A positive blood test is a positive result for a relevant item in a blood test. A positive result is relative to a negative result, which is qualitative, and qualitative means simply judging whether the relevant test item can be detected. If the test is for a certain kind of antibody, such as hepatitis B surface antibody, a positive result means that this antibody can be detected, and after the hepatitis B vaccination, it is likely that this antibody can be produced, and a positive result means that immunity to the hepatitis B virus has been acquired. However, if the positive result refers to antibodies against hepatitis C, AIDS or the corresponding antibodies against syphilis spirochetes, it means that it is likely to be infected with these diseases, because these three antibodies are not protective antibodies.