3 minutes read3 minutes readThe field of medical testing can feel complex and difficult to understand. Thousands of tests exist for millions of different conditions.
Even when we limit the scope to STI testing, the picture can still seem a bit complicated.
Testing for STIs tends to use three different types of tests – antibody, antigen, and PCR testing.
This blog will cover antibody testing, what it is, and how it works.
Let’s get started.
What is an STI?
To understand how antibody testing works, we must understand what STIs are.
STIs (sexually transmitted infections) are infections that pass between people during sexual contact. “Sexual contact” can mean a lot of things, but it tends to refer to vaginal, anal, and oral sex, as well as sharing sex toys.
But “sexual contact” can also refer to other intimate acts, like foreplay, digital stimulation, and other activities.
These infections can also be transmitted in other ways. Infections like HIV, Hepatitis B, Hepatitis C, and Syphilis have to be transmitted through blood transfusions, needle stick injuries, sharing needles for drug use, and sharing personal hygiene items like razors.
What is antibody testing?
Now that we know what STIs are, we need to know how to detect them.
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 or oral fluid. 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 antibody, they can also detect the virus itself.
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.
When should I get an antibody test?
The accuracy of an antibody test depends largely on the window period of the infection.
The window period for an infection refers to the time between when a person becomes infected and when that infection can be reliably detected on a test.
Most infections must be in the body for a certain period of time (usually a few days to several weeks) before a test can reliably detect antibodies.
Are antibody test always accurate?
If we wait out the window period before getting tested, some antibody tests are extremely sensitive, like the antibody test of HIV.
Others are less precise, such as Syphilis tests. Some antibody tests (like trying to detect Gonorrhoea or Chlamydia through blood tests) have no value at all. For those other infections, we have PCR tests available. Ask us for advice if you do not know which tests is best for you.
Final thoughts
We have many tools to detect STIs; antibody tests are just one.
If you’re concerned that you may have been exposed to an STI, you should get tested.
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This article has been medically reviewed by Dr. Steve Chapman, 30/10/2024.
