Chlamydia is a common bacterial infection transmitted through sexual contact. While testing is generally accurate, no medical test is completely without error. Understanding factors that influence test results is important, as both negative and positive outcomes can sometimes be misleading.
When a Negative Result Might Be Misleading
A negative result indicates absence of infection, but can be misleading if the infection is present but undetected (a false negative). A common reason is testing too soon after exposure. Chlamydia requires 7 to 21 days to multiply to detectable levels. Testing within 1-2 weeks after exposure may yield a false negative due to low bacterial load.
Sample collection errors also contribute to false negative results. Insufficient sample material, incorrect swabbing, or sampling from an uninfected site can lead to missed bacteria. For instance, if an infection is present in the throat or rectum, but only a urine sample is collected, it might go undetected. Specific collection guidelines, such as providing a “first-catch” urine sample, are important as this initial stream contains the highest concentration of bacteria.
A low bacterial load can prevent detection; if the infection is at a very low level, the test might not detect it even with proper timing and collection. Nucleic Acid Amplification Tests (NAATs) are highly sensitive and considered the gold standard for chlamydia detection. However, older or rapid tests may have lower sensitivity, increasing the chance of a false negative. Improper handling or storage of the sample before laboratory analysis can degrade the sample and lead to inaccurate results.
When a Positive Result Might Be Misleading
A positive chlamydia test result means an active infection is present, but can rarely be a false positive, indicating no infection exists. Laboratory errors can cause false positives, including contamination during processing, mix-ups, or issues with reagents and equipment calibration. Laboratories follow strict protocols to minimize these, but errors are not entirely eliminated.
Cross-reactivity is another reason for false positives, though uncommon with modern NAATs. This occurs when the test mistakenly detects genetic material from other bacteria or organisms structurally similar to Chlamydia trachomatis. For example, bacteria like Mycoplasma or Ureaplasma, or conditions like urinary tract infections (UTIs) or bacterial vaginosis (BV), can cause cross-reactions.
A test might detect genetic material from non-viable chlamydia organisms. This occurs if a person was previously infected and treated, but fragments of bacterial DNA or RNA remain. While no longer infectious, the test can still yield a positive result. Retesting too soon after treatment (typically within three weeks) is not recommended, as it can show residual bacterial DNA. The false positive rate for chlamydia tests is very low (0% to 2%).
Next Steps If You Doubt Your Results
If concerned about chlamydia test accuracy, discuss results with a healthcare provider. They can review symptoms, sexual history, and exposures to determine if further action is warranted. Communicating persistent symptoms or new exposures helps interpret results within your complete clinical picture.
Retesting may be recommended. If the initial test was too early, a healthcare provider might suggest retesting after the appropriate window period. Retesting is also advised if symptoms persist despite a negative result or if there is strong suspicion of infection based on exposure history.
In ambiguous cases (e.g., positive with no symptoms, or negative despite clear symptoms), confirmatory testing might be pursued. This could involve using a different chlamydia test, sending a sample to a different laboratory, or testing an alternative anatomical site. A healthcare provider will consider all available information, including health history and symptoms, rather than relying solely on a single test result, to ensure accurate diagnosis and appropriate treatment.