A modified IgG avidity assay for reliability improvement of an in-house capture ELISA to discriminate primary from secondary dengue virus infections.

Although discrimination between primary and secondary dengue infections can be performed using commercially available immunoassays or in-house tests, the evaluation of these methods is important, but is often problematic due to incomplete clinical data. In many cases, patients' sera submitted to the laboratory may not include the date of onset of illness which is necessary to discriminate primary and secondary dengue infections. This study reports improvement of an in-house capture ELISA using IgG avidity to discriminate primary and secondary dengue virus infection. Modified definition criteria were applied to characterize 99 single sera based on their IgM/IgG ratios. Regressive analysis indicated that the avidity test results (avidity index of 60 % as cutoff) for the discrimination showed good agreement (96 %) and a high correlation (r = -0.81) with those of the in-house capture ELISA (IgM/IgG ratio at 1.2 as cutoff). To further evaluate the in-house tests, 318 convalescent sera were compared with a Focus Diagnostics' anti-dengue IgM ELISA. Compared with the Focus Diagnostics system, the sensitivity of an in-house IgM determination was 83 %, whereas using both IgM and IgG capture ELISAs the sensitivity increased to 95 %.