The window period for 4th generation antibody based tests range from 4-6 weeks. Among them ELISA (rapid) is more sensitive than Western blot in picking up positive cases. Thats why ELISA is done first usually and Western blot reserved for confirmation. In your case, you are out of window period and tested negative. However if there were any symptoms suggestive of seroconversion after the exposure (usually first week), it is highly recommended to test again with ELISA after 3 months. PCR based viral load detection tests are available in all major cities now which can detect HIV RNA/DNA even during window period. These are for people who want to know their status sooner than window period and are very accurate. But due to cost constraints, these are not recommended routinely for screening.