An HIV test will not detect the presence of the HIV virus immediately after exposure. Statistics show that 96% (perhaps higher) of all infected individuals will test positive within 2 to 12 weeks. In some cases, this may take up to six months.
Think about this: if you got a negative test at six weeks, would you believe it? Would it make you less anxious? If so, go for it. But to be certain, you will need to be tested again at six months.