Problems related to CPD preserved blood used for NEQAS trials in haematology.

A study of the results of the Belgian National External Quality Assessment Scheme (NEQAS) in haematology, revealed marked differences in MCV values obtained with instruments of different types. For this reason the citrate-phosphate-dextrose (CPD) preserved blood used as NEQAS material was analysed daily with a Coulter S-Plus IV and with a cross-calibrated Ortho ELT-800. The observed differences in MCV are apparently due to differences in measuring apparatus and also to the changing characteristics of the erythrocytes. Ageing of CPD blood during 1 to 4 days (postal delivery) influenced markedly the MCV as measured with the Ortho ELT systems. These time- and instrument-related factors influence the measurements, they interfere with the statistical evaluation of NEQAS results and can lead to errors in the evaluation of individual performers.