Why pre-transfusion testing does not decrease the incidence of hepatitis?

Possible reasons for post transfusion hepatitis by CMV in spite of screening: CMV persists in blood monocytes and 50-60% of adults are lifelong carriers of the virus. Hence it can be transmitted by transfusion of cellular blood components. Transfusion transmitted CMV can cause problems in premature neonates, allograft recipients, splenectomized patients. Problem with Hepatitis B testing and our strategy to bypass it HBsAg is not detected during the window period between disappearance of HBsAg and appearances of IgM Ab HBs. Anti HBc IgM is detected in this window period which is hence used in screening methods.