We have developed a technique which permits high-yield fabrication of microbridges and low noise YBa2Cu3O7 superconducting quantum interference devices (SQUIDs) in epitaxial thin films. These SQUIDs operate over a wide temperature range extending from 4 K to close to the superconducting transition temperature. Measurements of an rf SQUID operating at 77 K give a peak-to-peak flux sensitivity of 36 μV/Φ0 and a flux noise at 10 Hz of 1.5× 10−4 Φ0/√Hz. Device yields over 80% have been obtained.
Reducing agents dramatically alter the specificity of competitive assays for antibody to hepatitis B core antigen (anti-HBc). A specificity improvement was demonstrated with a new assay which utilizes microparticle membrane capture and chemiluminescence detection as well as a current radioimmunoassay procedure (Corab; Abbott Laboratories, Abbott Park, Ill.). The effect was most noticeable with elevated negative and weakly reactive samples. In both systems, reductants increased separation of a negative population (n = 160) from assay cutoffs. With a selected population (n = 307), inclusion of reductant eliminated apparent anti-HBc activity in 54 of 81 samples in the 30 to 70% inhibition range. Reductant-stable anti-HBc samples were strongly associated with the presence of antibody to hepatitis B surface antigen (21 of 27). The association persisted below the detection limits of current assays to 0.3 to 0.4 Paul Ehrlich Institute units per ml. Only 1 of 54 reduction-sensitive borderline samples was confirmed to be positive for antibody to hepatitis B surface antigen. The modified procedures had unchanged or slightly improved sensitivity for immunoglobulin G (IgG)associated anti-HBc activity. Although IgM anti-HBc detection was reduced from fourto eightfold in the presence of reductants, sensitivities remained at least twofold greater than that of an enzyme immunoassay (Corzyme M; Abbott) designed to detect acute-phase levels of IgM anti-HBc. The use of reducing agents should significantly improve the reliability of anti-HBc testing, especially near assay cutoffs.
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