Infection with the human immunodeficiency virus ( HIV ) is routinely diagnosed by detecting the presence of specific antibodies in the patient's serum . Although the presence of the virus itself can be now be detected these tests remain expensive and require laboratory techniques that are not routinely available . The diagnosis of HIV infection begins with an enzyme immunoassay ( ELA ) . The optical density ( OD ) of the patient's ELA is compared to a control specimen ( OD ration If the OD ratio is - above the established cutoff for that control sample on repeat testing the specimen is termed " repeatedly reactive " The ELA is relatively sensitive fast , simple and inexpensive which makes it an appropriate screening test However , if one examines the ODS for a large group of samples from patients with and without true HIV infection you can see that there is some overlap in their ELA results if a value of A is used for the cutoff HIV- HIV Low High OD Ratio Hypothetical distribution of On ratios for patients with and without HIV Infection Hypothetical distribution of OD ratios for patients with and without HIV infection . Patients with HIV infection are depicted with the thick line , and the patients without HIV are depicted with a thin . RED line