T. vaginalis is a pathogenic protozoan with an estimated worldwide incidence of 170 million cases per year.¹ As a sexually transmitted disease T. vaginalis infection causes vaginitis in women and nongonococcal urethritis and prostatitis in men and has been linked to preterm bioti, infertility and acquisition of HIV.^2,3 Neonatal trichomoniasis is uncommon, but can occur in the form of vaginitis⁴ or lower respiratory tract infection.^5,6 Women infected during pregnancy are predisposed to premature rupture of the placental membranes, premature labor, and low birth weight infants.¹ Trichomonas infection might also be linked to cervical cancer, pelvic inflammatory disease and infertility.¹ This infection might also enhance venereal acquisition of HIV in women.¹ Diagnosis based on clinical findings alone, e.g., vaginal discharge, is unreliable.⁷

Traditional laboratory diagnosis of trichomoniasis is by direct microscopic examination. The saline wet mount provides excellent specificity of 100%, but poor sensitivity of 32 to 82%.^1,8 Contamination of saline with flagellate protozoa can cause false positives. Due to loss of mobility of the trichomonads, wet mounts should be examined immediately.⁹ The sensitivity of the saline wet mount is improved by phase contrast microscopy.⁸ Direct acridine-orange fluorescence staining has a sensitivity of 33 to 71%.^1,8 The direct fluorescence antibody assay utilizes monoclonal antibodies F3 and H10 conjugated with fluorescein. This test has a sensitivity of 81 to 86% with a specificity of 98 to 99%.¹⁰ Papanicolaou-stained smears have a sensitivity of 56 to 78%.¹¹ Besides its poor sensitivity, the Pap smear has a serious problem with false positives for T. vaginalis.¹² Culture methods for detection of T. vaginalis have a sensitivity of 30 to 80% with a specificity of 99%.¹³ Modified Diamond’s media or Feinberg and Whittington media permit the best recovery of T. vaginalis, but have the disadvantages of not being readily available and of a long turnaround time of up to 7 days.¹³ Modified Columbia agar is sensitive, reliable and easy to handle.¹⁴ A commercial system, InPouch TV^TM culture, compares favorably with other culture methods.^15-17

At least eight distinct serotypes of T. vaginalis are identified, with as many as 88 strains of this organism in North America alone.¹ This heterogeneity makes serological diagnosis of this disease impractical. Tests for antigen detection typically require a "cocktail" of monoclonal antibodies to various cellular components in order to provide adequate sensitivity.¹ Antigen detection using a latex agglutination test has a sensitivity of 95%, but there are many equivocal results due to difficult-to-define endpoint.¹² Antigen detection using ELISA technology has a sensitivity between 89 to 93% with a specificity of 97 to 98% and should be replaced by PCR, if cost can be controlled.¹⁸

PCR and vaginal swabs^19,20 (including single intravaginal swabs for multiple STIs)²⁰ or PCR on first fraction urine^21-23, perform very well for men and women and are preferred. PCR results show that TV is significantly undertreated by standard algorithms for metronidazole therapy.²⁴

Treatment failures with metronidazole can be due to a number of causes with noncompliance or re-infection by a sexual partner being the most likely problem.¹ Other possibilities include inactivation of the drug by vaginal bacteria, poor intestinal absorption and zinc deficiency.¹ Approximately 5% of T. vaginalis in the United States has some degree of drug resistance; this equates to 30,000 cases per year.²⁵ In vitro minimum lethal concentrations can be measured both aerobically and anaerobically; better correlation to clinical treatment failure occurs with aerobic minimum lethal concentration (MLC).^1,25 Methods for determining the MLC include conventional tube dilutions, a disk broth method, a microtiter plate method and a method using the InPouch TV^™ culture system.²⁶

Using the microtiter plate methods, aerobic MLC values of 25µg/ml or anaerobic MLC values of 1.6µg/ml represent the cutoff values between sensitivity and resistance.²⁷ In highly resistant cases, the aerobic MLC value is typically 100µg/ml or greater and the anaerobic MLC value is at least 3.1µg/ml.²⁷ The MLC cutoff value between sensitive and resistant using the InPouch TV^TM system is 3.1µg/ml.²⁶ Measurement of the MLC does not correlate with blood drug levels needed to eliminate an infection, but the MLC does provide documentation of resistance and assistance in estimating the dosage of metronidazole necessary for re-treatment.¹