And changing the liberal use of transfusions is going to be difficult despite the evidence showing it is usually not essential.'

Blumberg and co-author Mary Rogers analysed patient records in 40 hospitals, from admission to 30 days after discharge.

All had received coronary artery bypass graft surgery from 2003 to 2006. They found that 30 percent of variation in transfusion practices seemed to be due to widely varied practices among hospital sites.

Also, blood use among women patients ranged from 72.5 percent to 100 percent, and blood use among men varied from about 50 percent to 100 percent.

Transfusions with donor blood were associated with infections of the genitourinary system, respiratory tract, bloodstream, digestive tract and skin, the study said.

The study was published in BMC Medicine.