Serum Osmolality Calculator - Calculated Serum Osmolality and Osmolal Gap

A serum osmolality calculator is a clinical tool that turns four standard blood values into the calculated serum osmolality in mOsm per kilogram of water, and, when a measured value is provided, the osmolal gap that screens for unmeasured osmoles such as methanol, ethylene glycol, and isopropanol. The calculator applies the Purssell 2001 formula 2 times sodium plus BUN over 2.8 plus glucose over 18 plus alcohol over 3.7, with the 275 to 295 mOsm per kilogram reference window and the -14 to +10 mOsm per kilogram osmolal gap range.

• • Pre-rounds chart review: Enter the latest basic metabolic panel values and any serum ethanol before a shift so the conversation with the clinician starts from a published number.
• • Repeat scoring after a basic metabolic panel: Paste a fresh sodium, BUN, and glucose into the form and watch for movement between the reference range and the high or low bands.
• • Toxic alcohol workup: Add a measured value and the serum ethanol to compute the osmolal gap, the published screen for methanol, ethylene glycol, and isopropanol.
• • Walk-through of a worked example: Recheck the calculation with sodium 140, BUN 14, glucose 90, and alcohol 0 to see how the four routine inputs add up to 290 mOsm per kilogram.

When a measured value is added, the calculator also reports the osmolal gap. The Purssell 2001 ethanol term keeps the gap interpretable in an intoxicated patient.

A second clinical chemistry tool that pairs naturally with this result during the metabolic or toxic alcohol workup is the Plasma Osmolality Calculator, which uses the same 2 times sodium plus glucose over 18 plus BUN over 2.8 equation for a panel-only read without the ethanol term.

The calculator multiplies serum sodium by 2, divides blood urea nitrogen by 2.8, divides blood glucose by 18, divides serum alcohol by 3.7, and sums the four terms. When a measured value is provided, the calculator subtracts the calculated value from the measured value to produce the osmolal gap, the published screen for an unmeasured osmole.

• sodium: Serum sodium in mEq per liter. The 2 multiplier counts both the cation and its accompanying anion.
• bun: BUN in mg per deciliter. The 2.8 divisor converts the routine unit to mmol per liter.
• glucose: Blood glucose in mg per deciliter. The 18 divisor converts the routine unit to mmol per liter.
• alcohol: Serum ethanol in mg per deciliter. The 3.7 divisor is the Purssell 2001 conversion factor.
• calculatedOsmolality: Calculated value in mOsm per kilogram of water. Normal band 275 to 295 mOsm per kilogram.
• osmolalGap: Measured minus the calculated value, in mOsm per kilogram. Normal band -14 to +10 mOsm per kilogram.

The 2.8, 18, and 3.7 divisors convert the routine mg per deciliter units on US lab slips into mmol per liter.

According to Omni Calculator Serum Osmolality reference, the calculated osmolality uses the Purssell 2001 formula 2 times sodium plus BUN over 2.8 plus glucose over 18 plus alcohol over 3.7.

According to MedlinePlus Osmolality Blood Test, the normal osmolality is 275 to 295 mOsm per kilogram and the test is ordered for low blood sodium, water loss, or suspected poisoning from ethanol, methanol, or ethylene glycol.

When the same basic metabolic panel you used here also printed sodium, chloride, and bicarbonate, the Anion Gap Calculator reads those values to flag acid-base disturbances that often accompany a high or low band.

Four concepts drive the result. None is a stand-alone diagnostic criterion, but together they explain why the same numbers move the band in opposite directions during dehydration, intoxication, and liver failure.

These four concepts are the framework most emergency medicine texts use to teach the result, and they map directly to the three band labels the calculator returns.

Because the BUN term carries the most weight among the kidney-derived solutes in the formula, the BUN Creatinine Ratio Calculator is a useful companion when you want to see whether the same BUN value is also driving a prerenal or intrinsic renal pattern.

The form works from a small set of basic metabolic panel values and two optional values. Each input should come from the same draw when possible.

1. 1 Enter the serum sodium: Type the sodium from the latest basic metabolic panel, in mEq per liter. Most adult reports list a reference range of 135 to 145 mEq per liter.
2. 2 Enter the blood urea nitrogen: The basic metabolic panel usually lists this as BUN. Adult range 7 to 20 mg per deciliter.
3. 3 Enter the blood glucose: The basic metabolic panel usually lists this as glucose. Adult fasting range 70 to 99 mg per deciliter.
4. 4 Enter the serum alcohol if available: Add the serum ethanol from a toxicology panel, in mg per deciliter, to apply the Purssell 2001 ethanol term.
5. 5 Enter the measured value if available: Add the measured value, in mOsm per kilogram, to turn on the osmolal gap.
6. 6 Read the calculated value and the bands: The result panel shows the calculated value in mOsm per kilogram, the band label, the osmolal gap, and a one line clinical interpretation.

A patient with sodium 152, BUN 28, glucose 110, alcohol 0, and measured osmolality 340 enters those five numbers and gets a calculated value of 320 mOsm per kilogram, a high band label, and an osmolal gap of 20 mOsm per kilogram in the elevated band.

A bedside kidney function review that uses the same lab panel and pairs naturally with the high band pathway is the GFR Calculator, which turns serum creatinine, age, and sex into a single kidney function number for the same metabolic workup.

Using a calculator offers several practical advantages over mental math during a fast-moving chart review or a toxic alcohol workup.

• • Standardized mOsm per kilogram unit: The calculator returns the calculated value in mOsm per kilogram of water, the unit used in every published fluid and electrolyte guideline.
• • Purssell 2001 ethanol term: The optional alcohol input applies the published 3.7 divisor so the osmolal gap stays interpretable in an intoxicated patient.
• • Quick bedside math: All four routine inputs come from a basic metabolic panel plus an optional ethanol value, so the result can be computed in under a minute.
• • Transparent calculation: The formula is shown in plain English, so the contribution of sodium, BUN, glucose, and ethanol is visible.
• • Built-in high, normal, and low bands: The result is paired with the published 275 and 295 mOsm per kilogram cut-offs and the -14 to +10 mOsm per kilogram osmolal gap range.
• • Optional osmolal gap: The osmolal gap is shown whenever the measured value is entered, the published screen for an unmeasured osmole.

The same calculated value is used in emergency, inpatient, and outpatient settings, so it serves as a shared language across the clinical team and reduces the chance of miscommunication between shifts.

A second bedside tool that uses the same basic metabolic panel and pairs naturally with this result is the Acid Base Calculator, which turns an arterial blood gas into a pH, PCO2, and bicarbonate reading for the acid-base assessment that often follows a high or low band.

The output depends on the four entered values. Five variables move the result up or down independent of true osmolality.

• • The calculated value is a screening tool, not a stand-alone diagnosis. The blood gas, measured value, serum ethanol, toxicology panel, and a clinical exam are still required.
• • Children, pregnant patients, and older adults can have atypical patterns, and the reference window is calibrated for the general adult population.

An osmolar gap above 10 mOsm per kilogram is the published trigger for an unmeasured-osmole workup, so a wide gap on this calculator should be paired with a methanol, ethylene glycol, isopropanol, and salicylate panel.

According to Merck Manuals Water and Sodium Balance, plasma osmolality is normally between 275 and 290 milliosmoles per kilogram, and an osmolar gap of 10 milliosmoles per kilogram or more is caused by unmeasured osmotically active substances such as ethanol, methanol, isopropanol, and ethylene glycol.

When the band is borderline and the serum ethanol is doing most of the work, the Alcohol Units Calculator helps you see whether the same ethanol exposure is also driving an alcohol unit count for the same intoxicated patient workup.