Because of hyperglycemia-induced osmotic diuresis and natriuresis, patients with diabetic ketoacidosis usually present with a marked contraction of the extracellular fluid volume. This factor affects the assessment of their acid-base status and in some cases their therapy. Determination of the severity of metabolic acidemia is usually based on the extent of the decrease in the plasma bicarbonate concentration. Nevertheless, as shown in the equation below, the plasma bicarbonate concentration may be only moderately reduced when there is both a large deficit of bicarbonate in the extracellular fluid and a severe contraction of the volume of extracellular fluid.
Extracellular fluid bicarbonate concentration [HCO3−] = extracellular fluid HCO3− content (divided by) extracellular fluid volume.