Measurement of residual leukocytes in platelet concentrate units.

Leukocyte contamination of various blood products is implicated in febrile transfusion reactions, in alloimmunization phenomena and immunosuppression, and in the transmission of viral infections. The biggest difficulty in interpreting published reports lies in the methods used to quantify residual leukocytes. As automated cell counters are not precise and sensitive enough to detect low cell counts, we compared two techniques: i) the classical method using the Nageotte haemocytometer; and ii) fluorescent staining with propidium iodide (PI). We performed these two techniques to evaluate the residual white cell count in 41 platelet units collected by cytapheresis. The difference in results obtained with the two techniques was borderline significant (P = 0.05). To evaluate the accuracy of both techniques, we added a known number of lymphocytes to platelet concentrates that had been filtered three times. There was a good correlation between the values obtained using the two techniques (r > 0.99). However, though the difference between the known value and PI results was not significant, the difference for Nageotte cell counter results was significant (P < 0.0001). From a practical point of view, the PI fluorescent technique is simple, rapid, and easy to carry out. Our results demonstrate the reliability of the PI technique for the evaluation of residual leukocytes.