Understanding PCR in COVID-19 with Kevin McKernan

One thing you can be certain of with COVID-19 is that there is no shortage of controversy. Whether it is lockdowns, treatment, vaccines, or testing there have been volumes written about what should or should not happen. So today, we're going to learn about the most common test in the middle of the controversy - the PCR.

What is PCR?

"PCR (polymerase chain reaction) is a very sensitive test that can detect down to one molecule of DNA or RNA," said founder and CSO of Medicinal Genomics, Kevin McKernan. I contacted Kevin to help us get to the bottom of what PCR is, its limitations, how to interpret results, and what is the best path forward to use the test in the midst of a pandemic. PCR is an inexpensive, quick, and very sensitive test that amplifies the target proteins in the sample. By running multiple cycles, the target is doubled each time until it is detectable. Once a sample turns "positive," the test runs a few more cycles to verify the positive result. Usually, the PCR for SARS-CoV-2 (the virus responsible for COVID-19) returns to patients as either positive or negative.

Why Does Cycle Time Matter?

The number of times a sample has to be cycled through the doubling process is a way of determining not only the presence of a particular protein, but also its prevalence. Low cycle times suggest that there is a lot of the target while high cycle times suggest that there is not nearly as much. Since we are doing doubling you can imagine that for every 3.16 cycle times you have 10 times as much target substance. For today's SARS-CoV-2 PCR tests patients and clinicians are only given a qualitative answer of positive or negative.

But, it is almost certainly true that the cycle time is important for understanding how infectious an individual is. The human body will shed inactive virus particles for sometimes months after an infection which can be detected by the PCR. This means that one could have a positive result even though it has been months since you could transmit the virus to others. As you can imagine, cycle times offer a window into how likely you are infectious (have enough targets suggesting an active infection) and perhaps we should insist the lab provide a qualitative and quantitative result for us.

What are Other Concerns  About the PCR test?

One of Kevin's concerns is that we are centralizing the testing of PCR by only having it occur in a few locations. This groups both symptomatic and asymptomatic people together causing a potential hot spot for transmitting the virus. Additionally, by centralizing and not allowing home testing, the costs for the PCR test are driven up. Finally, part of the quality control for PCR tests is to determine if the host DNA is acquired in the sample (this shows that the test was done properly) and there is not usually any way of knowing whether the DNA is being stored (something I talked to Twila Brase about before with regards to HIPAA).