Ideal tonicity of WBC diluting fluids?

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Tonicity of the diluting fluid is tested by how it affects cells during WBC counting. To get an accurate WBC count, you want red blood cells to be removed from view while white cells stay intact. A hypotonic diluting fluid causes red cells to take up water, swell, and lyse, leaving the white cells free to be counted. White blood cells are more resistant to this osmotic change and remain intact in the dilute environment, enabling a clearer, more accurate count. If the diluent were isotonic, red cells would stay present and interfere with counting; if it were hypertonic, both red cells and white cells could be affected, potentially shrinking or damaging them and skewing results. Isometric isn’t a standard term used to describe tonicity in this context.