Sexually transmitted diseases (STDs) constitute the most common infectious diseases around the world and bear significant consequences for both the individual and public health of the community. More than 20 STDs have now been identified, and they affect more than 13 million men and women in the United States each year (CDC, 2002). Data from the Centers for Disease Control and Prevention (CDC) show that more than 7 million cases of Chlamydia trachomatis infection and more than 350,000 cases of Neisseria gonorrhoeae infection were reported in 2000 (CDC, 2001). In the past decade, the rapid development of molecular techniques has gradually shifted the paradigm of laboratory diagnosis from traditional biological to molecular amplification and detection of major causative agents of sexually transmitted infections.

A milestone in biotechnology that heralded the beginning of molecular diagnostics was the development of the polymerase chain reaction (PCR) by Mullis and colleagues (Saiki et al., 1988). Since then, numerous molecular detection techniques have been designed to detect specific nucleic acids without relying on the ability to culture or directly observe intact organisms. As a result, stringency in transport of clinical samples, in terms of preserving organism viability, has become less strict. With automation, a faster turn-around time of molecular tests became an advantage that significantly enhances this paradigm shift. Silent pathogens such as human papillomavirus (HPV) that cannot be cultivated in vitro can now be detected and typed by using molecular detection techniques that can also determine oncogenic potential and prognostic outcome of different infections. These powerful molecular techniques have a significant impact on strategies and public health programs designed for the control and prevention of STDs worldwide.

An estimated 50% of STDs occur asymptomatically, and this forms a major reservoir of infectious source that persists in the community. More sensitive detection techniques are often required for detecting asymptomatic individuals with low microbial load (Yoshida et al., 2002). Currently available molecular techniques using nucleic acid amplification can now offer high sensitivity in screening for these infections and disrupt the transmission chains within the community. This would, in turn, lead to decrease in case burden and ultimately eliminate the reservoir of infection.

Because of difficulties in selection of representative samples for evaluation of commercially available test kits, interpretation of test performance (specificity, sensitivity) must be done with clear definition of the prevalence of disease in the test population. Also, because of obvious social stigma that can be associated with various STDs, accuracy (positive predictive values) of tests should be of highest priority in selection of those that are appropriate for the patient. Presence of nonviable gene fragments can, in principle, generate positive signals in all the molecular tests. This means that vigorous evaluation and correlation studies must be done before the significance of their presence can be adequately interpreted.

Although rapid tests do significantly reduce turn-around time and this will ex-pectedly generate pressures from both clinician and patient, the choice of molecular tests should not depend on this consideration alone. In less well developed countries, cost implications of some molecular tests can become an insurmountable one that must be balanced with available resources. However, with rapid evolution and refinement of different test platforms, it is to be expected that unit prices of many molecular tests will decline significantly in the very near future.

This paper intends to review the currently developed and available molecular diagnostics of common STDs including (1) Neisseria gonorrhoeae; (2) Chlamydia trachomatis; (3) Treponema pallidum; (4) Haemophilus ducreyi; (5) Mycoplasma and Ureaplasma; (6) Trichomonas vaginalis; (7) herpes simplex virus; and (8) human papilloma virus.

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