With the expansion of the detection base, there are indeed more and more false positive cases in the initial screening. Recently, I met a considerable number of patients who were HIV-positive, HIV-negative or were not sure about the diagnosis. It should be said here that both the initial screening and the confirmed experiment are to check whether there are antibodies against HIV in the body. Preliminary screening experiments generally include enzyme-linked immunosorbent assay (ELISA) and rapid detection (rapid test paper and gelatin particle agglutination test). The confirmed experiment is western blot (WB), but it is only for antibodies against certain HIV proteins, including p24, gp4 1, gp 160/gp 120. From the epidemiological point of view, it should be made clear that in order to diagnose or exclude a certain disease, medical staff will use certain experimental means to check it, which is called diagnostic test, and the degree of coincidence between diagnostic test and the actual situation of the subjects is called "authenticity". The most common methods to evaluate authenticity are of course sensitivity (1- false negative rate) and specificity (1- false positive rate). After the blood is drawn, the blood will be sent to the laboratory for a preliminary screening experiment with high sensitivity and low specificity, so as to dig out all possible patients. If the initial screening test is negative, the possibility of illness is quite low, and the corresponding false positive rate (misdiagnosis) is relatively high. Suspected cases with positive initial screening need to be diagnosed. The WB experiment mentioned above is used to detect antibodies to various HIV-specific proteins (antigens), and the specificity of the experiment is very high. Antibodies to multiple specific proteins were detected at the same time, so this case can be diagnosed as infection, and only a few antibodies were detected. Those who do not meet the diagnostic criteria will be diagnosed as "HIV uncertain". In this case, the sensitivity and specificity of the initial screening experiment are 99.7% and 98.5%. That is to say, the false positive rate can be about 1.5%, and the subject is diagnosed as negative. This situation can generally rule out the possibility of infection. If it is more than 3 months since the last high-risk behavior (unprotected sex, needle use, etc.). ), there is no need to do more.