Interlaboratory variation of leukocyte differential counts: results from the Finnish proficiency testing programme in haematological morphology, 1974-1977.

The interlaboratory variation of leukocyte differential counts in the Finnish proficiency testing programme in haematological morphology is presented. During 1974-1977 altogether twenty-four different blood smears with neither leukocytosis nor abnormal leukocytes were examined by the participants. During this period 206 laboratories participated in the voluntary programme. The interlaboratory variation of leukocyte differential counts was analyzed for band neutrophils, segmented neutrophils, eosinophils, basophils, lymphocytes and monocytes. Except for band neutrophils and monocytes the observed variations were found to closely match the known random sampling variation which is inherent in the method for estimating the proportions of the various leukocytes of peripheral blood. It is suggested that the results point to variations in the definitions to differentiate band vs. segmented neutrophils and monocytes vs. lymphocytes. Interlaboratory proficiency testing is considered to offer an effective means to investigate the quality of morphologic analyses as practised on a day to day basis in the participating laboratories.