When should some one be considered hiv negative after exposure when antibody only and duo combo tests used?

Depends: Antibody tests are positive in most infected people by about three months after exposure. In a small number the test may not turn positive for a year. Combo test is more reliable and at about 6 months after exposure, a negative result should rule out HIV. However, the best test is for HIV nucleic acid, if it is negative at three months after exposure that would rule out HIV.