Serum Osmolality & Osmolal Gap Calculator

About the Serum Osmolality & Osmolal Gap Calculator

Serum osmolality is the concentration of all dissolved particles in blood. Most of it comes from sodium and its anions, glucose and urea, so it can be calculated from those three values — and comparing the calculated value with the laboratory’s measured osmolality gives the osmolal gap, which rises when an unmeasured substance such as ethanol, methanol or ethylene glycol is present.

This calculator gives the calculated osmolality by the Smithline–Gardner equation (2 Na + glucose + urea) or the Dorwart–Chalmers equation, the effective osmolality (tonicity, the part that moves water out of cells), the osmolal gap before and after the share of ethanol, and sodium corrected for a high glucose by the classic Katz factor and the Hillier factor. Glucose can be entered in mg/dL or mmol/L, urea as BUN in mg/dL, urea in mmol/L or urea in mg/dL, and ethanol in mg/dL or mmol/L. Nothing you type leaves your browser.

Without a pass, the free preview shows the calculated osmolality in full with what it means and every warning, and the first rows of the working table; a Pro pass unlocks the osmolal gap, the corrected sodium, every other figure and Copy result.

Common uses

• Checking a measured osmolality against the calculated one for an osmolal gap.
• Accounting for ethanol before reading the gap in a suspected poisoning.
• Working out effective osmolality and corrected sodium in very high glucose.
• Teaching how osmolality, tonicity and the osmolal gap are built.

The formulas

• Smithline–Gardner: 2 × Na + glucose ÷ 18 + BUN ÷ 2.8, with glucose and BUN in mg/dL, or 2 × Na + glucose + urea with all three in mmol/L. Urea in mg/dL (not urea nitrogen) is divided by 6.
• Dorwart–Chalmers: 1.86 × Na + glucose ÷ 18 + BUN ÷ 2.8 + 9 — of 13 published methods compared on 715 hospital sera, the one that came closest to the measured osmolality.
• Ethanol’s share (Purssell): ethanol (mg/dL) ÷ 3.7, or 1.25 × ethanol (mmol/L), added to the calculated osmolality.
• Effective osmolality (tonicity) = 2 × Na + glucose (mmol/L): urea is left out because it crosses cell membranes freely.
• Osmolal gap = measured osmolality − calculated osmolality.
• Sodium corrected for glucose = Na + factor × (glucose − 100) ÷ 100, with glucose in mg/dL: 1.6 mmol/L per 100 mg/dL by the classic Katz factor, or 2.4 by Hillier’s experimental study, which found 1.6 fitted up to 400 mg/dL and about 4 above it.

The usual range of measured serum osmolality is 275–295 mOsm/kg.

The osmolal gap and toxic alcohols

A gap above 10 mOsm/kg is the usual threshold to suspect an unmeasured osmole. In a study of patients tested for methanol and ethylene glycol, this threshold found every patient who needed dialysis but missed some who needed an antidote, and it was high in many patients who did not have a toxic alcohol level that needed treatment. Ethanol should be counted first: the Purssell factor 1/3.7 explained ethanol’s share best in emergency patients. In the same study the gap left after ethanol varied from about −18 to +18 mOsm/kg, so the gap is a screening clue — a normal gap does not rule out poisoning, and poisoning needs emergency care whatever the gap.

High glucose: HHS

In the hyperglycaemic hyperosmolar state the consensus report on hyperglycaemic crises requires all four of: a plasma glucose of 600 mg/dL (33.3 mmol/L) or more, an effective osmolality above 300 mOsm/kg (or a total of 2 Na + glucose + urea above 320), no significant ketones, and no acidosis (pH 7.3 or above and bicarbonate 15 mmol/L or above). The calculator shows a warning when the glucose and osmolality criteria are met; only a clinician can make the diagnosis.

Sources

• Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations — Clin Chem
• Purssell RA et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap — Ann Emerg Med
• Lynd LD et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning — BMC Emerg Med
• Katz MA. Hyperglycemia-induced hyponatremia — calculation of expected serum sodium depression — N Engl J Med
• Hillier TA et al. Hyponatremia: evaluating the correction factor for hyperglycemia00055-8) — Am J Med
• Umpierrez GE et al. Hyperglycemic crises in adults with diabetes: a consensus report — Diabetes Care
• Osmolality blood test — MedlinePlus: the normal range

Limitations

• Calculated osmolality is an estimate; only an osmometer measures osmolality.
• The osmolal gap is a screening clue: it cannot confirm or rule out poisoning, which needs emergency care.
• The sodium corrections are approximations, least certain at very high glucose.
• Accepts sodium 100–200 mmol/L, glucose 10–2,500 mg/dL (0.5–140 mmol/L), BUN 1–300 mg/dL, urea 0.3–110 mmol/L and a measured osmolality of 200–500 mOsm/kg.

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