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PCR Tests
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

PCR Tests

PCR (polymerase chain reaction) tests are highly accurate diagnostic tools that detect infectious diseases, cancer, and genetic changes by identifying small amounts of genetic material.

What are PCR tests?

PCR (polymerase chain reaction) tests detect the genetic material—DNA or RNA—of pathogens or abnormal cells. Unlike other tests, PCR can identify disease in its earliest stages by using a process called amplification, which creates billions of copies of a specific genetic segment, making it much easier to detect even trace amounts of a virus, bacteria, or cancer cell.

How are they used?

PCR tests are used to:

  • Diagnose infectious diseases.
  • Identify genetic changes that cause disease.
  • Detect small amounts of cancer cells that other tests might miss.

What happens during a PCR test?

Samples are typically collected via blood tests or nasal swabs.

  • Blood test: A professional draws blood from a vein in your arm.
  • Nasal swabs: These include anterior nares (front of the nostrils), nasal mid-turbinate (NMT), or nasopharyngeal (NP) swabs. These involve rotating a swab inside the nostril to collect mucus or cells.

Once collected, the sample is placed in a machine called a thermal cycler, which uses heating and cooling cycles to amplify the genetic material. For RNA-based viruses like COVID-19, the RNA is first converted to DNA through reverse transcription PCR (rtPCR).

Are there any risks?

Blood tests carry minimal risk, such as slight pain or bruising. Nasal swabs may cause temporary tickling, coughing, or gagging, particularly with nasopharyngeal swabs.

Frequently Asked Questions

PCR for influenza has a sensitivity of 95‑99 % and specificity >98 %, detecting even low viral loads. Rapid antigen tests have lower sensitivity (50‑70 %) and may miss early or mild infections, though they provide results in minutes. A negative rapid test in a symptomatic patient is often confirmed with PCR to rule out false‑negatives.
No fasting is required for most DNA‑based PCR assays because the test measures genetic material, not metabolites. However, certain RNA‑based tests (e.g., for viral load) may ask you to avoid recent meals that could affect blood viscosity. Always follow the specific lab instructions, but generally you can take routine medications and eat normally.
KRAS mutation can guide targeted therapy decisions. Your oncologist will review the specific variant, stage of disease, and overall health to determine eligibility for KRAS‑inhibitor drugs or to adjust chemotherapy regimens. The result may also prompt referral to a clinical trial or additional imaging to assess disease burden.
PCR can detect viral RNA as early as 1–2 days after infection, but the viral load may be too low for reliable detection in the first 24 hours. Testing at least 48 hours post‑exposure maximizes sensitivity, and a repeat test 2–3 days later is recommended if the initial result is negative but symptoms develop.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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