A GENERAL-METHOD FOR CONCENTRATING BLOOD-SAMPLES IN PREPARATION FOR COUNTING VERY-LOW NUMBERS OF WHITE CELLS

BACKGROUND: To count extremely low levels of white cells (WBCs) in WBC -reduced blood components, a larger volume of sample must be processed . The goal was to develop an all-purpose method for concentrating the samples obtained from WBC-reduced red cells or platelets. The method w as designed to be compatible with a variety of counting techniques. ST UDY DESIGN AND METHODS: Coded samples of red cell concentrates with an expected WBC concentration of 200, 100, 50, and 10 per mt and of the diluent (undetectable WBCs/mL) were sent to three sites on five occasi ons and counted by the use of the concentration method, crystal violet stain, and a Nageotte counting chamber. Additional samples were teste d by flow cytometry, polymerase chain reaction, and volumetric capilla ry cytometry. RESULTS: The results from the three test sites showed go od linearity, with an overall r(2) = 0.9994. The lower limit of accura te detection of the assay was 10 WBCs per mt. The results were biased toward underestimation, particularly at one of the test sites (Site A) . There were no significantly different results in Sites B and C. The intra-assay CV was acceptable. Precision (reproducibility) at the thre e test sites varied. CONCLUSION: This method allows reliable determina tion of WBC concentrations as low as 0.01 per mu L in blood. Despite t he use of technologists trained in Nageotte chamber counting, validati on testing demonstrated that one test site's performance was significa ntly different from that of the other two sites, because of both under estimation bias and variation in count results. The sample concentrati on method, when used in conjunction with an automated assay for WBC id entification, she-lid permit larger volume analysis with a greater deg ree of precision and a lower limit of detection than is found in assay s that do not concentrate the sample before counting.