A patient comes to hospital with a history of sore throat, diarrhoea and sexual contact 2 weeks before. The best investigation to rule out HIV is-(

The standard blood screening test for HIV infection is the ELISA, also referred to as an enzyme immunoassay (EIA). This solid-phase assay is an extremely good screening test with a sensitivity of >99.5%. The fouhgeneration EIA tests combine detection of antibodies to HIV with detection of the p24 antigen of HIV. The most commonly used confirmatory test is the Western blot. (Harrison's Principles of internal medicine, 20 th edition, page 1538)