Trichomoniasis, or “trich,” is the most common curable sexually transmitted infection (STI), caused by the parasite Trichomonas vaginalis. Diagnosis can be challenging because many infected individuals experience no symptoms yet can still transmit the parasite. While testing is generally reliable, it is possible to receive a negative result while still being infected, a situation known as a false negative. This outcome usually stems from limitations in the specific diagnostic method used or factors related to the timing of the test.
Understanding Trichomoniasis and Testing Methods
The Trichomonas vaginalis parasite is a microscopic, single-celled organism that primarily infects the lower genital tract. In women, the infection may cause symptoms like a frothy, foul-smelling discharge, genital itching, and discomfort during urination. However, approximately 70% of infected individuals do not exhibit noticeable symptoms, which allows the infection to spread unknowingly. Testing is the only way to confirm or rule out the presence of the parasite, especially because symptoms often mimic other common infections.
There are two main categories of diagnostic tools used by healthcare providers. The older, more traditional method is wet-mount microscopy, where a sample of genital fluid is examined immediately under a microscope to look for the live, moving parasites. While quick and inexpensive, this test is significantly less sensitive than newer options, with accuracy often reported between 50% and 70%.
The current standard for diagnosis is the Nucleic Acid Amplification Test (NAAT), which detects the parasite’s genetic material (DNA or RNA) in a sample. NAATs are highly sensitive, reporting accuracy between 95% and 99%. This makes them superior for detecting low-level or asymptomatic infections, and this difference in test sensitivity is the primary factor in why a false negative result may occur.
Technical Reasons for False Negative Results
A false negative result means the parasite is present in the body, but the test failed to detect it. The most common technical reason this occurs is the use of wet-mount microscopy. Because this method relies on a provider physically seeing a live, motile parasite, the result is highly dependent on the quality of the sample and the speed of analysis. If the sample is not analyzed within 10 to 20 minutes of collection, the organisms may stop moving and die, making them nearly impossible to identify correctly, which results in a false negative.
The inherent low sensitivity of the wet mount means it struggles to detect infections where the parasite load is low. This contrasts sharply with NAATs, which can amplify and detect even trace amounts of genetic material. A low parasite load is a frequent cause of false negatives, even with highly sensitive tests. If the infection is very new or mild, the number of organisms in the collected sample may be too small for the test to register a positive result. Studies suggest that wet mounts may require a threshold of at least 100 copies of T. vaginalis DNA to be detectable.
External factors related to sample collection can also compromise the test’s accuracy. The use of certain products or activities before testing can effectively wash away the organisms from the collection site. Recent douching, for example, can dilute the sample and reduce the number of parasites available for testing. Similarly, urinating just before a sample is collected can flush out the organisms from the urethra, leading to an artificially low count in the specimen. Inadequate swabbing technique, where the provider does not collect enough material, also contributes to an insufficient sample.
When to Seek Retesting and Follow-Up Actions
If you have a negative Trichomoniasis test but your symptoms persist, or if you have a known high-risk exposure, retesting is a necessary next step. Persistent symptoms, such as unusual discharge, irritation, or pain, should always prompt a discussion with a healthcare professional, regardless of a previous negative result. Retesting is the only way to rule out a false negative.
A strong indication for retesting or treatment is a positive diagnosis in a sexual partner. If your partner has tested positive for Trichomoniasis, you should be treated immediately, even if your own initial test was negative. The probability of a false negative in this scenario is high, and treatment is necessary to prevent re-infection of your partner and to clear your own infection.
If your initial test was a wet mount, you should specifically request to be retested using a Nucleic Acid Amplification Test (NAAT). Given the superior sensitivity of NAATs, a second test using this technology significantly reduces the likelihood of a false negative result. The Centers for Disease Control and Prevention (CDC) recommends that if a wet mount is negative but the infection is still suspected, a NAAT should be utilized to confirm the diagnosis.
If you believe you were tested too soon after exposure, wait for the recommended testing window before retesting. While the incubation period can range from 5 to 28 days, a test is generally considered most reliable when performed about one week to one month after a potential exposure. In cases where clinical suspicion remains high and a NAAT result is not immediately available, a healthcare provider may opt for empiric treatment based on symptoms and exposure history.