How does PCR testing work?

To understand how we test for STIs, you need to understand what STIs are in their most basic form.

Sexually transmitted infections (STIs) are infections that pass between people primarily through sexual contact. This can refer to a range of activities, including vaginal, anal, and oral sex, along with sharing sex toys. During these activities, infected bodily fluids come in contact with mucous membranes or blood, thus passing on the infection.

Other acts can also pass on certain infections, including foreplay, genital-to-genital touching, kissing, and more.

Some STIs can be passed on through blood-to-blood contact, like blood transfusions, needle stick injuries, and sharing needles for drug use.

One of the main forms of testing for STIs is PCR testing.

PCR (polymerase chain reaction) testing is a form of NAAT (nucleic acid amplification) testing that uses thermal cycling processes and enzymes to detect pathogens within a provided sample. Its aim is to find DNA and make copies of it.

A sample—usually a blood, urine, or swab sample—is taken from the patient. A thermal cycler heats and cools the sample several times while primers and polymerase enzymes are added at different stages.

This creates conditions for the DNA of the pathogen in question to be copied millions (or sometimes billions) of times to reach detectable levels. The DNA is then detected using fluorescent tubes or dyes.

In this way, rather than checking for the markers of an infection, like with an antibody test, a PCR test can detect the actual presence of the virus, bacteria, or parasite within the sample.

PCR tests precisely detect even small amounts of genetic material within a given sample. That is why we use them to test for many infections caused by bacteria, viruses, and parasites.

For example, if we try to culture the Gonorrhoea bacteria, only about 70% detection is reached (when there are very few bacteria in the sample, when some bacteria never made it to the laboratory dying in transit etc). But just with a simple urine test, the PCR for Gonorrhoea will probably detect more than 99% of infections.

In the case of HIV, the classic test was waiting for antibodies to be produced by the immune system which the test would detect. This could takes weeks or months before being able to say that the person is definitively infected or not. But PCR for HIV will detect just as well but only 10 days after the sexual contact.

If you think you may have been exposed to an STI, you may want to consider getting a PCR test. However, it’s also really important to know when to get tested.

There is a period of time between when you are infected and when an infection can be reliably detected on a test. This is known as the window period.

The length of a window period depends on the infection, but also depends on the test method used.

Since PCR tests look for the actual genetic material of the infecting organism, this method usually enjoys a shorter window period than other alternative tests.

Finding out as soon as possible if you are infected is always better, allowing you to start treatment and halt the damage the infection will do. Some infections can be detected after a few days, while others may require several weeks.

Here are some STIs that can be detected with a PCR test:

• HIV
• Syphilis
• Chlamydia
• Gonorrhoea
• Herpes I & II
• Hepatitis A
• Hepatitis B
• Hepatitis C
• HPV
• Trichomonas
• Mycoplasma
• Ureaplasma
• Gardnerella

NOTE: While many infections can be detected through a PCR test, some infections can be more quickly detected with other tests, like antibody or antigen tests. PCR tests can only detect current infections, so they can’t tell you if you’ve had an infection in the past.

If you’re concerned about STIs, we want to ensure you get the right test.

Click the button below to begin an online booking for an STI test at a sexual health clinic near you.

You can also call the number at the top of this page to speak to one of our dedicated Sexual Health Advisors. They can talk you through your situation, book you an appointment, and help you choose a test that will meet your needs.

This article has been medically reviewed by Dr. Steve Chapman, 16/07/2025.