<prompt> A patient comes to hospital with a history of sore throat, diarrhea and sexual contact 2 weeks before. The best investigation to rule out HIV is:p24 antigen assayELISAWestern blotLymph node biopsy  <response>: Ans. is a i.e., p. 24 antigen assay Ref: Ananthanarayan, 9the, p. 577 - 578 and 8the, p. 576 p24 antigen is the earliest virus marker to appear in blood and is the one tested for. It may be detectable in blood after about 2 weeks.Remember: Nucleic acid tests RT - PCR, BONA, PCR are the earliest test to become positive even before p24 antigen detection.TestPositive after infectionNucleic acid test12 daysp24 antigen test16 daysAntibodies test22 days 