White blood cell counts may be done either manually using a hemocytometer or with automated cell counters. The error produced by hemocytometer counts is about 4%-8% but may be higher with inexperienced personnel. Automated counters have approximately 2%-4% error. The machine has the disadvantage that WBC counts more than 100,000/mm3 become increasingly inaccurate unless a dilution is used. In addition, some of the abnormal lymphocytes of lymphocytic leukemia are exceptionally fragile and may be destroyed when the specimen is prepared for a machine count, thus giving a reading falsely lower than the true value. With either hemocytometer or machine counting, nucleated RBCs are counted as WBCs, and a correction must be made on the basis of the percentage of nucleated RBCs (to 100 WBCs) found on the peripheral smear.
Reference values are most often quoted as 5,000-10,000/mm3(5-10 Ч 109/L). Several studies suggest that 4,500-11,000/mm3 would be more correct. However, there is a significant overlap of normal and abnormal between 4,500-5,000/mm3 and 10,000-11,000/mm3, especially in the latter area. There is some evidence that the normal range for African Americans may be at least 500/mm3 (.5 Ч 109/L) lower than the normal range for Europeans.