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More than 2 decades of developments in the field have led to a wide variety of test kits available for addressing the different needs of surveillance, diagnosis, and monitoring of human immunodeficiency virus (HIV) infection and antiretroviral therapy (20, 23). These test kits are based on different tech- nologies, including immunoassays and molecular testing. Typ- ically, enzyme-linked immunosorbent assays (ELISAs) and rapid tests for detecting anti-HIV antibodies are used for blood screening and/or surveillance purposes. The former in- clude third-generation ELISAs that utilize the antigen sand- wich design for improved sensitivity, while the latter term re- fers to immunochromatographic tests or other simple antibody tests requiring less than 30 min of assay time. Since the viral capsid (core) p24 protein can be detected earlier than anti- HIV antibodies (93), attempts had also been made to develop a new generation of ELISA capable of simultaneously detect- ing the p24 antigen and anti-HIV antibodies (34, 52, 76, 101, 103). Significant reductions of the window period were achieved, which is ideal for blood screening and surveillance (102), but users were also advised to be mindful of possible limitations when utilizing such a combination assay (52, 81, 83). With specific panels, a delay in detection by a fourth- generation ELISA was evident compared with a dedicated p24 antigen assay (81). This shortcoming of fourth-generation ELISAs is understandable, because the surface area of the solid phase for detection is constant and limited regardless of whether it is utilized for a single test or combined assays (101). It should also be noted that an additional procedure for the dissociation of immune complexes employed by dedicated p24 antigen assays but not by combined assays was found to facil- itate sensitive detection (85). However, this does not mean that using ELISA dedicated for detecting p24 antigen has no short- comings. The viral p24 protein is a transient marker that can fall to an undetectable level after initial acute infection and only reemerges at the advanced stage of AIDS in patients (93, 107). Hence, tests utilizing this marker are more valuable for situations such as screening blood products from individuals before antibodies are produced, detecting early infections in persons who have been exposed but are seronegative, or mon-

Frequency, Causes, and New Challenges of Indeterminate Results in Western Blot Confirmatory Testing for Antibodies to Human Immunodeficiency Virus