Because the establishment and testing of an EBV transformed B cell line suitable for use or as an antigen-presenting cell require 4-5 weeks of in vitro culture, it is important that this sample be obtained as early as possible for patients at risk for a CMV infection. Because patients receiving HSCT from unrelated or HLA disparate donors are particularly at risk for severe CMV infections in the first 2-3 months after transplant, this blood sample should be obtained from the donor prior to donation of the hematopoietic progenitor cell transplant whenever possible.