text: With regards to a previous answer, several tests are NOT done on blood. For example, gonorrhea is usually cultured from exudate's. Trichomonas from a urinalysis (Trichomonas can only be detected via urinalysis. Trichomonas are protozoan pathogens that are motile flagellates which can be seen during a microscopic urinalysis.)\n\nChlamydia are a intracellular parasite, very similar to bacteria which are sometimes classified as gram-negative, though they are hard to stain. Unlike Trichomonas, it does respond to antibiotics similar to many other gram-negative bacteria.\n\nIt is usually confirmed via a test on vaginal, anal, or urethral exudate's or from a urine sample.\n\nJust because something is a localized infection, doesn't mean it doesn't show up in blood work. Syphilis is localized for some time before it starts to become more systemic. Blood tests are usually against antibodies to the infection, or from reactions to the infection (like the RPR to test for syphilis).\n\nI worked quite a few years in a clinical laboratory, testing for things like this.