Accuracy of Immunohistochemistry and Flow Cytometry in Estimating the Proportion of Leukemic Cells in S Phase in Chronic Myeloid Leukemia Patients

We compared the proportion of S phase cells in bone marrow and peripheral blood samples obtained from 17 patients with chronic myeloid leukemia (CML). Before sampling all patients received a one hour IV infusion of iododeoxyuridine (IdUrd). The proportion of S phase cells was studied by immunohistochemistry (IHC) in bone marrow biopsies, and by flow cytometry (FCM) in bone marrow aspirates and peripheral blood samples. The IdUrd labelling index (LI) in bone marrow biopsy sections (27.5 ± 1.8%) was significantly higher than the proportion of IdUrd labelled cells in bone marrow aspirate (15.1 ± 2.0%). The percentage of S phase cells in peripheral blood was approximately the same as that in the aspirate (12.4 ± 1.3%) and was correlated with that of bone marrow aspirate indicating a high degree of the aspirate dilution by peripheral blood. It is likely that the differences in % S phase cells in the aspirate and the biopsy result from this dilution. Estimates of the % S phase cells in the peripheral blood study by IHC and FCM were essentially the same. Samples labelled for one hour in vitro resulted in 1.5 fold higher LI than the same samples labelled in vivo. We conclude that estimates of the 8% S phase cells in the bone marrow of patients with CML should be made by infusing patients with IdUrd or BrdUrd with immunohistochemical evaluation of a marrow biopsy. Additionally in vitro labelling is not reflective of the percent S phase cells in vivo in patients.