Serum Osmolality and Osmolal Gap

Reference ranges

Severity & critical values

Conversion: To convert glucose, divide mg/dL by 18 to get mmol/L. The formulas below use that conversion for the glucose term. Serum osmolality reads the same number in mOsm/kg and mmol/kg.

In a normal result, the calculated value sits close to the measured value. Most laboratories report this simplified form, but some use other coefficients.

Leave urea (BUN) out. It crosses cell membranes and does not shift water. HHS uses this effective value, with a threshold of >300 mOsm/kg.

A normal gap is commonly under 10 mOsm/kg, though the cutoff is lab-specific. Flag anything above 10 mOsm/kg. Above 20 mOsm/kg, suspicion for toxic alcohols climbs (see Clinical Pearls).

Use the coefficients your lab reports. The ethanol term is only an estimate. See Physiology & Test Basics.

High (hyperosmolality above 295 mOsm/kg)

Low (hypotonic, below 275 mOsm/kg)

A low measured osmolality means low effective solute, and most often that means hyponatremia from water excess. The value rarely needs treatment on its own. Treat the cause of the sodium disorder (see the Sodium page).

Low sodium with normal osmolality (pseudohyponatremia)

A low sodium with a normal measured osmolality points to pseudohyponatremia. The sodium is falsely low because of the method, most often with marked hyperlipidemia or hyperproteinemia. Hyperglycemia is different. It draws water out of cells, which lowers sodium (translocational hyponatremia). It also raises the measured osmolality. So it is not pseudohyponatremia. The expert panel's classification is what draws this line (see References).

Step-by-step approach

1. Is it real? Check the specimen for hemolysis and line contamination, and confirm the tube was sealed and analyzed promptly. Repeat if the value conflicts with the clinical picture.
2. How sick is the patient? Assess mental status, seizures, hypotension and respiratory pattern before the laboratory detail.
3. Calculate both forms. Compute calculated and effective osmolality from the same draw as sodium, glucose and BUN. Add the ethanol term if an ethanol level is available.
4. Interpret the gap. A gap above 10 mOsm/kg means looking for toxic alcohols, ethanol, ketoacidosis, renal failure, shock and recent mannitol.
5. Match the picture to HHS or DKA criteria. HHS requires glucose ≥600 mg/dL with effective osmolality above 300 mOsm/kg, pH ≥7.30, bicarbonate ≥15 mmol/L and no significant ketosis (β-hydroxybutyrate below 3.0 mmol/L). DKA criteria are separate and are not defined by osmolality; mixed DKA/HHS presentations occur.
6. Escalate or dispose. Suspected toxic alcohol: poison center and medical direction. HHS: ED or ICU protocol with serial sodium.

Pattern and what it suggests

One data point comes from a retrospective academic cohort published in 2012. In it, 851 patients had a gap of 9 or more on initial labs. Of 20,669 patients screened, 341 had a gap above 14 mOsm/kg. At a cutoff of 20 mOsm/kg, sensitivity was 0.82 and specificity 0.85 for GC-confirmed toxic alcohol ingestion. Roughly 18% of ingestions were missed at that cutoff. Cutoffs are laboratory-specific, so this is not a universal threshold.

A normal osmolal gap does not rule out toxic alcohol poisoning. Here's why. Early on, the gap may not have risen yet. Later, as the parent alcohol breaks down, the gap can drop. Those breakdown products are acids, and they push the anion gap up. Read the gap with the anion gap, not on its own.

Toxic alcohol antidote (fomepizole)

Start empirically: Give fomepizole once ingestion is strongly suspected, from the history or from a gap with unexplained anion gap acidosis. Don't hold it for confirmatory levels. Make the decision with the poison center and medical direction.

Hemodialysis timing (per label): At the start of dialysis, give no dose if the last dose was less than 6 h earlier; give the next scheduled dose if it was 6 h or more earlier. During dialysis, dose every 4 h. At the end of dialysis: no dose if less than 1 h has passed since the last dose; half the next scheduled dose if 1 to 3 h; the next scheduled dose if more than 3 h. Off dialysis, the next dose is due 12 h after the last dose.

Osmotic therapy

Under protocol, mannitol and 3% hypertonic saline treat raised intracranial pressure or other hyperosmolar states. Find the dosing on the Mannitol and Hypertonic Saline 3% pages. The mannitol label asks for serum osmolarity monitoring and sets no numeric cutoff. Your ceiling is local, set by whatever protocol you run.

Hyperglycemic hyperosmolar state

HHS care is built from fluids, insulin, potassium replacement and serial sodium. The consensus protocol and local policy decide the details. For insulin dosing, go to the Insulin page.

Definitive care & disposition

• Toxic alcohol with acidosis or organ injury: Call the poison center and medical direction. Extracorporeal treatment is decided against EXTRIP criteria (see the Toxic Alcohols page and References).
• HHS: Disposition is the ED or ICU, per local criteria. Track neurologic checks, fluid balance and sodium with each set of labs.

Doses are typical references. Follow local protocols, medical direction and institutional policy.

Special populations

Collection & handling

Medications & osmotically active agents

Physiologic & biologic

Common misinterpretations

• Measured osmolality alone can't diagnose HHS. You need glucose and effective osmolality too.
• BUN counts toward total calculated osmolality. It is left out of effective osmolality and the HHS criteria.
• Many gaps get read with no prior baseline. In an acutely ill patient, a single gap may not reflect a baseline.
• Every lab sets its own gap cutoff. Don't carry a cutoff over from another hospital's report.

What it measures

Serum osmolality reads the total concentration of dissolved particles in serum. The osmolal gap sets that measured number against the number calculated from sodium, glucose and BUN. It's the gap, not the osmolality alone, that screens for unmeasured solutes such as toxic alcohols.

Physiology & regulation

• Sodium is the main driver of serum osmolality. That's why the calculated value doubles it.
• Water follows effective osmolality across cell membranes, meaning sodium and glucose. Urea crosses membranes freely, so BUN doesn't drive water shifts.
• ADH and thirst hold osmolality in a narrow range. If either one fails, osmolality rises or falls.
• If ethanol behaved as an ideal solute, every 4.6 mg/dL would add about 1 mOsm/kg. A prospective volunteer study found an empirical coefficient of about 4.25, so the ideal term is only an estimate.

How it is measured

Use serum from a clotted (serum) tube, sealed and analyzed promptly. Most laboratories measure osmolality by freezing-point depression on an osmometer. The method is lab-specific. Draw sodium, glucose, BUN and, if ordered, ethanol from the same draw.

Why it matters

In emergency and critical care, osmolality answers two quick questions. Is the patient hyperosmolar enough to need HHS treatment? Is an unmeasured solute pointing to toxic alcohol ingestion? The answer drives fluid, insulin and antidote decisions.