Diagnosis

STI tests may test for a single infection, or consist of a number of individual tests for any of a wide range of STIs, including tests for syphilis, trichomonas, gonorrhea, chlamydia, herpes, hepatitis and HIV tests. No procedure tests for all infectious agents.

STI tests may be used for a number of reasons:
as a diagnostic test to determine the cause of symptoms or illness

as a screening test to detect asymptomatic or presymptomatic infections

as a check that prospective sexual partners are free of disease before they engage in sex without safer sex precautions (for example, when starting a long term mutually monogamous sexual relationship, in fluid bonding, or for procreation).

as a check prior to or during pregnancy, to prevent harm to the baby

as a check after birth, to check that the baby has not caught an STI from the mother

to prevent the use of infected donated blood or organs

as part of the process of contact tracing from a known infected individual

as part of mass epidemiological surveillance

Not all STIs are symptomatic, and symptoms may not appear immediately after infection. In some instances a disease can be carried with no symptoms, which leaves a greater risk of passing the disease on to others. Depending on the disease, some untreated STIs can lead to infertility, chronic pain or even death. Early identification and treatment results in less chance to spread disease, and for some conditions may improve the outcomes of treatment.

There is often a window period after initial infection during which an STI test will be negative. During this period the infection may be transmissible. The duration of this period varies depending on the infection and the test.

Diagnosis may also be delayed by reluctance of the infected person to seek a medical professional. One report indicated that afflicted people turn to the Internet rather than to a medical professional for information on STIs to a higher degree than for other sexual problems.

STD Wizard
The STD Wizard is a publicly available expert system for determining which screening tests, vaccinations, and evaluations are recommended, related to sexually transmitted diseases. The information included within the STD Wizard is based on the Centers for Disease Control and Prevention "Sexually Transmitted Diseases Treatment Guidelines - 2006". The system has English and Spanish language interfaces.

The STD Wizard runs in a web browser. The program asks a series of questions, related to demographics, behaviors, and symptoms. There are potentially over 100 questions, but most users get asked about 20. The exact questions asked depend on the user's responses. Typical questions include:

Demographics - How old are you, in years?
Behaviors - Have you had more than one sex partner within the past six months?
Symptoms - Are you having a discharge from your penis or burning during urination?

Typical recommendations include:
Screening tests - e.g., HIV screening test
Vaccinations - e.g., Hepatitis B vaccination
Evaluations - e.g., Seek medical attention in the next week for rash