I once took an STD test series for all the typical STDs and everything came back negative….except for HSV-1 (cold sores aka oral herpies). It returned an ‘abnormal’ result of 1.05….not at the level of true positive (1.09 or higher) but not at ‘normal’ of 0.9 or lower. Of course I freaked, had a few days of intense anxiety and suicidal thoughts, then decided to get a second test. This second one came back full normal. WTF? Please explain how this works.
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All medical tests (all tests in general, really) can produce false positives or false negatives. Your body is a very complicated system, it changes over time, and it isn’t quite the same as anyone else’s body. Similarly, most diseases are complicated and can develop in different ways in different people. For example, a disease might usually be associated with a specific substance appearing in your blood, but there might be a rare form of the disease that doesn’t produce that substance, or there might be something completely different going on in your body that produces that substance without the disease being present.
On top of that, there are inevitably some mistakes. The person carrying out the test might forget a step, or they might mix your sample up with someone else’s, or a machine might have a fault. If the lab is run well, these mistakes should be very rare, but nobody is infallible.
Anyway, different tests can have very different false positive and false negative rates. Ideally, medical professionals are supposed to put them into context and explain how definitive they are. Often they will combine multiple different tests before deciding on a diagnosis.
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