PCR Test

What is a PCR test?

A PCR (polymerase chain reaction) test is a way to look for genetic material (DNA or RNA). After taking a body fluid sample (like blood) or a swab from a body site (like deep in your nose), scientists use PCR to make many copies of DNA (amplification) that was in the original sample. By amplifying with PCR, scientists can better analyze the DNA or RNA that was in the original sample.

There are many PCR tests. Each is different and is designed for a certain purpose. For example, some PCR tests can determine if a specific virus is present in the sample, which might be making you sick. Other PCR tests can determine if certain genes in your body might have important changes.

Most of us know the term “PCR test” because of testing for COVID-19. But it’s been used in medicine for a long time and has many other uses. Any test that uses a polymerase chain reaction technique is a PCR test. PCR is a type of nucleic acid amplification test (NAAT).

What is a COVID-19 PCR test?

Healthcare providers can use a PCR test to determine whether you have a COVID infection. They use a swab to get a sample of mucus or secretions from deep in your nose. PCR is then used to determine if genetic material (RNA) of SARS-CoV-2 (the virus that causes COVID) in your sample.

What can PCR detect?

Healthcare professionals perform PCR tests for many reasons. PCR can diagnose or detect:

• Infectious diseases (viruses, bacteria, fungi and parasites)
• Genetic diseases, like cystic fibrosis, Huntington’s disease, fragile X syndrome and others
• Types of cancer, like lymphoma
• Gene changes (variations) in cancer that can be targeted with specific medications.
• Gene changes that can put you at higher risk for certain diseases (like genetic testing for cancer risk)
• Gene changes in prenatal testing

Forensic labs can also use PCR to create detectable amounts of DNA found at crime scenes.

What kind of tests or procedures use PCR?

Many types of tests that use body fluids to look for genetic changes or infectious diseases use PCR. The type of test or procedure you need in order to get a sample depends on what you’re being tested for. Tests and procedures that use PCR include:

• Blood tests. Providers use a sample of blood to look for gene changes and some infectious diseases.
• Nasopharyngeal swab. A lab can use a sample of mucus from your nose to look for common infections, like influenza (the flu), COVID or respiratory syncytial virus (RSV).
• Preimplantation genetic diagnosis/PGD or preimplantation genetic screening/PGS. For preimplantation testing during in vitro fertilization (IVF), a lab will test a few cells from embryos before implantation. This usually happens a few days after egg retrieval.
• Amniocentesis. Providers use a sample of amniotic fluid for prenatal testing.
• Biopsy. To diagnose genetic mutations in tumor cells (molecular testing), you’ll need a tumor or bone marrow biopsy.

How to prepare for this test

Most of the time, you don’t need to prepare for a PCR test. But it depends on what you’re being tested for and the procedure you’re having. Ask your healthcare provider if there’s anything you need to do before the test.

How does it work?

PCR identifies a very specific sequence of molecules that’s unique to the DNA it’s looking for (target DNA). For example, this sequence could be a specific gene change in a tumor or the genetic code of a virus or bacteria.

The name “polymerase chain reaction” refers to how the lab amplifies (makes more copies of) the target DNA. It uses an enzyme called DNA polymerase to build strings of DNA that copy the target DNA, if it’s present. The chain reaction happens when many cycles of heating and cooling a solution make more and more copies, potentially doubling the amount of target DNA each time. This means that PCR works with samples of DNA (or RNA) that are so small that they might be missed in other tests.

Often, after PCR is complete, scientists might have to run other tests to analyze the sample — now with potentially millions of copies of the target DNA.

Other times, the solution has labels that allow a healthcare professional to see whether the sample is positive or negative for the target DNA during the PCR amplification. In testing for infectious diseases, like viruses, this allows for quicker results, since you don’t need any additional testing.

Types of PCR

Some types of PCR require additional steps to amplify or analyze the target DNA:

• Reverse-transcriptase PCR (RT-PCR). Scientists use RT-PCR when the genetic information they want to copy is in the form of RNA rather than DNA. There are extra steps that make the RNA into complementary DNA (cDNA), so that a PCR can be performed. (Without this step, PCR can only be performed on DNA.)
• Real-time PCR/quantitative PCR (Q-PCR). In real-time PCR, scientists add fluorescent labels to the DNA that mark the target DNA. This allows them to see whether the target DNA is in the sample without an additional test. Confusingly, some people also refer to this as “RT-PCR,” and it can also be combined with reverse-transcriptase PCR (real-time RT-PCR).

Healthcare professionals use real-time RT-PCR for nasal swab tests for COVID-19 and the flu. SARS-CoV-2 (COVID-19) and influenza viruses are RNA viruses.

What do the results of a PCR test mean?

PCR tests might use different language in your results, depending on what you’re being tested for. For example, if your results say “detected” or “present,” it generally means you have the gene change or infection that the test was looking for.

If the results say “not detected,” it means the test didn’t find the target DNA or RNA. In the case of an infection, this can sometimes mean that you were tested too early in an infection. You could still have an infection, but there weren’t enough viruses, bacteria or other diseases to be detectable, even with PCR.

What do COVID PCR test results mean?

A positive COVID PCR test result — if you see the word “detected” on your result — means that it’s likely that you have COVID or that you’ve recently recovered from it.

A negative (“not detected”) COVID PCR test result means you probably didn’t have COVID at the time of your nasal swab. But sometimes, PCR tests can be negative very early on in an infection, especially if you don’t have symptoms yet.

When should I know the results of a PCR test?

The results of nasal swab tests for common viruses like influenza and COVID usually take about a day to come back. The results of genetic testing through blood tests, biopsies and other procedures can take at least a week or longer.

When should I call my doctor?

Contact your healthcare provider if you have any questions about a PCR test or its results.