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Serum Osmolality Calculator

Calculate serum osmolality and osmolal gap using laboratory values. Enter sodium, glucose, and BUN to evaluate fluid balance, detect electrolyte disorders, and identify potential toxins.

Calculate Serum Osmolality

Enter laboratory values to calculate osmolality

Sodium (Na+) mEq/L

Normal: 135-145 mEq/L

Glucose mg/dL

Normal (fasting): 70-100 mg/dL

BUN mg/dL

Normal: 7-20 mg/dL

Measured Osmolality (Optional) mOsm/kg

For osmolal gap calculation

Your Results

290.4
mOsm/kg
Normal

Interpretation

Normal serum osmolality. Your fluid and electrolyte balance appears within normal range.

Sodium:140 mEq/L
Glucose:90 mg/dL
BUN:15 mg/dL

Normal Reference Ranges

📖 Reference Standards: Values based on Clinical Laboratory Standards Institute (CLSI) guidelines and peer-reviewed medical literature. Individual lab ranges may vary slightly based on analytical methods.

Laboratory Values

Sodium (Na+)135-145 mEq/L

Primary serum electrolyte

  • • Major determinant of osmolality
  • • Regulated by ADH and kidneys
Glucose70-100 mg/dL

Blood sugar level (fasting)

  • • Elevated in diabetes
  • • Affects osmolality significantly
BUN7-20 mg/dL

Blood Urea Nitrogen

  • • Kidney function indicator
  • • Elevated in renal disease

Osmolality Categories

Low<275 mOsm/kg

Hypo-osmolar state

  • • Overhydration, SIADH
  • • Hyponatremia risk
Normal275-295 mOsm/kg

Normal fluid balance

  • • Proper hydration
  • • Normal electrolyte balance
High>295 mOsm/kg

Hyperosmolar state

  • • Dehydration, hypernatremia
  • • Hyperglycemia, uremia

How to Calculate Serum Osmolality

Standard Formula

Calculated Osmolality = 2 × Na + Glucose/18 + BUN/2.8
Units: Na (mEq/L), Glucose (mg/dL), BUN (mg/dL)

📚 Authority Reference:

Source: National Institutes of Health (NIH) - MedlinePlus Medical Encyclopedia

Formula Origin: Established clinical chemistry standard based on physiological principles and validated through extensive clinical use since the 1960s.

Key References:

  • Smithline N, Gardner KD Jr. Gaps—anionic and osmolal. JAMA. 1976;236(14):1594-1597. PMID: 989112
  • Bhagat CI, et al. Calculated vs measured plasma osmolalities revisited. Clin Chem. 1984;30(10):1703-1705. PMID: 6478607
  • Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659. PMID: 11719745

Normal Range: 275-295 mOsm/kg (established by clinical laboratory standards)

Calculation Steps:

  1. 1
    Obtain laboratory values
    Sodium, glucose, and BUN from blood test
  2. 2
    Apply the standard formula
    Calculate using: 2×Na + Glucose/18 + BUN/2.8
  3. 3
    Calculate osmolal gap (optional)
    Subtract calculated from measured osmolality
  4. 4
    Interpret results
    Compare with normal ranges and assess clinical context

Important Considerations

⚠️ Medical Disclaimer

This calculator is for educational purposes only. Always consult healthcare professionals for medical diagnosis and treatment.

🔬 Laboratory Accuracy

Use recent lab values for accuracy

  • • Values change rapidly in critical illness
  • • Hemolysis affects results
  • • Sample handling matters
🚨 Osmolal Gap Alert

Gap >10 requires urgent evaluation

  • • Methanol, ethylene glycol toxicity
  • • Alcoholic/diabetic ketoacidosis
  • • Lactic acidosis
💊 Medication Effects

Many drugs affect osmolality

  • • Mannitol increases osmolality
  • • Diuretics affect electrolytes
  • • Inform clinician of all medications
🏥 Clinical Context

Results must be interpreted clinically

  • • Patient symptoms matter
  • • Consider acute vs chronic changes
  • • Serial measurements helpful

Clinical Example Cases

Case 1: Normal Hydration

Laboratory Values:
Na: 140 mEq/L
Glucose: 90 mg/dL
BUN: 15 mg/dL
Results:
Calculated: 290 mOsm/kg
Category: Normal
Interpretation: Normal fluid balance

Clinical Context: Healthy patient with normal electrolyte and fluid status. No intervention needed.

Case 2: Hyperglycemia (Uncontrolled Diabetes)

Laboratory Values:
Na: 145 mEq/L
Glucose: 450 mg/dL
BUN: 25 mg/dL
Results:
Calculated: 324 mOsm/kg
Category: High
Interpretation: Hyperosmolar state

Clinical Context: Diabetic patient with severe hyperglycemia. Requires urgent glucose management and rehydration.

Clinical Applications

1

Fluid & Electrolyte Disorders

Evaluate hyponatremia, hypernatremia, and assess fluid balance in hospitalized patients. Guide fluid resuscitation strategies.

2

Toxicology Screening

Detect toxic alcohol ingestion (methanol, ethylene glycol) through elevated osmolal gap. Critical for emergency departments.

3

Diabetic Emergencies

Monitor hyperosmolar hyperglycemic state (HHS) and diabetic ketoacidosis (DKA). Track treatment response and hydration status.

4

SIADH Diagnosis

Confirm syndrome of inappropriate antidiuretic hormone secretion with low serum osmolality despite concentrated urine.

5

ICU Monitoring

Track critically ill patients' fluid status, guide parenteral nutrition, and assess effectiveness of osmotic therapy.

Frequently Asked Questions

What is serum osmolality?
Serum osmolality is the concentration of dissolved particles (osmoles) in blood serum, measured in mOsm/kg. It reflects the balance of water and solutes in your body, primarily sodium, glucose, and urea. Normal range is 275-295 mOsm/kg.
What is osmolal gap and why is it important?
Osmolal gap is the difference between measured and calculated osmolality. A normal gap is ≤10 mOsm/kg. An elevated gap (>10) suggests the presence of unmeasured osmoles like alcohols, ketones, or toxins, requiring immediate medical attention.
When should serum osmolality be measured?
Serum osmolality testing is ordered to evaluate fluid and electrolyte imbalances, unexplained altered mental status, suspected poisoning (methanol, ethylene glycol), hyponatremia/hypernatremia investigation, or diabetic ketoacidosis assessment.
What causes high serum osmolality?
High osmolality (>295 mOsm/kg) can result from dehydration, hypernatremia, hyperglycemia, uremia, alcohol ingestion, or mannitol administration. Symptoms may include thirst, confusion, seizures, or coma depending on severity.
What causes low serum osmolality?
Low osmolality (<275 mOsm/kg) can occur with overhydration, SIADH (syndrome of inappropriate ADH secretion), hyponatremia, adrenal insufficiency, or psychogenic polydipsia. This may cause headache, nausea, confusion, or in severe cases, seizures.

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