Abstract
In a diagnostic laboratory performing analyses for about 40 hospitals and 2,000 physicians treating outpatients results of HIV 4th generation combined antibody/antigen screening assays were monitored over a period of 50 months. In period A (Jan 2007 - Mar 2009) 37,986 serum samples were examined by Architect and in period B (Apr 2009 - Feb 2011) 38,178 samples by Modular system. In period B1 (Apr 2009 - Jun 2010) 24,756 samples were analyzed only by Modular system while in period B2 (July 2010 - February 2011) 13,422 samples were examined in parallel by Modular and Axsym system. Sensitivity and negative predictive value of each was 100%. Specificities ranged from 99.69-99.88% and positive predictive values (ppv) from 35.1-65.9%. Architect test results obtained a better reliability than Modular test results while Axsym test results were similar to that of Architect system. However, specificity and ppv of Modular system was markedly improved in period B2. In summary our study shows that long term monitoring of HIV combined antibody/antigen screening test results allows discovering of impairment and improvement of HIV testing quality. We also show that in a low prevalence region specificities of > 99% are accompanied by relatively low ppv. Increase of cut off values to define reactivity of the tests will increase specificities and ppv without affecting sensitivity.
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