Can an hiv antibody screening test be positive but the western blot negative?

Yes: There are 2 reasons for this kind of result: (1) a false positive, or (2) early infection before seroconversion. In someone at low risk for HIV without symptoms, another negative WB in 1 month means there's no infection. In someone at risk or with symptoms of acute infection, a viral load can be helpful. The new 4th generation antigen/antibody tests will eliminate this problem.