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Background: Screening assays are needed in order to guarantee safety of donated blood, but a significant number of safe donations are removed from blood supply because of reactive screening results. It is important to evaluate the positive predictive value (PPV) of screening assays in order to modulate confirmatory algorithm and implement an adequate counseling., Methods: An analysis of 17,912 blood donations has been conducted at Transfusion Medicine at Second University Naples, Italy, in 2009-2012. Serological screening for syphilis, hepatitis B virus (HBV), hepatitis C virus (HCV), and human immunodeficiency virus (HIV) was performed by ARCHITECT (Abbott Diagnostics, Wiesbaden, Germany); repeatedly reactive (RR) samples were checked by respective confirmatory tests. The relationship between sample/cutoff and confirmed seropositivity were analyzed., Results: RR rates were low as expected in blood donors: 0.47% for syphilis, 0.42% for HBV, 0.50% for HCV, and 0.15% for HIV. The specificity on RR + gray zone (GZ) was 99.67%, 99.79%, 99.77%, and 99.88%, respectively; due to the low prevalence, PPV value was 30.6% for syphilis, 50.7% for HBV, 42.2% for HCV, and 18.5% for HIV. These values increased substantially reaching a plateau of 89.3% for syphilis, 94.6% for HBV, 85.7% for HCV, and 100% for HIV at the threshold established by receiver operating characteristics curve analysis., Conclusions: Supplemental testing on samples with high signal by screening assays seems to add little information. GZ settings and confirmatory testing for positive screening results should be designed taking in account several factors, including difference in the natural history among blood-borne infections, the characteristics of first- and second-level tests, and, when available, the results of nucleic acid amplification testing., (© 2014 Wiley Periodicals, Inc.)

Recently enzyme immobilization techniques have been proposed that are mainly founded on the formation of an enzyme-gel layer onto the active surface of an ultrafiltration membrane within an unstirred ultrafiltration cell. If the membrane molecular-weight cutoff is less than the enzyme molecular weight and hence such as to completely prevent enzyme permeation (once the enzyme solution has been charged into the test cell and pressure applied to the system), a time progressive increase in enzyme concentration takes place at the upstream membrane surface that can eventually lead to gelation and hence to enzyme immobilization. However, depending on the total enzyme amount fed, the maximum enzyme concentration achieved in the unsteady state could be less than the gelation level. In this situation, no immobilization occurs and the enzyme still remains in the soluble form although it is practically confined within a limited region immediately upstream the membrane and at fairly high concentrations. In this paper, the experimental conditions that allow gelling to occur are discussed together with a theoretical analysis of the soluble enzyme membrane reactor which is obtained when no gelling takes place. Such a system could be usefully employed in performing kinetic analyses at high enzyme concentration levels that are still in the soluble form.