[1] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Int Hlth, Baltimore, MD USA
[2] Johns Hopkins Univ, Dept Med, Div Hematol, Sch Med, Baltimore, MD 21287 USA
[3] Johns Hopkins Univ, Dept Pathol, Sch Med, Baltimore, MD 21287 USA
[4] Amer Red Cross, Div Biomed Serv, Baltimore, MD USA
[5] Johns Hopkins Univ, Dept Med, Div Gen Internal Med, Sch Med, Baltimore, MD 21287 USA