STD testing

An STD test is a medical test for the presence of any of a number of sexually transmitted diseases (STDs). Most STD tests are blood tests. STD tests may test for a single disease, or consist of a number of individual tests for any of a wide range of STDs, including tests for syphilis, gonorrhea, hepatitis and chlamydia, and HIV tests. Not all STD tests test for all diseases, so it is important to be aware what diseases a given test tests for.

STD tests may be used for a number of reasons:
 * as a diagnostic test for the diagnosis of illness
 * as a check that prospective long-term sexual partners are free of disease before they engage in sex without safer sex precautions (for example, in fluid bonding, or to attempt to have a baby). Some states require STD testing of both prospective partners before marriage.
 * as a check prior to or during pregnancy, to prevent damage to the baby
 * as a check after birth, to check that the baby has not caught an STD from their 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 STDs appear right away. In some instances a disease can be carried with no outward symptoms, which leaves a greater risk of passing the disease on to others.

Dependence fallacy
A common fallacy is the view of STD testing as a kind of safe sex practice which overrides the actual practice of safe sex. Tests by definition are only informative and not preventative, and while important in keeping oneself and others informed of a partner's STD status, they do not take the place of preventative safe sex practice -- caution in selecting partners and refraining from promiscuity. They are however essential in the early treatment of STD's, and most STD's are, by current medical science, inherently treatable once discovered.