Why still some people asked to check hiv after 6 months? how about 4th gen test reliability?

Seroconversion: The "window period" is inherently tied to the bodies ability to mount an immune response to HIV. The tests are looking for antibodies to HIV's GP shell. After 6 months time, 99% of people exposed to HIV will have serosconverted. There are other testing modalities..HIV RNA/DNA PCR for example. Branched RNA is another excellent modality. These will show infection virtually immediately.