Estimated Serum Osmolality Calculator
Use this calculator to estimate serum osmolality based on common laboratory values. This calculation is often used in clinical settings to assess fluid and electrolyte balance.
Understanding Serum Osmolality
Serum osmolality is a measure of the concentration of all chemical particles found in the fluid part of the blood (serum). It reflects the body's fluid balance and is primarily determined by the concentration of sodium, glucose, and urea (BUN).
Why is Osmolality Important?
Clinicians use serum osmolality to diagnose and monitor various conditions, including:
- Dehydration or Overhydration: High osmolality can indicate dehydration, while low osmolality might suggest overhydration.
- Diabetes Mellitus: Extremely high glucose levels in uncontrolled diabetes can significantly increase osmolality.
- Kidney Function: BUN, a waste product, contributes to osmolality and can indicate kidney issues.
- Electrolyte Imbalances: Sodium is the primary determinant of serum osmolality, so imbalances directly affect it.
- Toxic Ingestions: Certain substances like ethanol or methanol can increase osmolality, leading to an "osmolar gap."
The Calculation Formula
The calculator uses a commonly accepted formula to estimate serum osmolality (in mOsm/kg):
Estimated Osmolality = (2 * Sodium) + (Glucose / 18) + (BUN / 2.8)
- Sodium (Na): Measured in mEq/L. It's multiplied by 2 because sodium is typically accompanied by an anion (like chloride), effectively doubling its osmotic contribution.
- Glucose: Measured in mg/dL. It's divided by 18 to convert it from mg/dL to mmol/L, as 1 mmol of glucose weighs 18 mg.
- Blood Urea Nitrogen (BUN): Measured in mg/dL. It's divided by 2.8 to convert it from mg/dL to mmol/L, as 1 mmol of urea nitrogen weighs 2.8 mg.
Normal Ranges and Interpretation
A typical normal range for serum osmolality is approximately 275 to 295 mOsm/kg. Deviations from this range can indicate underlying medical conditions. Always interpret these results in the context of a patient's overall clinical picture and other laboratory findings. This calculator provides an estimate and should not replace professional medical advice or direct laboratory measurements.
How to Use the Calculator:
- Enter the patient's Sodium (Na) concentration in mEq/L.
- Enter the patient's Glucose concentration in mg/dL.
- Enter the patient's Blood Urea Nitrogen (BUN) concentration in mg/dL.
- Click the "Calculate Osmolality" button.
- The estimated serum osmolality will be displayed below.
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function calculateOsmolality() {
var sodiumStr = document.getElementById("sodiumConcentration").value;
var glucoseStr = document.getElementById("glucoseConcentration").value;
var bunStr = document.getElementById("bunConcentration").value;
var sodium = parseFloat(sodiumStr);
var glucose = parseFloat(glucoseStr);
var bun = parseFloat(bunStr);
var resultElement = document.getElementById("osmolalityResult");
if (isNaN(sodium) || isNaN(glucose) || isNaN(bun) || sodium < 0 || glucose < 0 || bun < 0) {
resultElement.innerHTML = "Please enter valid positive numbers for all fields.";
resultElement.style.color = "#dc3545"; /* Red for error */
return;
}
// Formula: Estimated Osmolality = (2 * Sodium) + (Glucose / 18) + (BUN / 2.8)
var estimatedOsmolality = (2 * sodium) + (glucose / 18) + (bun / 2.8);
resultElement.innerHTML = estimatedOsmolality.toFixed(1) + " mOsm/kg";
resultElement.style.color = "#218838"; /* Green for success */
}