Kinetics of delta antigen and delta antibody in acute delta hepatitis: evaluation with different enzyme immunoassays

The kinetics of delta antigen (HDAg) and anti-delta antibody (anti-HD) was analyzed in 22 acute delta hepatitis infections (11 coinfections and 11 superinfections), with an enzyme immunoassay developed by Organon Teknika and with two commercially available assays: Deltassay, a test for HDAg from Noctech and Abbott anti-delta enzyme immunoassay, a test for anti-HD from Abbott Laboratories. In seven cases, HDAg was detected with the Organon assay but not with Deltassay. These discrepancies were not related to the type of hepatitis delta virus infection. All samples from these patients taken beyond week 4 of illness were found anti-HD positive with both the Organon and Abbott anti-HD assays. These data reflect the lack of sensitivity of Deltassay. In no instance were HDAg and anti-HD present simultaneously when tested with the Organon assays. On the contrary, 10 sera among the 28 that were HDAg positive with the Organon assay also were found anti-HD positive with the Abbott test. We suspected that the test procedure recommended by Abbott (a one-step competitive assay) may have yielded false-positive anti-HD results when HDAg present in the sera reacted with peroxidase-labeled anti-HD of the kit. To determine the specificity of the simultaneous presence of HDAg and anti-HD, these 10 sera were retested with the Abbott anti-HD assay, but by a modified two-step procedure that avoided contact between sera and labeled antibody. For six sera a negative result was obtained with the second procedure, suggesting that a false anti-HD reaction occurred with the standard test procedure. For four sera a positive result with both procedures was indicative of the simultaneous presence of HDAg and anti-HD. In conclusion, assay for HDAg was found very convenient for the early diagnosis of acute hepatitis delta virus infections. Seroconversion to anti-HD could be used for a late diagnosis 2 to 5 weeks after the beginning of illness. However, anti-HD results obtained with one-step competitive assays have to be interpreted carefully in HDAg-positive sera.