What is PCR and why is it the “gold standard” in molecular diagnostics?

May 1, 2020

What Is PCR Testing?

Recognized as one of the most important scientific advances of the 20th century,¹ polymerase chain reaction (PCR) is a quick, easy way to create unlimited copies of DNA from just one original strand. These millions of copies of a section of DNA are made in just a few hours with the goal to recreate enough DNA for multiple testing uses, such as sequencing or infection identification. It is also used for food safety, forensics, epidemiology and many other disciplines beyond diagnostics testing.

There are two important components in PCR testing – the Primers and the Probes.

Primers are the short sequences of nucleotides that our DNA is made of that binds together in a unique way. The “Primer” is the starting point, or region of DNA, that is identified for copy. Two primers are used and are bound to each end of our template DNA. The area in between these primer regions is then replicated by the polymerase. Like a copy machine, it keeps printing the sequence over and over, creating a significant amount of genetic material.
Probes bind very specifically to the amplified material after replication and give off a light signal that can be detected. The more the sequence is present (from replication) the more light it will give off providing a clearer picture for a more accurate test. The very specific nature of the primers and probes binding produce that light, and the amount generated is what gives us confidence in identifying positive or negative results.

PCR — The Gold Standard in Diagnostic Testing

The “gold standard” refers to the highest quality, or benchmark, of a specific practice, product or technology. Understanding this definition, PCR technology is considered that benchmark in many aspects of diagnostic testing, since it can theoretically identify and detect a target with a single copy present in the sample. The ideal diagnostic is both specific and sensitive, meaning that targets, even at an extremely low concentration, test positive and false negatives do not slip through the cracks. For example, one gene that detects chlamydia, may not be as good as two genes specific for chlamydia.

Multiple targets in the test design, when positive, provide more confidence that there is a correct diagnosis. PCR technology is also rapid, and can be performed in hours to minutes compared to traditional methods such as culture, which is labor intensive and can take days to produce a result

Other diagnostic testing methods, like culture or serology, may not provide the same level of sensitivity as PCR. Therefore, the risk of false negatives increases in critical testing scenarios where organisms or viruses may be difficult to grow, or detect an immune response to. This is why PCR is considered the gold standard by many across the diagnostic community. Roche Diagnostics has a lengthy history of creating high quality PCR-based molecular tests across a wide spectrum of diseases and utilized this technology to rapidly develop the SARS-CoV-2 test in response to the evolving pandemic.

Reference:

Mark R. Hughes, Deputy Director of the National Center for Human Genome Research at the National Institutes of Health (Human Genome Project).