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Every day in the USA, an estimated 50 million complete blood counts (CBC) are ordered clinically. These are ordered as part of annual physical exams, for cancer screening and management, or for diagnosis of infection or allergy. Today, CBCs are automated, although visual examination of stained blood smears is still used for identification of specific disease pathologies. Each automated CBC cytometer is uniquely calibrated, and that machine's reference range for each cell type is reported alongside the patient's cell count.
Despite the fact that circulating leukocytes fall and rise predictably with circadian cortisol cycles as shown in figures 1 and 2, CBC results are compared to broad reference ranges that encompasses this temporal variation as well as normal variation among healthy adults.1––4 The crudity of broad reference ranges is well recognised and consequently, inpatient CBC monitoring protocols minimise both sources of variation: blood is drawn at the same time each day and the trend in …