Predictive value of two commercial human immunodeficiency virus serological tests in cases with indeterminate Western blot results.

Background and Purpose: Serodiagnosis of human immunodeficiency virus (HIV) infection typically requires repeatedly reactive positive screening test followed by Western blot (WB) assay. When WB assay result is indeterminate, the results of follow-up WB assay may remain inconclusive. This study evaluated use of enzymelinked immunosorbent assay (ELISA) and particle agglutination test (PAT) as sequential screening tests with WB assay for the diagnosis of HIV infection. Methods: From January 1, 2000 to December 31, 2003, a total of 565 serum samples collected from individuals with a previous positive or borderline positive ELISA test for HIV at regular check-up (281 samples) and a second group of individuals with known risk of HIV exposure or suspected infection based on clinical presentation (284 samples) were tested for HIV infection by ELISA, PAT and WB assay. Results: The result was positive for HIV infection and confirmed by WB assay in 197 samples (22.5%), indeterminate in 127 samples (22.5%) and negative in 241 samples (42.6%). The sensitivity and specificity of ELISA were 100% and 21.6% and of PAT were 99.5% and 95%, respectively. Among the 197 HIV-infected cases, all ELISA and PAT results were concordant with WB assay except 1 (1/197) with negative PAT. Positive ELISA, positive PAT and indeterminate WB assay results were found in 9 of the 284 samples (7 individuals) from at-risk patients. Among these 7 individuals, 5 were later proved to have HIV infection. WB assay in 1 of the 7 individuals remained indeterminate 1 year later, and the remaining case was lost to follow-up. Conclusion: We suggest initial ELISA followed by PAT as sequential screening for HIV infection. When both screening tests are concordant but subsequent WB assay is indeterminate, further evaluation (such as nucleic acid amplification test) should be arranged as soon as possible.