FWIW, a list of STD’s and how they’re diagnosed:
Gonorrhea Gram stain of infected material, PCR on infected material*
Chlamydia trachomatis Detection of antigens in infected tissue or PCR to detect C. trachomatis genome
Ureaplasma urealyticum Culture of infected material
Syphilis Dark field examination of infected material, blood test (VDRL,RPR,FTA)
Haemophilus ducreyi Culture of infected material
C. granulomatis Examination, culture of infected material
HIV Blood test for anti-HIV anitbody, HIV antigens, and/or HIV genome
Herpes Culture of virus from infected material, microscopic exam of infected material
Papillomavirus PCR detection of viral genome in infected material
Hepatitis B Blood test for antibodies and antigens
Trichomonas vaginalis Microscopic examination of infected material
Candida albicans Microscopic examination of infected material
*Techniques designed to detect gonococcal infection by testing of a single serum sample for antibody to N. gonorrhoeae have been limited by an inability to differentiate antibody due to past gonorrhea from antibody due to current infection and by false-positive results caused by cross-reactive antibody to N. meningitidis. For these reasons, serologic tests for gonorrhea have had a very low predictive value and are not used in clinical practice.
Anyhow, many STD’s are idenitfied primarily by clinical presentation and examination of infected material. In clinical practice, the only STD’s routinely identified by a blood test are syphilis, HIV, and Hepatitis B. To diagnose most STD’s, you gotta go to where the action is.