What is polymerase chain reaction (PCR)?


Polymerase chain reaction (commonly called PCR) is a laboratory technique used to produce millions of copies of a specific DNA segment. This process allows doctors to detect even very small amounts of genetic material in a sample. PCR is widely used in medicine, including in pathology, to diagnose infections, identify cancer-related mutations, and help guide treatment decisions.

Why is PCR important in pathology?

Pathologists often use PCR when they need to identify or examine specific DNA changes in a tissue, blood, or other body fluid sample. Because PCR can detect very small amounts of DNA, it is especially useful in:

  • Detecting viruses or bacteria that might be causing an infection.
  • Detecting genetic mutations associated with specific cancers.
  • Monitoring residual disease after treatment.
  • Confirming the diagnosis of inherited genetic conditions.

PCR is quick, sensitive, and very specific, making it a valuable tool in modern diagnostic medicine.

How does PCR work?

PCR works by mimicking the natural process your body uses to copy DNA, but in a controlled lab setting. It begins with a very small amount of DNA and involves repeated heating and cooling cycles to produce millions of exact copies of a specific DNA region. Each cycle doubles the amount of DNA, so after many cycles, there is enough material to analyze in detail or detect through special testing methods.

The process requires:

  • A small DNA sample (from tissue, blood, or other fluid).
  • Short pieces of DNA called primers that target the region of interest.
  • An enzyme called DNA polymerase that copies the DNA.
  • Nucleotides (the building blocks of DNA).
  • A machine called a thermal cycler that controls the temperature changes.

What types of diseases are diagnosed using PCR?

PCR is utilized for various medical conditions. Some examples include:

  • Infections: Detecting viruses such as human papillomavirus (HPV), Epstein-Barr virus (EBV), cytomegalovirus (CMV), and bacteria like tuberculosis.
  • Cancer: Detecting mutations in cancers such as leukemia, lymphoma, lung cancer, and colon cancer.
  • Inherited disorders: Verifying the existence of inherited genetic mutations.
  • COVID-19 testing: PCR became widely recognized during the COVID-19 pandemic for its role in detecting the virus that causes the disease..

What does it mean if PCR results are mentioned in a pathology report?

If your pathology report mentions PCR, it means this method was used to test your tissue or fluid sample. The report will often indicate whether a specific gene, mutation, or infection was detected. These results can help confirm a diagnosis or determine if certain treatments, such as targeted therapies, might be effective.

For example, in cancer, PCR might detect mutations in genes like EGFR or KRAS, which could assist your doctor in deciding if a targeted drug is suitable for you. In infectious diseases, PCR can quickly confirm the presence of a virus or bacteria in the tissue.

Questions to ask your doctor

  • Was PCR used as part of my diagnosis?
  • What did the PCR test detect?
  • Does this result affect my treatment options?
  • Are any follow-up tests needed?
  • Should my family members also be tested?
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