat, or a sputum sample, and the fluid is subsequently screened for genetic material that is unique to the pandemic strain (its viral RNA, to be more specific). This genetic material can typically be detected starting on Day 1 of symptoms, and is most likely to be present in the first week of symptoms. After that, there is increased risk of what we call a ‘false negative’, when the test results are negative despite true infection. False negatives are possible and common at any point in the illness, especially with suboptimal timing or inadequate sample collection.
So exactly how accurate is the PCR test? This partly depends on how the sample is collected. The best way to obtain a good sample is through what is called ‘bronchoalveolar lavage (BAL)’, which is only performed in intubated patients where it is easy to obtain a fluid sample from inside the lungs. The sensitivity of the test decreases with more traditional sputum, nose, or throat swabs. In a study of 205 patients with confirmed COVID-19 infection, PCR positivity was highest in the bronchoalveolar lavage specimens (93%), followed by sputum (72%), nasal swab (63%), and pharyngeal swab (32%)1.
The ‘antibody’ test has been a more recent development, is less widely available, and takes more time for results to return. This is performed with a sample of the patient’s blood, and screens for the presence of the host’s immune response to the virus (more specifically, two types of antibodies, referred to as IgG and IgM). These antibodies are undetectable in the early stages of infection, but remain present in the blood well beyond active infection (and the highest sensitivity is typically in the second and third week of symptoms). It follows that this is helpful in patients who present late in their illness course, or after resolution of their symptoms. This has become particularly attractive to people who have been exposed to the virus and need to know if they have been infected, and is also likely to provide utility in population studies to understand spread and extent of virus. We are still studying how accurate or useful this test will be for diagnosis, but initial studies show that in combination with the traditional PCR test, the sensitivity is over 98%2.
The following figure, obtained from a JAMA article3, may provide a helpful visual to understand how the sensitivity of these tests change over the course of an illness depending on the testing mode. If this just looks like a bunch of colorful lines in the shapes of ski slopes, breeze right on past it to the testing information below.
How do patients get tested?
The PCR test is widely available at local hospitals and outpatient testing centers, and Governor Cooper is even working to set up testing centers at retail pharmacies. Testing is free, and the easiest place to start your screening is through your regular outpatient doctor. If you don’t have a primary care physician, do not worry. Both Novant and Atrium systems have plenty of resources in place to help. An easy place to start is by visiting this online resource from Novant, or call 877-9NOVANT.
The antibody test is starting to become available through some outpatient clinics, though may require an out-of-pocket payment depending on your insurance. If you are interested in obtaining this test, please call your doctor’s office.
Please note that in the emergency department, we are still unable to test asymptomatic patients, and are providing these patients to the outpatient resources listed above.
Stay safe, Charlotte. We are here for you.
- Wang W , Xu Y , Gao R , et al. Detection of SARS-CoV-2 in different types of clinical specimens. JAMA. 2020. Published online March 11, 2020. doi:10.1001/jama.2020.3786
- Guo L, Ren L, Yang S, et al. Profiling early humoral response to diagnose novel coronavirus disease (COVID-19). Clin Infect Dis. 2020;ciaa310. Published online March 21, 2020. doi:10.1093/cid/ciaa310
- Sethuraman N, Jeremiah SS, Ryo A. Interpreting Diagnostic Tests for SARS-CoV-2. JAMA. Published online May 06, 2020. doi:10.1001/jama.2020.8259