- Leukapheresis This procedure for collecting larger numbers of circulating blood precursor cells is optional and may take the place of the larger blood draw described above. Patients 5 years old or older may have leukapheresis. Whole blood is collected through a needle in an arm vein. The blood circulates through a machine that separates it into its components. The desired cells are then removed and the rest of the blood is returned to the body, either through the same needle or through a second one placed in the other arm. The cells obtained will be used to purify blood precursors for growing in culture and to examine the ability to transfer new genes into these precursor cells. For patients whose arm veins are too scarred to for needle placement, a vein in the groin area (femoral vein) may be used instead.