Corrected Sodium Calculator

Glucose is an osmotically active particle that, when elevated, pulls water out of cells and into the vascular space by osmosis, diluting the measured serum sodium without any actual change in the body's total sodium content. This means a diabetic patient in hyperglycemic crisis can show a falsely low sodium on a basic metabolic panel even though their sodium homeostasis is otherwise intact — a distinction that matters because true hyponatremia and glucose-diluted pseudo-hyponatremia call for very different management. Correcting for this dilution is essential in diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS), where both severe hyperglycemia and genuine free-water losses can coexist and need to be teased apart. The classic Katz correction adds 1.6 mEq/L to the measured sodium for every 100 mg/dL that glucose exceeds the normal 100 mg/dL reference. A widely cited but empirically-derived revision by Hillier and colleagues found the true dilutional effect is closer to 2.4 mEq/L per 100 mg/dL, particularly at the very high glucose levels (well above 400 mg/dL) typical of severe DKA and HHS, where the simpler Katz coefficient tends to underestimate the correction. Clinicians use the corrected value, not the raw measured sodium, to judge free-water deficit and decide on the tonicity of replacement IV fluids as glucose is brought back down.

Morton Katz described the linear glucose-sodium correction formula in a short 1973 New England Journal of Medicine letter, drawing on the well-established physical-chemistry principle that osmotically active solutes redistribute water across cell membranes according to Van't Hoff's law. His empirical coefficient of 1.6 mEq/L per 100 mg/dL glucose above normal was derived from clinical observation and became the formula taught in medical education for the following several decades. Hillier, Abbott, and Barrett revisited the relationship in a 1999 American Journal of Medicine study using controlled hyperglycemia data, finding that the true dilutional slope was steeper than Katz's original estimate — closer to 2.4 mEq/L per 100 mg/dL — especially once glucose climbed into the severe hyperglycemia range seen in DKA and HHS. Both coefficients remain in clinical use today; many emergency medicine and endocrinology references now favor the higher Hillier factor for glucose values above roughly 400 mg/dL, while the original Katz factor is still commonly cited for milder hyperglycemia.