Transfusion-associated transmission of West Nile virus--Arizona, 2004.

Blood transfusion-associated transmission (TAT) of West Nile virus (WNV) in the United States was first identified in 2002. In 2003, blood collection agencies (BCAs) responded by screening donations for WNV by using nucleic acid-amplification tests (NATs). The majority of BCAs use a two-tiered NAT-screening algorithm. On the basis of the test manufacturer's format, NATs are conducted on minipools of samples from either six or 16 blood donations. If a minipool is nonreactive, its constituent donations are released for transfusion. If a minipool is reactive, the constituent donations undergo individual testing. If an individual donation is reactive, associated blood components are impounded, and the donor is notified for further testing to confirm the infection. In 2003, blood-donation screening for WNV resulted in the impounding of approximately 800 blood components potentially containing WNV. However, six reported cases of transfusion-associated WNV disease were associated with units of blood components with viral concentrations too small to be detected by minipool NAT. In 2004, to improve the sensitivity of WNV screening, BCAs implemented systems to trigger a switch from minipool NAT to individual NAT in areas with epidemic WNV transmission. This report describes the first transfusion-associated WNV infection identified in 2004; the implicated blood donation was collected before the switch to individual testing. Clinicians should remain aware of the risk for WNV transmission through blood-product transfusion and alert state health officials to hospitalized patients with WNV disease symptoms who have had a transfusion during the preceding 28 days.