Multiple screening tests are performed on each donation to detect the presence of transfusion-transmissible infectious agents. The screening tests performed may detect antigens, antibodies or nucleic acids of the infectious agents. Table 1 provides the screening and confirmatory tests performed at Canadian Blood Services.

Antibody and antigen tests are done on individual donor samples while nucleic acid testing (NAT) is primarily done on pools of six samples.5, 6 The multiplex assay used for NAT enables the simultaneous detection of HIV RNA, hepatitis C virus (HCV) RNA and hepatitis B virus (HBV) DNA.

You forgot an important part of that paragraph there. Sure I'm a factor of five off but it still describes the point. That is how blood is tested. That's why if any one of those six samples have either HIV, Hep C or Hep B they have to do the whole thing all over again.