The effect of red cell shape on the measurement of red cell volume. A proposed method for the comparative assessment of this effect among various haematology analysers

Shape changes of abnormally deformed red cells in aperture impedance haematology analysers are known to affect MCV, MCHC and haematocrit estimation. However, different counters vary in the manifestation of this effect. We performed a comparative study among five analysers. Three of them are based on impedance without hydrodynamic focusing (Coulter STKR, Cell-Dyn3000 Abbott and K-1000 Sysmex). The other two use hydrodynamic focusing, either with impedance (NE-8000 Sysmex) or two angle laser scatter (H*1 Bayer). A novel method of analysis was applied. Two hundred and three specimens with abnormal red cells and 50 normal specimens (according to ICSH criteria) were assayed. In all samples the PCV was estimated by the reference method without correction for trapped plasma. A true MCHC value was estimated from the mean haemoglobin value and the PCV. The shape effect was assessed by three linear regressions: 1) haematocrit deviations from PCV (corrected for any calibration bias) versus true MCHC; 2) analyser MCHC vs. true MCHC; 3) MCV vs. MCH. The regressions for the analysers with hydrodynamic focusing indicated no significant shape effect. Aperture impedance analysers without focusing varied in their behaviour. The Coulter STKR and the Cell-Dyn3000 both showed strong correlation of haematocrit deviations with true MCHC, poor MCHC correlations and linear MCV-MCH regressions. The K-1000 showed minor indications of such an effect. We conclude that comparative studies are needed to quantitate red cell shape effect errors among various impedance analysers.