Plasma Osmolality Calculator - Calculated and Osmolal Gap

A plasma osmolality calculator turns a basic metabolic panel into the calculated plasma osmolality, the standard first step in the workup of dehydration, hyperglycemia, hyponatremia, and a suspected toxic alcohol ingestion. The calculator takes serum sodium in mEq per liter, blood glucose in mg per deciliter, and blood urea nitrogen in mg per deciliter and returns a single calculated plasma osmolality in mOsm per kilogram with a high, normal, or low band from the published 275 to 295 mOsm per kilogram reference window.

• • Pre-rounds chart review: enter the latest basic metabolic panel values before a shift so the conversation with the clinician starts from a published osmolality number.
• • Repeat scoring after a basic metabolic panel: paste a fresh sodium, glucose, and BUN into the form and watch for movement between the reference range and the high or low plasma osmolality bands.
• • Toxic alcohol workup: add the measured plasma osmolality 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, glucose 90, and BUN 14 to see how the three routine inputs add up to a normal band.

The calculator is most useful when all three routine inputs are recent and come from the same basic metabolic panel. Mixing a sodium from one draw with a glucose from another draw is a common source of error.

When the measured plasma osmolality is added, the calculator also reports the osmolal gap, the published screen for an unmeasured osmole such as methanol, ethylene glycol, or isopropanol. The ethanol adjusted osmolal gap refines the screen in an intoxicated patient.

A second lab-to-single-number pattern that pairs naturally with the plasma osmolality during a metabolic or toxic alcohol workup is the Anion Gap Calculator, which turns the same basic metabolic panel into a serum anion gap with a published high, normal, or low band.

The plasma osmolality calculator multiplies serum sodium by 2, divides blood glucose by 18, divides blood urea nitrogen by 2.8, and sums the three terms to get the calculated plasma osmolality in mOsm per kilogram. When a measured plasma osmolality is provided, the calculator subtracts the calculated value from the measured value to produce the osmolal gap, and optionally subtracts the ethanol contribution to produce the ethanol adjusted osmolal gap.

• sodium: serum sodium from the basic metabolic panel, in mEq per liter.
• glucose: blood glucose from the basic metabolic panel, in mg per deciliter.
• BUN: blood urea nitrogen from the basic metabolic panel, in mg per deciliter.
• calculatedOsmolality: calculated plasma osmolality in mOsm per kilogram, equal to 2 times sodium plus glucose divided by 18 plus BUN divided by 2.8.
• osmolalGap: measured plasma osmolality minus calculated plasma osmolality, in mOsm per kilogram. The published normal band is less than 10 mOsm per kilogram.
• ethanolAdjustedGap: osmolal gap minus ethanol divided by 4.6, in mOsm per kilogram.

The 18 and 2.8 factors convert the routine mg per deciliter units on a US lab slip into the mmol per liter units that count toward osmolality.

The osmolal gap is a screen for an unmeasured osmole rather than a diagnosis, and the published normal band of less than 10 mOsm per kilogram is the trigger for an expanded toxicology workup.

According to MedlinePlus Osmolality Blood Test, plasma osmolality is normally 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.

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

Four concepts drive the result.

A second clinical chemistry tool that pairs with the osmolal gap during a toxic alcohol evaluation is the BAC Calculator, which turns a drinking history into an estimated blood alcohol percentage for the same intoxication workup.

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

1. 1 Enter the serum sodium: type the sodium from the latest metabolic panel, in mEq per liter. Most adult reports list a reference range of 135 to 145 mEq per liter next to the result.
2. 2 Enter the blood glucose: the basic metabolic panel usually lists this under the glucose field. A typical adult fasting range is 70 to 99 mg per deciliter.
3. 3 Enter the blood urea nitrogen: the basic metabolic panel usually lists this as BUN. A typical adult range is 7 to 20 mg per deciliter.
4. 4 Enter the measured plasma osmolality if available: add the measured plasma osmolality from a clinical chemistry analyzer, in mOsm per kilogram of water, to turn on the osmolal gap.
5. 5 Enter the serum ethanol if available: add the serum or blood ethanol from a toxicology panel, in mg per deciliter, to turn on the ethanol adjusted osmolal gap.
6. 6 Read the calculated plasma osmolality and the band: the result panel shows the calculated plasma osmolality in mOsm per kilogram, the matching band label, the osmolal gap and the ethanol adjusted osmolal gap when their inputs are provided, and a one line clinical interpretation.

A patient with sodium 152, glucose 110, BUN 28, measured osmolality 340, and ethanol 0 enters those five numbers and gets a calculated plasma osmolality of 324 mOsm per kilogram, a high band label, and an osmolal gap of 16 mOsm per kilogram. The result triggers the published hyperosmolar dehydration and toxic alcohol workup.

A second clinical screening tool that pairs with the high-plasma-osmolality pathway is the Insulin Dosage Calculator, which turns a current blood glucose and insulin sensitivity factor into a correction dose for the hyperglycemia side of the workup.

Using a plasma osmolality calculator offers several practical advantages over mental math.

• • Standardized mOsm per kilogram unit: the calculator returns the calculated plasma osmolality in mOsm per kilogram of water, the unit used in every published fluid and electrolyte guideline.
• • Quick bedside math: all three routine inputs come from a single basic metabolic panel, so the result can be computed in under a minute.
• • Transparent calculation: the formula is shown in plain English, so the contribution of sodium, glucose, and BUN is visible.
• • Built-in high, normal, and low bands: the result is paired with the published 275 and 295 mOsm per kilogram cut-offs.
• • Optional osmolal gap: the osmolal gap is shown whenever the measured plasma osmolality is entered, the published screen for a toxic alcohol ingestion.
• • Optional ethanol adjusted osmolal gap: the ethanol adjusted osmolal gap is shown whenever the serum ethanol is entered, the published correction for co ingested ethanol.

The same calculated plasma osmolality is used in emergency, inpatient, and outpatient settings, so it serves as a shared language across the clinical team.

A second clinical chemistry tool that pairs with the calculated plasma osmolality during the metabolic workup is the Acid Base Calculator, which turns an arterial blood gas into a pH, PCO2, and bicarbonate reading for the same acid-base assessment that follows a high or low band.

The output depends on the basic metabolic panel values entered and on the patient sitting in front of the calculator.

• • The calculated plasma osmolality is a screening tool, not a stand alone diagnosis. The blood gas, the measured plasma osmolality, the serum ethanol, the toxicology panel, and a clinical exam are still required before any clinical decision.
• • Children, pregnant patients, and older adults can have atypical osmolality patterns, and the calculator can overestimate or underestimate the gap in those groups.

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

According to MedlinePlus Hyponatremia, the calculated plasma osmolality is the standard first step in the workup of hyponatremia, with a low value under 275 mOsm per kilogram supporting a true hypotonic hyponatremia.