Flow cytometer for reference measurements of blood cell concentrations with low uncertainty

A method is described to determine cell concentrations for erythrocytes and leukocytes with typical uncertainties of 0.8% and 2%, respectively. The unique method is designed in such a way that results are traceable to SI derived units. The sample suspensions are prepared under gravimetric control. The sample volume measured in cell counting is determined by weighing, including separate density measurements. Random coincidences are a significant challenge in particle counting leading to underestimated cell concentrations. In the method described here this challenge is met by recording the integrated dead time in particle counting and applying dilution series. This method is now routinely applied to provide reference measurement values for external quality assurance of German hematology laboratories for the complete blood count.