We report a case of a falsely increased mean corpuscular volume (MCV) due to severe hyperglycaemia in a patient with diabetic ketoacidosis. This phenomenon results from in vitro swelling of hyperosmolar red blood cell size when diluted in an iso-osmolar buffer of a haematology analyser, and does not reflect a true macrocytosis in vivo. The magnitude of this effect is dependent on the glucose concentration of the sample and time of incubation prior to analysis. Haematology analysers from three different manufacturers were found equally sensitive to this phenomenon. Therefore, it is suggested to use reluctance when reporting and interpreting MCV results in the case of severe hyperglycaemia to avoid unnecessary additional investigation.

(BELG J HEMATOL 2019;10(6):250–4)