Chlamydia is a common bacterial sexually transmitted infection (STI), often called a “silent infection” because most people who have it experience no symptoms. Testing is the only way to confirm the infection, making diagnostic accuracy a primary concern. While current testing methods are highly reliable, no medical test is perfectly accurate 100% of the time. Chlamydia tests can occasionally yield an incorrect result, either suggesting an infection is present when it is not (false positive) or vice versa (false negative). Understanding the technology and potential for error is important for interpreting results.
Understanding the Types of Chlamydia Tests
The standard for chlamydia detection today is the Nucleic Acid Amplification Test (NAAT). These tests work by seeking and multiplying the specific genetic material (DNA or RNA) of the Chlamydia trachomatis bacterium. This process allows for the detection of very small amounts of the organism, making NAATs highly sensitive. The high sensitivity of NAATs makes them the preferred screening method, often exceeding 95% accuracy for both sensitivity and specificity.
NAATs have largely replaced older methods like cell culture, which was time-consuming and less sensitive. Unlike culture, NAATs can use non-invasive samples, such as a simple urine sample from men or a self-collected vaginal swab from women. The ability to use these easier-to-collect samples has made widespread screening more practical and acceptable for patients. While a healthcare provider may still collect a urethral or cervical swab, the high performance of NAATs ensures that testing is generally reliable, though the small possibility of error remains.
Reasons for False Negative Results
A false negative result occurs when a person has an active chlamydia infection, but the test incorrectly reports that the infection is absent. This error is often tied to the timing of the test relative to exposure, known as the “window period.” Chlamydia bacteria typically require seven to 21 days to replicate enough to reach a detectable concentration. Testing too early, before the bacterial load is sufficient, is the most common cause of a false negative result, as the test cannot find enough genetic material to register positive.
The quality of the sample collection is another significant factor, particularly if the specimen is self-collected. If a urine sample lacks the “first catch” or a swab does not collect sufficient cellular material, the bacterial DNA may be too diluted or sparse for the test to amplify. False negatives can also occur if the infection is present at an untested site, such as the throat or rectum, when only a genital sample was taken. Additionally, taking antibiotics for an unrelated condition can temporarily suppress the bacterial load below detectable levels.
Factors Leading to False Positive Results
A false positive result, while less common than a false negative with modern NAATs, indicates a chlamydia infection is present when the person is not actually infected. This can occur because NAATs are designed to be extremely sensitive in finding target DNA, but this sensitivity can sometimes be a drawback.
The primary cause of this error is laboratory contamination, where genetic material from a positive sample accidentally contaminates a negative one during processing. This cross-contamination, though rare due to strict protocols, can cause the NAAT to amplify the stray DNA and register a positive result. Environmental contamination in a clinic setting has also been shown to cause false positives, as the bacteria’s RNA can persist on surfaces and contaminate the sample.
Another factor is testing too soon after successful treatment. The NAAT detects residual, non-viable (dead) bacterial DNA that remains in the system following treatment. Since the test detects genetic material, not live bacteria, a test performed less than three weeks after treatment can yield a false positive. Non-specific reactions with other bacteria, such as those causing a urinary tract infection or bacterial vaginosis, are minimized by the high specificity of current NAAT technology.
Next Steps After Questionable Results
When a test result is unexpected, such as a negative result despite clear symptoms or a positive result in a low-risk patient, confirmation testing is often pursued. If the initial result contradicts the patient’s history, a healthcare provider may order a different type of NAAT or re-run the original sample. This practice ensures that treatment decisions are based on the most accurate information available.
For patients who tested negative but were recently exposed, retesting after the window period is essential. Experts recommend waiting one to three weeks after potential exposure to test for chlamydia. A “test of cure” is typically not recommended after treatment unless the patient is pregnant or symptoms persist.
If a test of cure is necessary, it should be performed no sooner than three to four weeks after treatment to avoid a false positive from residual bacterial DNA. Healthcare providers often rely on clinical judgment, choosing to treat a patient based on symptoms and exposure history even if an initial test result is ambiguous.