You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is 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 - and explain in detail?
following a single massive imfection the p24Ag is the earliest marker appear in blood REF:MICROBIOLOGY ANANTHA NARAYANAN NINTH EDITION PAGE.577