What gap is abnormal?

Often >10 is notable and ≥20 more concerning, depending on lab normals.

Serum Osmolality & Osmolar Gap

Nephrology Critical Care Emergency Medicine

Medical context

Calculate serum osmolality and optional osmolar gap for toxic alcohol and hyperosmolar workups.

Nephrology and medicine teams use this when interpreting renal function, electrolytes, or fluid plans.

This page explains the Osmolality formula, how to interpret the output, known limitations, and peer-reviewed references so you can verify results against primary sources and local protocols.

It is grouped under Nephrology because kidney function, electrolytes, and renal dosing helpers.

How to use this calculator

Enter the values requested by the Osmolality calculator using the units shown beside each field.
Review the calculated result and read the interpretation section before acting on the output.
Check limitations and references if the result will change management, dosing, or disposition.
Use the related calculators section when you need supporting values (for example electrolytes, weight, or complementary risk scores).

Calculator

Sodium (mEq/L)Glucose (mg/dL)BUN (mg/dL)Ethanol (mg/dL)OptionalMeasured osmolality (mOsm/kg)Optional for gap

Result

290.6 mOsm/kg

Calculated serum osmolality

2×Na + glucose/18 + BUN/2.8 (+ ethanol/4.6). Gap = measured − calculated.

Educational use only. Confirm results with clinical judgment, primary sources, and local protocols. Not a substitute for professional medical advice. Full disclaimer.

Formula

Calculated = 2×Na + glucose/18 + BUN/2.8 (+ ethanol/4.6). Gap = measured − calculated.

Interpretation

Elevated gap raises concern for alcohols, acetone, or other osmoles — integrate with clinical picture.

Limitations

Units and ethanol terms vary; confirm lab methods.

Frequently asked questions

What gap is abnormal?: Often >10 is notable and ≥20 more concerning, depending on lab normals.
Is the Osmolality result enough to make a treatment decision?: No. Calculators support medical decision-making but do not replace examination, shared decision-making, institutional protocols, or consultant input when stakes are high.
Does Calcmedical store the values I type in?: Inputs are processed in your browser for this session. We do not ask for patient names or medical record numbers. See our privacy policy for hosting and analytics details.

References

Classic osmolar gap toxicology approach