This is something I hadn't heard of and is not in our SOPs; we have a Sysmex. Apparently, the patient RBCs are diluted by the instrument with an isoosmotic diluent to quantitate for MCV. However, a patient with hyponatremia has an osmotic gradient that allows water to flood the RBCs to establish osmotic equilibrium when the dilution is performed on board. This does not reflect the true MCV in vivo, hence a dilution is performed before running on the instrument. We have a few patients in house with hyponatremia, so I'll be looking to see what our Sysmex manual says about this. That's a great point; we don't want to go "off label" with this practice. Thanks for the info, everyone!
Here is a great article I found on this:
Jens Peter Philipsen & Kirsten Vikkelsø Madsen (2015) Hypo- and hypernatremia results in inaccurate erythrocyte mean corpuscular volume measurement in vitro, when using Sysmex XE 2100,Scandinavian Journal of Clinical and Laboratory Investigation, 75:7, 588-594, DOI: 10.3109/00365513.2015.1062534