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.
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.
