<qa>  What is the benefit of double erythrocytapheresis?The project will determine the advantages of double erythrocytapheresis in the collection of preoperative autologous erythrocytes as compared to the standard collections of whole blood. The study will assess the total reduction in the number of procedures required to obtain the preoperative units ordered by the surgeon. Also, successfulness of erythrocytaphereses versus classical whole blood collection for these patients will be compared. 