The risks of transfusion-transmitted infection: direct estimation and mathematical modelling Steven H. Kleinman MD Clinical Professor of Pathology University of British Columbia, Vancouver, BC, Canada Michael P. Busch* MD, PhD Vice President Research and Scienti®c Aairs Blood Centers of the Paci®c, San Francisco, CA, USA and Professor of Laboratory Medicine University of California, San Francisco, USA Direct measurement of the risk of transfusion-transmitted infection (TTI) is practical and accurate only if the level of risk is high. Historically, studies that established frozen repositories of transfusion recipient and/or blood donor samples were important in establishing the risk of many TTI agents, including the human immunode®ciency virus (HIV), hepatitis B virus (HBV) and hepatitis C virus (HCV). However, given the current very low risk of TTI, mathematical modelling is necessary to estimate the magnitude of such a risk. For agents for which routine blood donor screening is performed, most of this risk comes from transfusion of units collected in the window period between donor infection and a positive blood screening assay. The incidence/window period model has been used to estimate the magnitude of such risks (of the order of 1 : 100 000 to 1 : 1 000 000) and for predicting the extent of risk reduction that can be expected with implementation of new tests. Direct estimation and mathematical modelling approaches are both important tools for future assessment of potential, new or emerging TTI agents. Key words: transfusion-transmitted infection; HIV; hepatitis. Accurate estimates of the risks of transfusion-transmitted infection (TTI) are important for several reasons: to reassure the public about the safety of transfusion, to provide an informed basis for patients and physicians to decide on allogeneic transfusion versus other therapeutic options, and to enable policy makers to evaluate the expected bene®t and costs of proposed interventions to reduce such risk further. 1521±6926/00/04063119 $12.00/00 * c 2000 Harcourt Publishers Ltd. BaillieÁre's Clinical Haematology Vol. 13, No. 4, pp. 631±649, 2000 doi:10.1053/beha.2000.0104, available online at http://www.idealibrary.com on 9 *Address for correspondence: Blood Centers of the Paci®c, Irwin Center, 270 Masonic Avenue, San Francisco, CA 94118, USA.