Background and Aims: HIV is spreading rapidly among people word wide. Infection with this virus leads to immune suppression and finally acquired immune deficiency syndrome (AIDS).Early HIV detection is depended on antibody screening against virus by Enzyme Linked Immunosorbent Assay (ELISA).Some confirmatory tests such as Western Blot and Recombinant Immunobloting Assay (RIBA), are used to verify viral infection. Many of confirmatory tests results are indeterminate. The aim of this study is comparing the frequency and patterns of indeterminate results in two groups blood donors and patients with high risk behaviors, in northeast of Iran.

Materials and Methods: From October 2009 to March 2014 total number of 1055 serum samples with previous positive HIV ELISA test history, were tested in our laboratory. Some by RIBA and some by western blot method.

Results: Most of indeterminate results belonged to blood donors that were tested by Western Blot analysis and were positive. The most reacting band was P24 in both methods and groups.

Conclusion: RIBA assay is more sensitive and reliable than western Blot but it’s necessary to use other supplementary tests with less indistinctive results.