There are variations of course - some people cheat on their partners and want to know if they need to say anything, others are regretting an encounter with a sex worker - but the basic question remains the same:
If I have been exposed to an STD, how long do I have to wait before I know any infection will be detected by an STD test?
Unfortunately, the question of when to get an STD test is not easy to answer. To start with, STD testing isn't perfect - even if you have theoretically waited long enough for a test to work you could still end up with a false positive or a false negative. Then you need to account for the fact that not all STD tests work in the same way. Some tests look directly for the presence of a pathogen, while others look for your body's immune response to the infection.
In theory, tests that look directly for the pathogen should become positive faster, since pathogens are there from the start of the infection, but they often require samples from an infected location to work. New tools have allowed doctors to use urine testing for chlamydia and gonorrhea, but other infections such as HPV may be more difficult to detect without the presence of an obvious sore or lesion.
In contrast, blood tests that look for antibodies don't require a doctor to know where to sample, but they take time to turn positive. This is because your body's immune system must first react to the infection and then produce detectable levels of antibodies. Different types of antibodies peak at different times after infection. In some cases, this fact can be used to determine how long you've been infected with an STD, but the delayed response also affects how long it takes for a test to become reasonably predictive of infection.
Being able to answer how long it would take for someone to definitively test positive or negative on an STD test after a risky sexual encounter would require knowing a number of things including:
- What STDs the person had been exposed to.
- What tests were being used to detect the infection.
Common practice suggests that people could go in for basic testing for bacterial STDs as soon as 2 to 3 weeks after an exposure - sooner if they have symptoms - but that they would need to be retested again at least three to six months out in order to feel relatively certain of their results. At a month out, some tests for chlamydia and gonorrhea would be reasonably accurate, but tests for other diseases such as herpes and HIV take longer to become conclusive.
STD Testing Isn't Everything
The other question that people are often asking - explicitly or not - in these e-mails is "do I have to tell current/future partners that I might have been exposed to an STD?" That question may be often modified by "what if we only had oral sex?" or "what if it didn't last long?" but my answer is usually the same.
These are discussions that everybody should be having before they have sex. Most people don't come to sexual relationships completely inexperienced, and so talks about testing and safe sex are not just appropriate but smart. If nothing else, if you can't bring yourself to have the discussion, it is always a good idea to practice safe sex until you are reasonably certain of your test results. Condoms may not be perfect, but they are far better than doing nothing at all.
The question of disclosure is certainly more complicated for people who have been unfaithful to a current partner, but I have to believe that more people would be willing to forgive an infidelity that didn't unknowingly expose them to STDs than one that did. At least if someone discloses an infidelity, they give their partner a chance to minimize their emotional and physical risk.
Although people have certainly used it as a tool of manipulation, infecting someone with an STD is not a healthy way to make a partner stay with you or to convince them to overlook an infidelity. Fortunately, once most people get over the initial shock and the stigma of an STD diagnosis, they realize that fear isn't love. Dating with STDs may not always be easy -- but it's better than staying with a partner who is emotionally or physically abusive.
Kettle H, Cay S, Brown A, Glasier A. Screening for Chlamydia trachomatis infection is indicated for women under 30 using emergency contraception. Contraception. 2002 Oct;66(4):251-3.