Quick Answer
It is possible to test positive for chlamydia without actually being infected. False positive test results can occur due to:
- Testing errors
- Cross-reactivity with other bacteria
- Prior treated infections
Confirmation testing is recommended if someone tests positive but has no symptoms or risk factors. Treatment should only be started once a positive test has been confirmed.
What is chlamydia?
Chlamydia is a common sexually transmitted infection (STI) caused by the bacteria Chlamydia trachomatis. It can infect both men and women and is spread through unprotected vaginal, anal, or oral sex with an infected partner.
Signs and symptoms
Many people with chlamydia don’t experience any symptoms, which is why it’s often known as a “silent” infection. When symptoms do occur, they may include:
- Pain or burning during urination
- Abnormal vaginal discharge or bleeding
- Abdominal or pelvic pain
- Pain during sexual intercourse
Symptoms usually appear 1-3 weeks after exposure. Even without symptoms, an untreated chlamydia infection can lead to serious complications like pelvic inflammatory disease in women and epididymitis in men. It can also cause infertility.
This is why screening is important, especially for sexually active women under 25 years old.
How are chlamydia tests performed?
There are two main types of tests that can detect a chlamydia infection:
Nucleic acid amplification tests (NAATs)
NAATs detect the genetic material (DNA) of the bacteria. These tests are very accurate and can use urine samples or swabs from the cervix, urethra, rectum, or eye.
Some common NAATs for chlamydia include:
- Polymerase chain reaction (PCR) tests
- DNA probe tests
- Transcription-mediated amplification (TMA) tests
- Strand displacement amplification (SDA)
Cell culture
This technique tries to grow (culture) Chlamydia trachomatis bacteria from a urine sample, genital swab, or eye sample. Cell culture was once the standard test, but NAATs are now recommended as the primary diagnostic test for chlamydia because of their higher accuracy.
Cell culture may still be done after a positive NAAT to confirm the diagnosis before starting treatment.
Why might I test positive if I don’t have chlamydia?
NAATs and cell culture are highly accurate for chlamydia diagnosis. But no test is 100% perfect. It is possible that these tests can sometimes detect the bacteria when it’s not actually there, leading to a false positive result in someone without a true infection.
Some reasons this may happen include:
Testing errors
With any test, there is always a small chance of a technical or procedural error leading to an incorrect result. This may be more likely if the lab doesn’t follow proper protocols or quality control measures.
Cross-reactivity
NAATs work by detecting specific sequences of Chlamydia DNA. Sometimes similar sequences are present in other bacteria, which could lead to a positive test result even if Chlamydia trachomatis is not present.
For example, some research suggests that M. genitalium—another STI bacteria—can occasionally be misidentified as chlamydia with certain NAATs.
Prior treated infections
After successful treatment with antibiotics, dead chlamydia bacteria may persist in the body for 3-4 weeks. NAATs detect DNA from both live and dead bacteria. So it’s possible to still test positive during this time even after treatment has cured the infection.
Low levels of bacteria
NAATs are very sensitive and may sometimes detect trace amounts of chlamydia bacteria at levels too low to cause infection. The clinical significance of a positive result with a low bacterial load is unclear.
Contamination
In rare cases, a sample may become contaminated at some point during collection, transport, or processing, leading to an erroneously positive result. Proper precautions reduce this risk.
How accurate are the tests?
In general, NAATs for chlamydia are considered highly accurate, with a sensitivity and specificity of over 95% in most studies. This means:
- Sensitivity – When you have chlamydia, the test will be positive over 95% of the time.
- Specificity – When you don’t have chlamydia, the test will be negative over 95% of the time.
So while false positives are possible with NAATs, they are uncommon in the absence of the issues mentioned above. The frequency of false positives varies by test method:
| Test Type | False Positive Rate |
|---|---|
| PCR | 1-2% |
| DNA probe | 2-4% |
| TMA | 3-5% |
| SDA | 3-7% |
Cell culture has a lower false positive rate of only 0.6-0.8%, but is limited by lower sensitivity compared to NAATs.
What should I do if I test positive?
Don’t panic if you get a positive chlamydia test result but have no symptoms and believe you are low risk. Take the following steps:
- Discuss the result with your doctor. Ask if it could be a false positive.
- Undergo confirmation testing, preferably with a different NAAT method.
- If both tests are positive, get treated. Antibiotics like azithromycin or doxycycline cure chlamydia.
- Make sure your partners get tested & treated too.
- Practice safe sex with condoms to avoid reinfection.
- Get retested 3 months after treatment to confirm you are cured.
You should not initiate treatment until confirmatory testing shows the positive result is valid.
Prevention
To avoid chlamydia and other STIs:
- Use condoms correctly every time you have sex.
- Limit your number of sexual partners.
- Ask partners to get tested before sex.
- Get annual STI screening if sexually active.
- Don’t douche, which can increase infection risk.
Early diagnosis through regular screening is important, even when no symptoms are present. If caught early, chlamydia is easily cured with antibiotics before complications develop.
Conclusion
Chlamydia tests like NAATs are very accurate, but false positives are possible in a small percentage of cases. If you have a positive test but a low likelihood of infection based on risk factors, consider confirmation testing. Treatment should only be started once a positive diagnosis has been confirmed. Practicing safer sex lowers your risk of acquiring chlamydia in the first place.