What is PCR Testing?

It’s likely only since the start of the COVID-19 pandemic that you have heard of PCR testing. But, unlike many of the newer tests used to detect COVID-19, PCR testing has been relied upon for accurate medical diagnostics for almost 30 years.

Before that point, laboratories had a difficult time detecting a virus or bacteria in a specimen, because there was simply too little of it. Cloning took weeks or months. Scientists needed a way to increase the detectability of small amounts of pathogens in less time.

At about this time, 1983, Kary B. Mullis, who was working as a chemist at Cetus Corporation, a biotechnology firm in California, was actually looking for another way to create new diagnostic tests. As a side effect of one of his experiments, he had unintentionally doubled the DNA of the pathogen.

After a long week at the lab, he was driving to his mountain cabin in Mendocino County when the idea occurred to him. If he could repeat the steps and double the DNA each time, he would raise the detectability of the pathogen.

“There were diagrams of PCR reactions on every surface that would take a pencil or a crayon in my cabin,” Kary remembers in a video from Biosearch Technologies, the company who makes the DNA synthesizer Mullis used to test his hypothesis. “I woke up in a new world.”

He called it Polymerase Chain Reaction or PCR. It relied on enzymes and temperature cycling, increasing and decreasing the temperature over and over, to generate millions of copies of DNA in a comparatively short time.

Both Nature and Science, prestigious scientific journals, rejected his paper on the subject for publication, but by 1987 more scientists were talking about the potential of this new reaction. It underwent increased study and improvement.

By 1988, multiplex PCR allowed the test to detect multiple targets in one reaction. By 1992, scientists were able to measure the amplification of DNA in real time as the reaction progressed. And in 1993, Mullis was awarded the Nobel Prize in Chemistry, the same year that the first PCR diagnostic test was approved by the FDA.

In his acceptance speech, he said, “Innovation is also, on its face, often marked with a bit of insanity. If no one mentions loudly that one thinks you’re out of your mind, then you probably are not being innovative.”

Since then, PCR has undergone many improvements. The discovery of a new enzyme that could withstand high temperatures de-necessitated adding new enzymes with each heat cycle. That innovation and the engineering of new enzymes made the process faster, easier, and more specific. Additional chemical innovations made the reaction even more specific and safer for the scientists who use it.

PCR has revolutionized diagnosis of genetic defects, AIDS, and Hepatitis B and C. It is used to find E.coli, a harmful bacteria, during food safety testing. In 2009, it was used to monitor the H1N1 pandemic, and by 2012, the FDA had approved more than 100 PCR-based molecular diagnostic tests for use in improving cancer care. And the list goes on. PCR has been used to detect salmonella, malaria, and the tick-borne disease babesiosis. It is used to identify genetic and autoimmune diseases.

Other amplification methods emerged for the same purpose, but none are in widespread clinical use today. The same method the scientific leadership originally rejected has now appeared in nearly a half a million peer-reviewed papers, while publications featuring the other methods of amplification number just over 1,000.

Interestingly, in addition to making significant contributions to medicine, PCR allowed huge advances in criminology and archaeology, where amplifying small amounts of DNA helps catch criminals and unlock the mysteries of our ancient past.

Today, PCR is considered one of the most significant scientific advances of the 20th Century. It remains the world’s most clinically validated molecular diagnostics technology. Thanks to Mullis and his distinguished colleagues, we are able to detect COVID-19 quickly and accurately.

Karen Bond directs laboratory services at Southwestern Vermont Medical Center in Bennington, VT.