###Human: Please explain the accuracy of HIV Ag/Ab test at 40th day after exposure. ###Assistant: Hello. The window period of the Ag/Ab test varies from 18 to 45 days. Therefore a negative test at six weeks (42 days) would be considered conclusive in 98-99 % individuals. However, a negative test at 12 weeks is still considered the golden standard (conclusive in 99.9 % of individuals). CDC (centers for disease control) recommends that a negative Ag/Ab test at six weeks is good enough to rule out HIV. Take care.