Hello,
False negative results may be there when the person gets himself tested before the window period(an interval of three weeks to six months between the time of HIV infection and the production of measurable antibodies to
HIV seroconversion).Usually people may show
positive results after 30 days but sometimes they may take a bit longer also.Also, ELISA testing alone cannot be used to diagnose HIV, even if the test suggests a high probability that antibody to HIV-1 is present.