system
Given the following post, respond as if you're commenting on Reddit:
human
When someone gets blood drawn and tested for STD's, such as HIV, how many infected cells need to be in the sample for the test to show up positive?

From what I understand, if someone recently infected with an STD gets tested it may come back negative (because the virus has not spread to enough of the body??). So basically my question revolves around this concept (which I am not sure is even correct); how many cells would need to be infected for there to be a good probability of the test showing up as a true positive as opposed to a false negative?
gpt
Typically, when we do infectious disease testing for viruses (I am thinking HIV here but Syphillis and HCV are largely tested in basically the same way) we aren't actually testing your cells at all. The blood is drawn and centrifuged- which means it is spun very fast. The cells move to the bottom of the tube and there is typically a layer of gel which is intermediate in density that settles between the cells and the liquid portion which is left- which is serum. It's this serum which we test.

Tests for these viruses are typically immunoassays, which is a fancy word for saying that we are using an antigen/antibody response to detect the presence of the virus in your body. When your body recognizes a pathogen, it activates an immune response and begins producing antibodies that are specific to the pathogen. We can test for these antibodies by using special antibodies made commercially which react against YOUR anti-HIV antibodies. Obviously, it's a little more complicated than that but hopefully you get the point. There are tons of different techniques for doing this but the basic idea is the same.

It takes some time for your body to make enough antibodies to produce a positive result on an immunoassay. Traditionally, the HIV screening test used is ELISA and it usually takes about 6 weeks after exposure for an ELISA screen to be positive. Newer tests, like nucleic acid testing, are able to detect HIV antibodies much earlier but the trade off is they are extremely expensive- given how many people are screened for HIV (and the vast majority of them are negative), it's not cost effective to test everyone with NAAT. Blood banks are now using NAAT to test donated units to reduce the risk of transmitting HIV in transfused blood from a donor who was unaware they had HIV and were within that 6 week window that ELISA wouldn't catch it.

Hopefully this answers your question. Before six weeks after a possible exposure, a normal HIV test ordered by your doctor or the public health department is likely not reliable. A positive result before that time is definitive, but a negative result may be a false negative.