A polymerase chain reaction (PCR) test detects genetic material from a pathogen or abnormal cell sample.

PCR testing allows researchers to make many copies of a small section of DNA or RNA, in a process that some call “molecular photocopying.”

This technique has many applications. Health experts can use PCR tests as a quick, accurate way to diagnose infectious diseases, spot genetic changes that can cause disease, and identify small amounts of cancer cells.

In this article, we explore what PCR tests are in more detail, including how they work and how doctors interpret the results.

An automated machine performing a PCR test for coronavirus.Share on Pinterest
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American biochemist Dr. Kary Mullis developed the PCR technique in 1983. It is a quick, inexpensive way to copy small segments of genetic material.

Usually, large amounts of DNA are necessary for molecular and genetic testing, but the PCR technique allows scientists to generate millions of copies from a very small amount of DNA.

PCR is a common technique in medical and biological research labs, and there are many applications.

However, a “PCR test” typically refers to a quick, accurate diagnostic test for the early signs of an infectious disease. For example, this is one way of testing for SARS-CoV-2, the virus that causes COVID-19.

Due to the COVID-19 pandemic, many people have become familiar with the term “PCR test.” But the technique has many medical uses beyond testing for COVID-19.

Health experts can also use a PCR test to detect small amounts of cancer cells and genetic changes that can cause disease. PCR tests can also detect other pathogens that can result in diseases such as:

A PCR test amplifies DNA sequences. It involves DNA primers, DNA bases, enzymes, a buffer solution, and thermal cycling to help replicate these sequences.

The first step is to collect a sample from the person undergoing the test. We describe the acceptable types of sample below.

Next, a laboratory researcher uses a specialized machine to heat the sample. This separates the DNA inside into two pieces of single-stranded DNA. The reaction then cools to allow primers to attach to the template DNA sequences. It then heats up again to allow an enzyme known called Taq polymerase to add DNA bases to the templates. This process duplicates the original DNA sample, creating two strands.

The machine can automate this entire process and repeat it as many times as necessary to create many exact copies of the original DNA segment.

In a diagnostic PCR test, the machine can detect the presence of a pathogen after replicating the genetic material. However, certain viruses, including SARS-CoV-2, consist of RNA rather than DNA. For these viruses, the RNA undergoes a process called reverse transcription PCR (rtPCR). This turns the RNA into DNA before copying it.

The time it takes to get results from a PCR test can vary from a few minutes to several days. With an onsite analyzer, the results are rapid. It can take longer for results to come back when doctors send samples to an off-site lab, due to processing delays.

PCR tests seem to reliably detect the presence of SARS-CoV-2. A 2021 systematic review and meta-analysis found that the tests for this virus were accurate in 97.2% of cases.

Depending on the reason for the PCR test, a positive result can indicate the presence of a pathogen, cancer cells, or genetic changes. A negative result suggests that these are not present.

Using the example of COVID-19, a positive PCR result indicates that the test found SARS-CoV-2 in the sample. This suggests that the person may develop COVID-19. Some people have the viral infection without developing symptoms of the disease.

A negative test result indicates that there was no SARS-CoV-2 in the sample. However, a false negative can occur if there was not enough viral material in the sample for the test to detect it. This may occur if a person undergoes the test too soon after exposure to the virus.

The types of PCR test differ based on the sample involved. Common types include:

  • Nasal swab: This involves swabbing to take a sample from the back of the nose and throat.
  • Nasal mid turbinate swab: Often called an “NMT” swab, this involves taking a sample from deep inside the nostril.
  • Saliva: Giving this sample involves spitting saliva into a tube.
  • Blood: Certain types of PCR tests require a blood sample, collected from a vein.

Giving a sample for a PCR test usually only takes a few minutes and requires no preparation. A person may need to fill out a form with, for example, their name and date of birth.

The next steps depend on the kind of sample the test requires. For example, a healthcare professional may need to insert a long swab into a person’s nostril, or the person may be able to do this themselves. The person taking the sample rotates the swab in the nostril for 10–15 seconds before removing and doing the same in the second nostril.

PCR tests typically pose few, if any, risks.

Adverse effects may depend on the type of sample. For example, slight pain or bruising can develop after giving blood, but these tend to resolve quickly.

A swab of the nose, throat, or both may cause some mild coughing, discomfort, and a slight gagging sensation. These should be mild and temporary.

A PCR test can check for the presence of pathogen, such as a virus, cancer cells, or genetic changes.

During the ongoing COVID-19 pandemic, a person may take a PCR test to check for the presence of the underlying virus, SARS-CoV-2.

Current guidelines suggest that a person should test for SARS-CoV-2 if they have:

  • symptoms of COVID-19, even after vaccination
  • had close contact with a person who has tested positive
  • taken part in activities that increase the risk of developing the infection
  • been asked to test by a healthcare professional or health department

Many tests can detect the presence of pathogens, including SARS-CoV-2. Both PCR and antigen tests are molecular tests that can detect a current infection.

An antigen test, also known as a “lateral flow test,” detects the presence of viral proteins, not viral RNA. This test is cheaper and much quicker than a PCR test, returning results in 15–30 minutes. However, antigen tests are generally less sensitive than PCR tests.

As such, it may be advisable to use an antigen test first, then request a PCR test for confirmation if the initial result was positive.

PCR testing is a common research technique. In a health context, it can help detect the presence of genetic changes, cancerous cells, or pathogens, such as SARS-CoV-2.

The test involves taking a sample of fluid from the body, then processing the genetic material in the sample to make many copies.

For COVID-19, a PCR test can take longer to return results than other tests, but the results are often more accurate.