A sensitive and specific radioimmunoprécipitation assay was developed for the detection and analysis of anti-HIV antibody response in human sera with the use of 125I-labelled purified HIV proteins with subsequent sodium-dodecylsulfate gel electrophoresis (125I-RIPA). The 125I-RIPA was shown to be as specific but at least 1 log more sensitive with respect to the detection of gp41^env and p24^gag than the immunoblot analysis as tested in serum samples from several risk groups. Sequential sera were obtained from 9 individuals who seroconverted for HIV antibodies. In 4 individuals, antibody to p24^gag was detected in earlier serum samples by the 125I-RIPA than by EIA or immunoblot; in the other 5 individuals, the detection of p24^gag concorded in enzyme-linked immunosorbent assay (EIA), immunoblot and 125I-RIPA. Moreover, in one of 78 randomly chosen EIA-negative sera from individuals at high risk, antibodies to p24^gag could be detected by the l25I-RIPA. This early seroconversion was confirmed 3 months later by means of immunoblotting and EIA. The specificity of the 125I-RIPA was further demonstrated by analyzing sequential EIA-negative serum samples from 10 individuals at risk for AIDS, collected during 2 years at 3-monthly intervals. All 80 serum samples were found to be negative in the 125I-RIPA and the individuals revealed no signs of HIV infection. The 125I-RIPA technique may be a valuable confirmatory assay in the serology of HIV infections. The sensitivity of this test provides a reliable measure of effective sensitivity when new-generation screening tests are evaluated. In addition, this technique appears to be a specific and sensitive assay to detect antibodies to p24^gag at very early stages of HIV infection.

Several variables that may affect accurate measurement of platelet-associated IgG (PA-IgG) were studied using a radioimmunoassay of the consumption type. The amount of PA-IgG of washed, unfixed normal donor platelets was 1.0 ± 0.9 fg IgG/platelet (mean ± 2 SD). Upon storage of washed platelets in a buffer containing EDTA, the amount decreased significantly to 0.2 ±0.2 fg IgG/platelet. Simultaneously, an increase in modal platelet volume was observed. Similar results were obtained when platelets were fixed with paraformaldehyde (PFA). We postulate that this decrease in PA-IgG is caused by the release of plasma IgG entrapped by the surface-connected canicular system of the platelet, when the platelets swell during storage in EDTA or fixation with PFA. This presence of varying amounts of entrapped plasma IgG may cause the wide discrepancies in PA-IgG found in normal donor platelets as well as platelets from ITP patients by other investigators. A good quantification of platelet-bound alloantibodies was possible with our assay when platelets were routinely fixed to diminish the amount of nonspecific PA-IgG. This was demonstrated with different anti-Zw^a (=anti-P1^A1), anti-Bak^a and anti-HLA sera. We also observed that fragments of platelets as well as fragments of cells of other types can cause aspecifically increased Pa-IgG values and can thus interfere with the proper measurement of platelet-bound antibodies in all kinds of immunoassays in general.