The DUO test is still considered accurate at 28 days and would guess you've asked this question before and your result will not change.
The HIV Antibody & P24 antigen Combi test is an excellent and accurate test that reliably detects HIV infection as early as 28 days post exposure.
This HIV test is known as the 'Duo' because it tests for both HIV antibodies as well as the 'HIV P24 antigen'.
The HIV p24 antigen is a core HIV viral protein which becomes detectable at approximately 10 days post exposure, peaks at 16 days or so and then remains at high level for 8 to 10 weeks post exposure. It will then fluctuate over the rest of the course of the illness.
HIV p24 antigen is extremely useful in combination with an HIV antibody test in determining early on whether an individiual has been infected with HIV. Hence, when tested at the 28th day, if the individual was actually infected and if p24 ag was present in his/her body it would have been detected and if detectable amount of antibodies were already produced, the AB part of the test would have been positive. The HIV DUO test is rated as 99.8% accurate at the 28th /29th day.
Time to move on.
4 weeks is conclusive so you should move on because you are safe. We rely on the opinion of expert doctors so this is actually their opinion, and don't pay attention ot whatever claim you can locate on another site, otherwise we will be here all day arguing about who is just theoretical and being conservative despite no incidences.