Explanation

Chlamydia trachomatis IgG: what is measured?

Chlamydia is an STI caused by the bacterium Chlamydia trachomatis. After an infection, the immune system makes IgG antibodies against the bacterium, which can remain present for years, even when the infection has long gone. This test measures those antibodies in the blood. So it shows whether you have ever had a chlamydia infection, not whether the bacterium is present now; for that, a PCR on urine or a swab is needed. According to the Dutch guideline on initial fertility testing, a positive result in women who are not getting pregnant fits an increased chance of damaged fallopian tubes.

What can it detect?

A past chlamydia infection, even if it never caused symptoms
In women who are not getting pregnant: an increased chance of damaged or blocked fallopian tubes due to a past infection

Result

What does the result mean?

Positive result

A positive result means that you have probably had a chlamydia infection at some point. The result says nothing about an infection at this moment: the antibodies remain for years, even after treatment or after the infection has cleared up on its own. According to the Dutch guideline on initial fertility testing, a positive result in women who are not getting pregnant goes with an increased chance of damage to the fallopian tubes; a positive result does not mean that the fallopian tubes are actually damaged. Depending on the test used, antibodies against a related bacterium that causes respiratory infections (Chlamydia pneumoniae) can make the result falsely positive.

Negative result

A negative result means that no antibodies against chlamydia were found. This makes a past infection less likely, but does not rule it out: not everyone makes detectable antibodies, especially not after a short or superficial infection. A negative result also does not rule out an infection at this moment, because antibodies only develop weeks after the infection was passed on. With symptoms or after a risky sexual contact, a PCR on urine or a swab shows the bacterium itself.

After an abnormal result

With a positive result and a wish to have children, the GP or gynecologist assesses, together with the medical history, whether testing of the fallopian tubes is needed, for example with an X-ray using contrast fluid (HSG). A positive antibody test is no reason for antibiotics; with symptoms or a risky sexual contact, a PCR comes first. Individual cases of chlamydia are not notifiable.

Reliability

How reliable is the test?

Antibodies against chlamydia show a reaction of the immune system to a past infection, not the bacterium itself; according to the LCI guideline (RIVM), the test for a current infection is a PCR on urine or a swab. According to the Dutch guideline on initial fertility testing, a positive result, or a past pelvic inflammation, goes with an increased chance of damage to the fallopian tubes, and further testing of the fallopian tubes is then the next step; with a negative result and no such history, that chance is low. The result is a weighing of the chance, not a diagnosis.

What can affect the result?

The antibodies remain present for years after an infection and say nothing about an infection at this moment.
Shortly after an infection is passed on, antibodies cannot be detected yet, and not everyone makes them.
Depending on the test, a cross-reaction with Chlamydia pneumoniae, a cause of respiratory infections, can give a falsely positive result.
The result does not say where the infection was, nor whether the fallopian tubes are actually damaged.

Who it is for

Who is the test useful for?

Useful for

The result says the most in women who have not become pregnant for more than a year, as part of the search for the cause; it then helps to estimate whether testing of the fallopian tubes is useful. With symptoms that fit chlamydia or after a risky sexual contact, this test says nothing about an infection at this moment; a PCR on urine or a swab then says more. In people without symptoms and without a wish to have children, the result changes little.

How common is it?

Chlamydia is the most common bacterial STI in the Netherlands. In 2024, the sexual health centers of the GGD (municipal public health service) found chlamydia in 20,174 people, two thirds of whom had no symptoms; GPs made the diagnosis about 43,000 times in 2023. Chlamydia is most common in young people under 25, and many infections are never noticed.

RIVM (GGD sexual health centers 2024, GPs 2023)

Drawbacks and risks

A positive result can cause worry about fertility, while most people with antibodies do not have damaged fallopian tubes.
The result can wrongly be read as a sign of an infection at this moment, or, on the contrary, wrongly reassure you after a recent risky sexual contact.

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