What is serum osmolarity?

Serum osmolarity measures the concentration of dissolved particles (solutes) in blood, expressed in milliosmoles per liter (mOsm/L). It reflects the balance of water and electrolytes in the body. Normal serum osmolarity is approximately 275-295 mOsm/L. Osmolarity is crucial for maintaining proper cell function, as cells swell or shrink when osmolarity is abnormal.

How is serum osmolarity calculated?

The most common formula is: Osmolarity = 2(Na+) + (Glucose/18) + (BUN/2.8), where Na+ is sodium in mEq/L, Glucose is in mg/dL, and BUN (blood urea nitrogen) is in mg/dL. Sodium is multiplied by 2 to account for its associated anions (mainly chloride). This calculated osmolarity usually closely matches measured osmolarity.

What causes high serum osmolarity?

High serum osmolarity (>295 mOsm/L) or hyperosmolality can result from: dehydration, hyperglycemia (high blood sugar in diabetes), hypernatremia (high sodium), uremia (kidney failure with elevated BUN), or ingestion of certain substances like methanol or ethylene glycol. Symptoms include thirst, confusion, and in severe cases, seizures or coma.

What causes low serum osmolarity?

Low serum osmolarity (<275 mOsm/L) or hypo-osmolality typically results from excess water retention relative to solutes. Causes include: overhydration, SIADH (syndrome of inappropriate ADH secretion), severe hyponatremia (low sodium), adrenal insufficiency, or hypothyroidism. Symptoms include nausea, headache, confusion, and in severe cases, seizures due to brain cell swelling.

What is osmolar gap and why does it matter?

Osmolar gap is the difference between measured osmolarity (from lab) and calculated osmolarity (from formula). Normal gap is less than 10 mOsm/L. A large gap suggests the presence of unmeasured osmotically active substances like ethanol, methanol, ethylene glycol, or isopropyl alcohol. An elevated osmolar gap is an important clue in poisoning cases and requires immediate medical attention.

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Serum Osmolarity Calculator

Calculate serum osmolarity based on Sodium, Glucose, and BUN levels.

Sodium (Na+) [mEq/L]

Glucose [mg/dL]

BUN [mg/dL]

How to Use This Calculator

Gather Laboratory Values

You'll need three blood test results: Sodium (Na+) in mEq/L, Glucose in mg/dL, and BUN (Blood Urea Nitrogen) in mg/dL. These values come from standard blood chemistry panels ordered by healthcare providers. Note: This calculator is for educational purposes and medical decisions should be made with a healthcare professional.

Enter Lab Values

Input your sodium, glucose, and BUN values into the calculator, ensuring you're using the correct units (mEq/L for sodium, mg/dL for glucose and BUN). Double-check your numbers for accuracy as calculation errors could lead to misinterpretation.

Calculate and Interpret

The calculator will compute your serum osmolarity using the formula: 2(Na+) + (Glucose/18) + (BUN/2.8). Compare your result to the normal range (275-295 mOsm/L). Values outside this range should be discussed with your healthcare provider to determine the cause and appropriate treatment. This tool does not replace medical advice.