PCR (polymerase chain reaction) is a method that repeatedly copies a target segment of DNA (or RNA) until there is enough of it to detect. In essence it is a “magnifying glass” for genetic material.
Thanks to this, PCR finds even single molecules of a pathogen's DNA or mutations — which is why it has become the gold standard for diagnosing many infections and for genetic tests.
What PCR detects
PCR is used to: detect an infectious agent by its DNA/RNA (viruses, including COVID-19, hepatitis, HIV, STIs, tuberculosis), assess viral load, identify genetic mutations and hereditary diseases, and also in oncology and forensics.
Its main advantages are high sensitivity (it finds very small amounts) and specificity (it accurately determines whose genetic material it is).
How it works and what is important to understand
In a test tube the sample is repeatedly heated and cooled: the DNA strands separate, an enzyme builds up copies, and with each cycle their number doubles. Over several dozen cycles a single molecule becomes millions.
Important: a positive PCR indicates the presence of genetic material, but not always a live active pathogen (DNA can remain after recovery). So the result is interpreted taking into account the clinical picture and other tests.
Frequently asked questions
What does PCR show?
The presence of a specific segment of DNA or RNA in a sample — for example, the genetic material of a virus or bacterium, as well as mutations. Thanks to repeated copying, the method picks up even single molecules, which is why it is very sensitive and accurate.
Why can PCR be positive after recovery?
PCR detects the pathogen's genetic material, which can remain in the body for some time after the pathogen itself is already inactive. So a positive result does not always mean infectiousness — it is interpreted together with symptoms and other tests.
How does PCR differ from an antibody test?
PCR looks for the genetic material of the pathogen itself (whether there is an infection right now), while an antibody test looks for the immune system's response (whether there has been contact with the infection or a vaccination). These are tests with different meanings.
Related terms
Sources
- Polymerase Chain Reaction (PCR) — StatPearls (NCBI)
- MedlinePlus (NIH) — What is a PCR test?
- National Human Genome Research Institute — PCR