This is a test for the antibody to HIV, not the presence of virus. It may take up to three months or more from contact to become positive. The ELISA (Enzyme-Linked Immunosorbent Assay) is a screening test that is widely used. If an ELISA test yields two or more reactive results, the Western blot is used to confirm these results The Western blot is more specific and takes longer to perform than the ELISA. Together, the two tests are more than 99.9 percent accurate for the HIV strain that is prevalent in the general population, strain M.