The test most commonly used for the initial screening of a blood sample for HIV infection is

Explanation

ELISA detects antibodies against HIV and is cheap and sensitive enough for mass screening, with a positive result confirmed afterwards by a more specific test such as Western blot. It can miss a very recent infection, because during the window period of a few weeks the antibodies have not yet been made. The Widal test is for typhoid and has nothing to do with HIV.