Discordant test results for trichomoniasis, where one partner tests positive and the other tests negative, are common and stressful. This discrepancy is fully explainable through the parasite’s biology and the limitations of diagnostic testing. The difference in results does not imply a testing error or infidelity; instead, it highlights the complex nature of this highly prevalent sexually transmitted infection. Medical protocols specifically account for this frequent situation in their treatment guidelines.
The Biology of Transmission and Asymptomatic Carriage
The infection is caused by Trichomonas vaginalis, a single-celled protozoan parasite transmitted through sexual contact. It primarily colonizes the lower genital tract, residing in the vagina and urethra in women, and the urethra and prostate gland in men. Due to anatomical differences, the parasite often finds it more difficult to establish a high parasite load in men.
Because of this lower parasite burden, men are far less likely to experience symptoms compared to women. Up to 80% of infected men may be completely asymptomatic, carrying the infection without discharge, irritation, or discomfort. Even without symptoms, an infected man can still transmit the parasite to a sexual partner. Studies show that when women are diagnosed, over 70% of their male sexual partners are also infected, and most of those partners are asymptomatic.
The parasite can survive in the male urogenital tract for months or even years, creating a “silent reservoir” that maintains the infection’s spread. Therefore, a negative test result is not definitive proof that your partner is uninfected. They may be carrying the parasite without any noticeable signs. The lack of symptoms does not equate to a lack of transmission capability.
Factors Influencing Diagnostic Discrepancies
The most direct explanation for a negative result relates to the sensitivity of the testing method used. Older, less sensitive methods frequently miss the parasite, especially when the parasite load is low, which is common in men. For example, traditional “wet mount” microscopy has low sensitivity, detecting only about 60% of infections even in women, and is less reliable for male samples.
The gold standard for diagnosis is the Nucleic Acid Amplification Test (NAAT), which detects the parasite’s genetic material. These molecular tests are significantly more sensitive than older methods and can detect the infection even with a very low parasite count. If a partner was tested using a less sensitive method, such as a wet mount or culture, the test could easily produce a false negative result.
Sample collection issues also contribute to diagnostic discrepancies between sexes. In women, the parasite is typically found in the vagina and is easily sampled with a swab. In men, the parasite resides in the urethra and sometimes the prostate, requiring a first-void urine sample or a urethral swab. If only a single specimen, such as urine, is tested, the infection may be missed because the parasite can be more concentrated in other areas like semen. A negative result from a suboptimal sample collection in a man does not rule out the presence of the infection.
Treatment Protocols and Follow-Up Testing
Due to the high rate of asymptomatic carriage and potential for false-negative results, medical guidelines strongly recommend simultaneous treatment for all sexual partners. This approach, often called partner treatment, is necessary even if the partner tests negative. Treating the partner prevents a cycle of reinfection, known as the “ping-pong” effect, and eliminates the source of potential reinfection for the individual who initially tested positive.
Standard treatment involves a course of antibiotics, typically metronidazole or tinidazole. For men, the recommended regimen is a single 2-gram dose of metronidazole. Women are often prescribed metronidazole 500 mg twice daily for seven days, as this multi-dose regimen shows higher cure rates. Abstinence from sexual activity is advised until both partners have completed the full course of medication and any symptoms have fully resolved.
Follow-up testing, known as a Test of Cure (TOC), is recommended for the individual who initially tested positive. Given the high rates of reinfection, the Centers for Disease Control and Prevention (CDC) advises rescreening women approximately three months after completing treatment. This delayed retesting ensures the infection has been eradicated and that no reinfection has occurred. If the TOC is performed too soon (within two weeks of treatment), residual non-viable genetic material from the dead parasite can still be detected by the highly sensitive NAAT, leading to a misleading positive result.