Screening blood for West Nile virus

A recent mandate to screen blood donations in all US states for West Nile virus (WNV) makes little sense from a public health point of view, say researchers from Columbia University and Harvard School of Public Health.

And while this is not yet the time of year to worry about mosquitoes, it seems a good time to take a look at the current policy on screening blood products for WNV, and to ensure that the best policy is in place for the inevitable 2006 season.

WNV can be detected in blood samples by recently developed and approved tests. These tests detect some but not all cases of contamination with the virus. This means that WNV infections that could result from contaminated blood products are potentially avoidable by screening donated blood. However, the majority of WNV infections are asymptomatic, so screening may have a small impact on clinical outcomes. The US Food and Drug Administration (FDA) has mandated screening of donated blood samples. However, the FDA has not prescribed specific screening strategies, and the decision on how to best screen blood samples has been left to the states and the blood collection agencies.

Caroline Korves (of Columbia University) in collaboration with Megan Murray and Sue Goldie (both at Harvard School of Public Health) have done a cost-effectiveness analysis of different screening strategies to see which of them would prevent cases through contaminated blood for a reasonable price. (In an ideal world, cost would not matter when it comes to protecting human life and health, but in reality there is limited money available for public health measures. Studies such as this one are therefore essential to help politicians decide how to spend their budget wisely.)

The analysis, published in the international open-access medical journal PLoS Medicine, revealed that in states with low WNV infection rates, the risk of an infected person donating blood was so low that screening was unlikely to prevent any cases of serious illness from WNV, despite very large costs. In states with intense WNV transmission, screening is likely to prevent cases of serious illness, but at a substantial cost. Where WNV is common, screening blood only from May to November (during the mosquito season) was as effective at identifying contaminated blood samples as screening throughout the year. These results suggest that a modification of the current policies would save large costs without significantly increasing the risk for blood recipients.

Korves and colleagues also propose and analyse an interesting alternative scenario: It turns out that one way to reduce costs dramatically and keep the risk of serious disease in blood recipients low would be to create a separate pool of blood reserved for those with a weakened immune system, and to screen only those samples. Given an increasing number of people living with compromised immune systems (recipients of organ transplants, patients with HIV/AIDS, and others), the idea of creating a 'supersafe' pool of blood products that is reserved for people who are less able to fight off infections contracted through contaminated blood seems well worth considering.

In an accompanying commentary, Bruce Lee (of the University of Pittsburgh) and Brad Biggerstaff (of the US Centers for Disease Control and Prevention) say that "The threat of disease certainly still exists, but with limited resources and other potential hazards to blood supply safety, there has been a real need for good, objective economic studies to determine what type and what degree of screening should be performed. "

Source : Public Library of Science