Significance of low peripheral blood CD34+ cell numbers prior to leukapheresis: What should the threshold required for apheresis be?

The pre-apheresis peripheral blood CD34+ cell count (PBCD34) is the strongest predictor of the number of stem cells collected. It is common practice to apherese only when PBCD34 is ≥5/μl to avoid poor yields. However, in patients with compromised marrow function, the only way to get enough cells is to collect small numbers daily several times. Another pitfall with this practice is not accounting for weight. We studied 285 autologous collections performed with PBCD34 <5/μl. The blood volume processed was 15l (n=68) or 20l (n=217). A collection of ≥0.3 × 10⁶ CD34+ cells per kg ideal body weight (IBW) was considered acceptable as this daily yield over a week could total ≥2 × 10⁶/kg-enough for an autograft. Two hundred and four harvests (72%) had yields of ≥0.3; with 59% being ≥0.4, 47% being ≥0.5 and 36% being ≥0.6. There were 23 patients whose harvests done when PBCD34 was low totaled ≥2 × 10⁶/kg. Assuming a 20-l apheresis with 70% collection efficiency, we suggest harvesting if 14n/w ≥0.3, where n=PBCD34 and w=IBW. This approach is more logical than using an absolute threshold, and will ensure that patients are not deprived of the opportunity of stem cell collection and autotransplantation.