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The average per-child cost for antihemophilic factor was about $1.3 million over the time span covered by the study.

One type of therapy, called antihemophilic factor, accounted for 98 percent of spending on children with hemophilia, or 41 percent of total pharmacy spending -- even though children treated with antihemophilic factor accounted for just 0.4 percent of all the children in the study.

Antihemophilic factor is a protein that is essential to normal blood clotting and is lacking or deficient in persons having hemophilia A, the authors explained.