Trichomoniasis, often called “trich,” is the most common curable sexually transmitted infection (STI) globally, caused by a single-celled parasite named Trichomonas vaginalis. This infection can affect both men and women, primarily residing in the lower genital tract. It is a frequent situation when one partner tests positive for this STI while the other’s test returns negative. This discrepancy is not unusual and can be explained by a combination of biological factors and technical limitations in diagnostic testing.
Asymptomatic Infection and Parasite Load
The primary biological reason for a partner testing negative is the high rate of asymptomatic infection, especially in cisgender males. Approximately 70% to 85% of people with trichomoniasis experience minimal or no symptoms, which means they can carry and transmit the infection without ever knowing it. While the infection typically resides in the urethra in men, it often does not cause the noticeable discharge or irritation seen in many symptomatic women.
In an asymptomatic carrier, the number of parasites, or the parasite load, is often lower than in a person experiencing symptoms. This lower concentration of T. vaginalis organisms in the sample makes them significantly harder to detect, even if the infection is present. The parasite load can fluctuate, meaning that a sample collected at one time might contain too few organisms for a test to register a positive result.
Limitations of Diagnostic Testing
The sensitivity of the test used is a major factor contributing to a false negative result in a partner. The traditional diagnostic method for trichomoniasis is wet mount microscopy, where a sample is examined under a microscope for the characteristic motile parasites. While quick and inexpensive, the wet mount has a low sensitivity, often detecting only 44% to 68% of infections compared to more advanced molecular tests.
This low detection rate is due to the need for the organisms to be both present in high enough numbers and actively moving. Delays in transport or analysis can cause the parasites to lose their tell-tale motility, leading to a negative result even from an infected person.
In contrast, Nucleic Acid Amplification Tests (NAATs) are the most sensitive diagnostic tool available, with detection rates exceeding 95%. NAATs work by detecting the parasite’s genetic material, or DNA, which means they do not rely on the organism being alive or motile. If a partner was tested with a less-sensitive method like a wet mount, or if the sample collection was suboptimal, a false negative is a very real possibility.
Understanding Transmission and Coordinated Treatment
Trichomoniasis is highly transmissible, and the presence of a positive result in one partner strongly suggests the other partner is also infected, regardless of their negative test. If only the positive individual receives treatment, the parasite can be passed back to them during future sexual activity with the untreated partner. This cycle of re-infection, often called a “ping-pong” effect, prevents a cure and can lead to persistent or recurrent infection.
This dynamic makes simultaneous, coordinated treatment medically necessary to break the chain of transmission. The goal is to eliminate the parasite from both individuals at the same time. High rates of infection among sexual partners of infected individuals support the practice of presumptive treatment for the partner. Even a person who tests negative should often receive treatment if their partner tests positive, to ensure the infection is fully eradicated.
Next Steps Following a Positive Result
Following a positive trichomoniasis test, a healthcare provider will typically prescribe a nitroimidazole antibiotic, such as metronidazole or tinidazole. For women, the recommended regimen is often metronidazole 500 mg taken twice a day for seven days, while men are commonly treated with a single 2-gram dose of metronidazole or tinidazole. It is imperative to complete the full course of medication, even if symptoms resolve quickly, to ensure the parasite is completely cleared.
Communication with the partner is a necessary next step so they can consult a provider about presumptive treatment. Both partners must abstain from sexual activity until they have both completed their full course of medication and any symptoms have entirely disappeared, which is generally for about seven days after the last dose. Because re-infection is common, retesting, known as a Test of Cure (TOC), is recommended for all women three months after their initial treatment.