706 to 0.18% [14]. In our study, the prevalence in asymptomatic people was 0.4%, and the result was easily obtained by PCR test with a urine or endocervical swab sample. Mycoplasma genitalium was first isolated in 1981 from two men with nongonococcal urethritis (NGU) [15]. Nowadays, Mycoplasma genitalium is known as a causa- tive pathogen of NGU. Results from a meta-analysis of 19 studies on patients with NGU showed that Mycoplasma genitalium was found in 21% compared with 7% of patients without NGU [16]. Ross et al reported a detection rate of 0.6% for Mycoplasma genitalium in asymptomatic British persons [17]. In Japan, the detection rate of this organism was 1% [11]. Our study showed a prevalence of this myco- plasma of 0.3% in asymptomatic people. Mycoplasma geni- talium is difficult to identify by routine culture. Currently, nucleic acid amplification tests (NAATs) are usually rec- ommended to detect this pathogen [1]. Wikström and Jensen reported that Mycoplasma genitalium is a common cause of persistent or recurrent urethritis among men treated with doxycycline, and erythromycin appears to be less efficient than azithromycin in eradicating the in- fection [18]. Therefore, when physicians encounter pa- tients who complain of continuous symptoms of urethritis, they should consider drug-refractory urethritis such as that caused by Mycoplasma genitalium. Trichomonas vaginalis is an anaerobic flagellated protozoan. It is another pathogen of STI of the urogenital tract, and men with this infection rarely exhibit symptoms [19]. In women, greenish-yellow frothy vaginal discharge and itching can develop. Trichomoniasis is treated and cured with metronidazole or tinidazole, which should be prescribed to any sexual partners as well because they may be asymptomatic carriers. Sutton et al reported that the prevalence of Trichomonas vaginalis in reproductive-aged women was 3.1% [20]. In our study, the total prevalence rate was 1.1% in asymptomatic people, and prevalence rates in asymptomatic men and women were 0.2% and 2.5%, respectively. Screening for these pathogens is important not only to identify infected symptomatic individuals for the diag- nosis and management of their infections but also to identi- fy asymptomatic individuals who serve as reservoirs for in- fection [1-4]. In our study, there were 50 (7.1%) infected peo- ple among 709 asymptomatic people; thus, this proportion should not be neglected. Among the 50 infected patients, 37 with Chlamydia trachomatis, 1 with Neisseria gonor- rhoeae, 1 with Mycoplasma genitalium, 8 with Trichomo- nas vaginalis, 2 with Chlamydia trachomatis with Neisseria gonorrhoeae, and 1 with Chlamydia trachomatis with Mycoplasma genitalium were confirmed. Thus, re- gardless of Chlamydia trachomatis, 13 patients should be considered as candidates for antimicrobial treatment. Moreover, there were 3 double-infections among the 709 asymptomatic people. These facts suggest the possibility of prescribing the wrong medicines if practitioners give symptomatic patients some regimens empirically. There- fore, the need to detect multiple species at once should be Korean J Urol 2011;52:703-708 Kim et al raised. In this sense, use of the multiplex PCR assay to screen asymptomatic people may be important because the treatment regimen could be optimized for people with pos- itive results, especially for those with multiple organisms. This concept can be also applied to symptomatic patients. Ureaplasma urealyticum has been recognized as a patho- gen for NGU since the 1950s [21]. However, Ureaplasma urealyticum is frequently isolated from the urethra of healthy men, and some studies have reported that there is no significant difference in its prevalence between men with NGU and men without NGU [22]. Nevertheless, there are some reports that Ureaplasma urealyticum serves as a cause of persistent NGU [23,24]. For that reason, Ureaplasma urealyticum is sometimes recognized as a pathogen and sometimes as a commensal organism. Therefore, Ureaplasma infection must be considered in pa- tients with treatment failure for NGU or patients with mul- tiple sexual partners. If no species except Ureaplasma are detected, symptoms and leukocyte numbers in first voided urine samples are helpful for clarifying the diagnosis of Ureaplasma infection [25]. Mycoplasma hominis is frequently identified from the genitourinary tract. In general, it is known as a commensal species but it can work as a pathogen in special conditions such as in an immunocompromised state [6]. Generally, the rate of colonization of Mycoplasma hominis in the urogen- ital tract was reported to be between 4% and 13% in men and between 21% an 54% in women [26]. Our study showed that the incidence of this species was 9.1% in men and 15.4% in women. In our study, the incidences of Ureaplasma urealyticum and Mycoplasma hominis were 22.1% and 11.6%. Never- theless, those incidences were increased up to 35.0% and 22.5% when the samples had Chlamydia trachomatis. The reason for this increase with chlamydial infection is not clear, and a limitation of our study is that we did not include people who had symptoms. Therefore, comparison of the prevalence of these species in symptomatic urethritis pa- tients is necessary. As described previously, most pathogens causing STI as well as commensal microorganisms are difficult to culti- vate by routine microbiological diagnosis. However, NAATs, such as PCR, are useful for the identification of mi- croorganisms that are difficult to cultivate and for those that grow slowly [25]. In another recent study in Korea, the multiplex PCR kit (Seegene Inc., Seoul, Korea) was used to detect causative microorganisms of STI in patients with chronic prostatitis and vaginitis [27]. In multiplex PCR, more than one target sequence can be amplified by includ- ing more than one pair of primers in the reaction. Multiplex PCR has the potential to produce considerable savings of time and effort within the laboratory without compromis- ing test utility. Furthermore, when the clinical sample amount is limited, multiplexing allows more targets to be analyzed by using a single aliquot of sample material [28]. Although there are some worries that multiplex PCR in the clinical setting may have difficulties such as poor sensi-