Hypo - and hypernatremia results in inaccurate Erythrocytes mean Corpuscular Volume (MCV) measurement in vitro, when using Sysmex XE 2100

Introduction: Automated hematology analyzers dilute patient erythrocytes with an isosmotic diluent before quantitating the erythrocyte Mean Cell Volume (MCV). However, if patient plasma osmolality differs from the diluent, water will cross the erythrocytes membrane and establish a new equilibrium across the membrane. Since the new equilibrium is reached before the measurement of the MCV, the measured MCV may not reflect the true MCV in vivo.
Aim: Calculation of the theoretically change in MCV at changed P-Sodium/P-Osmolality and to investigate if the automated blood cell counter Sysmex XE 2100 measures MCV correctly in hypo- and hyperosmolality and hypo-and hypernatremia. In addition, to examine whether the theoretically calculated change in MCV corresponds with the experimentally determined MCV change.
Method: Theoretical calculation of the MCV inaccuracy at hypo- and hypernatremia, as well as at hypo- and hyperosmolality. Experimental studies with comparison of MCV measured at Sysmex XE 2100 to MCV found by using the manual measured packed cell volume method.
Results and conclusion: Measurement of MCV in hypo- and hypernatremia patients using the automated blood cell counter Sysmex XE 2100 resulted in inaccurate MCV. The experimental results also revealed a strong correlation between P-Osmolality/P-Sodium and MCV inaccuracy (R2= 0.70/0.85) similar to the theoretically calculated MCV inaccuracy. We suggest using MCH instead of MCV, MCHC and EVF. Alternatively, we suggest standardizing the measured MCV to a normal P-Sodium e.g. 140 mmol/L to estimate the in vivo MCV, using the following equation: MCVcorrected Na+=140mmol/L = MCVmeasured – (P-Sodium-140)*0.445 mmol/L.