PCR v. The Rest (of the Tests): a Comparison.
TL; DR:
The Abbott ID NOW(TM) rapid, point of care test is a reliable test that gives a same-day result in a person who has a high likelihood of active SARS-CoV-2 infection within 7 days of onset of symptoms which are concerning for COVID-19 disease. PCR is still the most accurate test to diagnose SARS CoV-2 infection, whether or not a person has symptoms, but it is significantly more inconvenient, and takes a lot longer to receive a test result. Either way, you need to have a conversation with your doctor.
The Questions:
What are the differences between the various types of COVID-19 testing available? Is the Polymerase Chain Reaction (PCR) test better than the rapid tests available?
You have probably heard about COVID-19 antibody tests, PCR tests, maybe saliva tests and antigen tests. First of all, none of the tests are 100% accurate. Secondly, how reliable the results are depends in part on what your risk is of having COVID-19. A PCR test can be done by a commercial lab or a hospital. PCR testing results usually take at least 48 hours to come back. In some cases, we have seen results take up to a week. “Rapid” test results are available the same day and so they are actionable more quickly.
The Take Home:
The rapid test, run on an Abbott ID NOW(TM) machine, performs favorably early in the disease process. If your likelihood of COVID-19 is high based on potential exposures (not carefully masking when in public, spending 15 or more minutes less than 6 feet apart from someone who is subsequently diagnosed as SARS-CoV-2 positive, etc.) and you have active symptoms, this test is a good choice to get results quickly, but we would also suggest testing other respiratory infections. If the rapid test results are negative and there is no good alternative diagnosis to explain active symptoms, we would recommend proceeding with PCR testing to add more confidence to a negative COVID-19 result. A positive Abbott ID NOW(TM) test indicates the presence of SARS-CoV-2 virus and must be correlated with clinical assessment to determine disease status. It does not rule out other viruses or bacterial causes of infection. In all of these situations we are available to provide guidance regarding testing, especially if you are having symptoms.
The Background:
Nucleic acid testing detects genetic material from the SARS-CoV-2 virus in a sample of fluid taken from the nose or the throat. PCR testing for active COVID-19 is the most reliable testing available but it must be performed by a hospital laboratory or public testing center and requires several days to obtain a result. The Abbott ID NOW(TM) test is a point of care, rapid nucleic acid test ideally suited to a primary care practice like ours.
Antibody tests are helpful to determine if you have recently been exposed to COVID-19 and have mounted an antibody response. We are hopeful that anyone with an antibody response has developed immunity to COVID-19 but the infection is too new to know that for sure and there are sporadic reports of patients recovering from COVID-19 and then later being re-infected.
Antigen testing has recently been approved and is reliable in the first 7 days of infection but is not commercially available. Saliva tests are currently available through some commercial labs. It is unclear how accurate the saliva tests are based on the limited data available.
How reliable are the PCR tests and the rapid tests? Both tests are very specific, which means a positive result almost always means you have COVID-19. The sensitivity however depends on pretest probability and the characteristics of the given test. Pretest probability means “how likely are you to have COVID-19?” This question depends on the rate of COVID-19 in the general public at the time you are tested, whether you have had close contact with a person who has COVID-19, and if you have symptoms consistent with COVID-19, especially in the absence of an alternative diagnosis. Sensitivity refers to the true positive rate. Studies so far show that PCR based COVID-19 nasopharyngeal swabs are 71-98% sensitive. In turn, the Abbott ID NOW(TM) test is almost as accurate. Accuracy of the test results also depend on the stage of disease, the degree of viral multiplication or clearance. If the disease prevalence is low then the sensitivity will be lower.
The Caveat:
If your Abbott ID NOW(TM) rapid results are not consistent with the expected results for any reason you should follow it with a PCR test.
The Evidence:
“ID NOW(TM) COVID-19 assay performed on the ID NOW(TM) Instrument is a rapid molecular in vitro diagnostic test utilizing an isothermal nucleic acid amplification technology intended for the qualitative detection of nucleic acid from the SARS-CoV-2 viral RNA in direct nasal, nasopharyngeal or throat swabs…” (reference FDA EUA document)
In a study of samples collected in viral transport media comparing ID NOW (TM) platform to two independent laboratories the sensitivity was 71.7% and specificity was 100%. The results were in agreement 78.8% of the time, meaning when the lab’s PCR results were negative the ID NOW(TM) result was negative 78.8% of the time. All of the false negative results occurred with weakly positive samples. These results in sensitivity are worse than in Abbott literature and in other clinic based studies but in this study the samples were not collected in the manner now recommended by Abbott and that may have had an adverse effect on the sensitivity. (1)
A comparison between results from the ID NOW(TM) and Abbott’s PCR test were similar. (2)
Analysis of 1939 paired pharyngeal and saliva samples from patients who were asymptomatic but at high risk, or who were mildly symptomatic, found that nasopharyngeal and oropharyngeal swabs detected more COVID-19 cases than saliva testing, but saliva testing was positive in some cases that were swab negative. Of the 70 patients who tested positive, 80% were detected positive by swabs and 68.6% by saliva, with only 34 (48.6%) testing positive on both their swab and saliva samples. (3)
The References:
1. Mitchell SL, George KS. Evaluation of the COVID19 ID NOW(TM) EUA assay. J Clin Virol. 2020;128:104429. doi:10.1016/j.jcv.2020.104429
2. Amanda Harrington, Brian Cox, Jennifer Snowdon, Jonathan Bakst, Erin Ley, Patricia Grajales, Jack Maggiore, Stephen Kahn. Comparison of Abbott ID NOW(TM) and Abbott m2000 Methods for the Detection of SARS-CoV-2 from Nasopharyngeal and Nasal Swabs from Symptomatic Patients. Journal of Clinical Microbiology Jul 2020, 58 (8) e00798-20; doi: 10.1128/JCM.00798-20
3. Lisa Caulley et al. Salivary Detection of Covid-19. Annals of Internal Medicine Aug 2020. doi.org/10.7326/M20=4738
Author: Alice L. Fuisz, M.D., M.A.C.P.
