Polymerase chain reaction (abbreviated PCR) is a laboratory technique for rapidly producing (amplifying) millions to billions of copies of a specific segment of DNA, which can then be studied in greater detail. PCR test involves using short synthetic DNA fragments called primers to select a segment of the genome to be amplified, and then multiple rounds of DNA synthesis to amplify that segment.
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.
Uses
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-19Trusted Source.
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:
- influenzaTrusted Source
- tuberculosisTrusted Source
- HIV
- EbolaTrusted Source
- hepatitis C
How does the test work?
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)Trusted Source. 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%Trusted Source of cases.
Interpreting the results
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 indicatesTrusted Source 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 negativeTrusted Source 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.
Types
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.
What happens during a test?
Giving a sample for a PCR test usually only takes a few minutesTrusted Source 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 secondsTrusted Source before removing and doing the same in the second nostril.
Risks
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.
Who may require a PCR test?
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.
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
