PCR And Serology Based Testing Explained
What is PCR Testing?
Polymerase Chain Reaction Testing (PCR) is laboratory test to diagnose the COVID-19 infection based on viral DNA. The test involves a nasopharyngeal swab that looks like a long Q-tip and draws mucus from the back of the patient’s nasal cavity where it meets the throat. The swab is then inserted into a vial and is shipped to a laboratory where lab techs or machines use reagents to extract the viral RNA. An enzyme, Reverse Transcriptase, is used to convert the RNA into DNA and the DNA is replicated many times to make it detectable.
PCR Testing Problems in The Current Environment
- PCR test results available (rapid) 15 minutes and (non-rapid) 3-5 days.
- There is a shortage of needed chemicals and materials required to run the tests, including swabs and reagents.
At this time, not nearly enough testing is being conducted to gain valuable insight into how many people have and have had the COVID-19 virus.
Why Serology Testing For COVID-19?
Serology testing will allow us to develop a better understanding of how the virus spreads and the true denominator to determine statistics like fatality rates. Serology tests use a patient’s blood, serum, or plasma to identify the presence of IV and IgG antibodies. Serology testing allows the determination of active infection and whether someone has built up an immunity to the virus based on past exposure. Unlike PCR based testing, Serology testing provides rapid results in 10 minutes.
- Typically, IgM peaks at day 5
- IgG peaks at 14-21 days
- IgM appears first and then slowly drops off. As IgM disappears and IgG picks up, this indicates the patient is likely entering recovery and at some point no longer contagious.
- IgM and IgG are helpful bio-markers to make determinations of patient’s immune status, whether or not they are contagious, recovering, etc.
- PCR tests are able to identify the virus before the antibody test. Serology tests are not very effective at identifying infection in the pre-symptomatic or early symptomatic stages. PCR may decline rapidly in 3-6 days.
- PCR begins to lose sensitivity by 3-6 days. However, IgM serology starts becoming very sensitive and showing positives at 5 days. This means, there are situations in which a patient comes in for testing around this time and a PCR based test could be negative, but IgM could be positive. There is a period of time where 101 becomes more sensitive than PCR.
- Rapid COVID tests are a great option to inexpensively and quickly have insight into what is going on. This is a solution that will allow hundreds of thousands of more patients to be tested while allowing for a better understanding of how the virus spreads and fatality ratios.
- Additionally, there is an application for IgG testing. The presence of IgG means that the patient has mounted an immune response following exposure to the virus. These individuals may have been asymptomatic and never tested or may have had mild symptoms that passed. This could be useful in making determinations for staffing surgical floors or for responders on the front-line dealing with the next wave of the virus. This will also allow a broader sample size of the population to be tested, helping to identify the spread of the virus as these individuals would unlikely have been tested using PCR methodology.
The best and most effective way to utilize these tests throughout this pandemic is in tandem. Because PCR and Serology testing have different advantages and limitations, leveraging both methods when it makes sense will likely provide us with the best and most comprehensive results.