PCR testing vs antibody testing vs antigen testing – what’s the difference?

To most people, a test is a test. They take one to get answers.

But not all tests are the same, and the answers they give are designed for certain people in certain situations.

If you’re worried about STIs, you need to know about the tests you’re taking. STI testing can mostly be broken down into three types: PCR, antibody, and antigen testing. But what’s the difference?

In this blog, we’ll explain what each test is, how it works, their benefits and drawbacks, and when you might need them.

Knowing the difference between tests will protect your sexual health.

What is PCR testing?

Polymerase chain reaction (PCR) testing remains the most widely used type of test for most STIs.

PCR testing is a highly accurate way to find infections. It looks for the genetic material of a virus, bacteria, or parasite in your sample. If it’s there, the test makes millions of copies of the pathogen’s DNA so it can be detected.

• A sample is taken—usually a swab, blood, or urine.
• In the lab, the sample is heated and cooled several times while special enzymes are added.
• This process copies the DNA of the infection many times until it reaches a level that can be measured.

Unlike antibody tests, which look for signs your body has fought an infection, PCR tests check for the actual presence of the infection itself.

Why do we use PCR testing for STIs?

PCR tests are very accurate and can find even tiny amounts of infection in your sample. That’s why they are used to test for many infections caused by bacteria, viruses, and parasites. With a simple urine sample, a PCR test for an infection like Gonorrhoea will detect over 99% of infections.

For HIV, older tests waited for your body to make antibodies, which could take weeks or months. PCR testing can detect HIV much sooner – around 10 days after sexual contact.

When should I get a PCR test?

If you think you may have been exposed to an STI, a PCR test could be the right choice. But timing matters.

There is a short period after infection when a test might not pick it up. This is called the window period. How long this lasts depends on the infection and the type of test.

PCR tests usually have a shorter window period because they look for the actual genetic material of the infection. This means they can often detect an infection sooner than other tests.

Getting tested allows you to start treatment quickly and prevent further harm. Some infections can be found after just a few days, while others may take a few weeks.

What STIs can be detected with a PCR test?

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

What is antibody testing?

Antibody testing is one method of detecting an STI in the body. But what are antibodies?

What are antibodies?

Antibodies are proteins the immune system produces when it detects antigens (protein structures on the outside of organisms foreign to your body, like bacteria or viruses). Antibodies are produced by white blood cells called B cells.

When an antigen comes into contact with a B cell, the cell divides and clones. The cloned B cells then release antibodies tailored to lock on to that specific antigen, tagging it for attack by other cells. Antibodies can also block antigens from affecting healthy cells.

There are five different types of antibodies:

• IgG: The most common antibody your immune system produces to protect you from viral and bacterial infections.
• IgM: IgM antibodies act as the first line of defence in your immune system and play a role in immune regulation.
• IgA: IgA is found in saliva, tears, mucous, breast milk, and intestinal fluid.
• IgE: Mainly found in your skin, lungs, and mucous membranes, these antibodies cause your mast cells to release histamines and other chemicals.
• IgD: The antibody found on the surface of B cells that supports B cell maturation and activation.

How do you test for antibodies?

The immune system creates specific antibodies for specific infections. STI tests look for these antibodies in your blood.

A negative antibody test result means there were insufficient antibodies in your blood to signal the infection. A positive result would mean certain antibodies are currently in your system, indicating a current or previous infection.

Most antibody STI tests detect IgM and IgG antibodies, which are activated in response to viral, bacterial, and parasitic infections.

What STIs can be detected with an antibody test?

There are different kinds of antibody tests. The kind you get will depend on the STI you’re being tested for.

Antibody tests are often used to detect HIV. They can detect the presence of antibodies in the blood. Most rapid tests for HIV are antibody tests.

However, laboratory tests for HIV often use a combined antigen/antibody test that looks for both antibodies and antigens. This means they will be positive if there are antibodies, but just in case there are low levels of antibodies, they can also detect antigens.

On the other hand, when dealing with Hepatitis B, antibody tests tend to be used to determine immunity to the virus, whereas screening for the presence of Hepatitis B usually uses an antigen test or a PCR test.

What is an antigen test?

Antigens are markers that help your immune system tell if something belongs in your body or not. They are found on the surface of things like viruses, bacteria, pollen, and other allergens.

When something foreign enters your body, your immune system spots its antigens and starts fighting back. It does this by making antibodies, which help your body recognise and attack the infection.

Antigens can also appear inside the body, such as in cancer cells or abnormal cell growth.

An antigen test—usually a blood test—looks for these markers in your sample. If antigens are found, it means the infection is present.

Why test for antigens?

Most STIs can be detected with antibody or PCR tests, but antigen tests can sometimes show an infection earlier than antibody tests.

For example, Better2Know uses a 4th-generation HIV test that checks for both antibodies and p24 antigens. This means we can detect HIV sooner, which is important for starting treatment quickly.

When should I get an antigen test?

Antigen tests are mainly used to detect STIs like HIV, Hepatitis B, and Hepatitis C in the early stages or to confirm ongoing infections.

Your doctor or clinician will recommend the right test for you. In some cases, an antigen test may be the best way to find an infection early.

PCR vs antibody vs antigen: key differences

Feature	PCR testing	Antibody testing	Antigen testing
Purpose	Detects DNA of pathogens, like bacteria and parasites	Detects antibodies created by the body to fight infections	Detects antigens present on pathogens
Sample type	Blood, urine, or swab	Blood	Blood
Timing	Best after pathogen has reached critical mass in the body, usually a few days to several weeks after infection	Best after body has established an immune response to the infection, or to determine immunity	Best for early detection, especially for HIV or Hepatitis B or C
Accuracy	Very high	Very high	High

Final thoughts

Getting tested can be a stressful moment, but with the right care and treatment, you don’t have to worry.

If you’d like to learn more about our different testing options, call the number at the top of this page. Or if you’d like to book a test at a nearby STI clinic, click the button below.  

Don’t leave your sexual health to chance.