Are STD Test Results Always Right?

Chlamydia screening smear test

Question: Are STD Test Results Always Right?

People often think that if they come away from their annual doctor's visit with a clean bill of health, they don't need to worry about whether they have an STD. That's a dangerous misconception.

Answer: There are several reasons why STD screening isn't an infallible way to avoid STDs. I would never want to discourage people from regular STD screening since it is one of the best things you can do to reduce your risk of acquiring an STD (along with practicing safe sex), but it isn't perfect.

Part of taking responsibility for your sexual health is acknowledging that even if you do everything right, sexual activity still has risks. Condoms and dental dams can sometimes fail, and STD tests don't always give the full picture. Here are some reasons that, even after getting an STD test, you may still occasionally end up asking yourself, "Do I have an STD?"

  1. You didn't actually get tested
    A lot of people think that their doctor screens them for STDs as part of their annual exam. This is, sadly, untrue. Many doctors don't regularly screen their clients for STDs, even when practice guidelines say they should. The only way to be certain you're getting tested for STDs is to ask your doctor to test you - and give them a list of what you want to be tested for.
  2. You got tested too soon
    Some STD tests are not effective for a newly acquired infection. Recent studies have, for example, shown that the standard blood test for syphilis is ineffective at detecting early cases of the disease. The types of HIV tests and other STD tests that look for an antibody response instead of hunting for the pathogen itself may be particularly susceptible to this problem since it takes time for an antibody response to develop.
  1. The test gave an inaccurate result
    When designing a diagnostic test, there is always a trade-off between sensitivity and specificity. Almost no test is going to be perfectly able to determine whether or not someone is infected. How accurately the results of any given test predict a person's disease state is dependent, in part, on the population that test is being used in (see this example about the accuracy of herpes blood tests.) Most tests are designed to be pretty good, and there are almost always ways to make their diagnoses more accurate, but both false positives and false negatives can be a problem depending on the disease in question and the test that is being used to detect it.
  1. You were given the wrong test
    There isn't always a right test, but sometimes there is a wrong one. As mentioned above, every diagnostic test has trade-offs, and there are often tests that are more or less accurate depending on the circumstance and the population. The problem is that the best test isn't always available or practical, and so doctors will sometimes end up having to use a less accurate method of diagnosis.
  2. Doctors don't test for what you have
    There are some diseases for which there are no commercial tests, or for which doctors simply do not bother to test - either because the disease is uncommon or because it is unlikely to cause serious problems if left untreated. For example, doctors don't test for molluscum contagiosum in part because they assume that anyone infected will have symptoms and because an infection will usually run its course without any serious side effects.

    On the other hand, doctors are probably unlikely to test for rectal chlamydia, anal cancer, and other rectal STDs both because of their relative rarity and because they may be uncomfortable asking the sexual history questions that would allow them to determine that someone is at risk. Even when reasonably accurate tests are available, they don't do any good if they're not being used.

    My Take

    The bottom line is that I am a great believer in the use of regular STD screening to reduce the likelihood of having an untreated STD that could be transmitted to others. I just worry that it may be too easy for some people to get caught up in the trap of believing that if you get regular STD testing and always practice safe sex, then sex will become a risk-free activity. It's not.

    With proper precautions, sex is a relatively low-risk way to show your affection for someone, but that doesn't mean it can't have consequences. Part of being responsible for your sexual health is keeping those potential consequences in mind. STD testing is a great tool for making better decisions about what levels of risk you personally find acceptable in any given situation.

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