Type of test Blood

Normal findings

 Adult/elderly: 285-295 mOsm/kg H₂O or 285-295 mmol/kg (SI units)

 Child: 275-290 mOsm/kg H₂O

Possible critical values

< 265 mOsm/kg H₂O

> 320 mOsm/kg H₂O

Test explanation and related physiology

Osmolality measures the number of dissolved particles in serum/plasma per unit volume. As the amount of free water in the blood increases or the number of particles decreases per unit volume of serum, osmolality decreases. As the amount of water in the blood decreases or the number of particles per unit volume increases, osmolality increases. Osmolality increases with dehydration and decreases with overhydration.

The simultaneous use of urine osmolality (see next test) helps in the interpretation and evaluation of problems involving osmolality.

The serum osmolality test is useful in evaluating fluid and electrolyte imbalance. The test is very helpful in the evaluation of seizures, ascites, hydration status, acid–base balance, suspected antidiuretic hormone (ADH) abnormalities, and suspected poisoning. Osmolality is also helpful in identifying the presence of organic acids, sugars, and ethanol.

Osmolality can be predicted based on calculations of serum sodium, glucose, and blood urea nitrogen (BUN)—the three most important solutes in the blood. The equation is

The measured osmolality should not exceed the predicted by more than 10 mOsm/kg. A difference of more than 10 mOsm/ kg is considered an osmolal gap or delta gap. Causes of a serum osmolal gap include mannitol, ethanol, methanol, ethylene glycol, and other toxins in very high concentration. Another measure providing similar data is the ratio of serum sodium to osmolality. Normally, the ratio of serum sodium, in mEq/L, to serum osmolality, in mOsm/kg, is between 0.43 and 0.5. The ratio may be distorted in drug intoxication.

Osmolality may have a role in evaluation of coma patients. Values greater than 385 mOsm/kg H2 O are associated with stupor in patients with hyperglycemia. When values of 400 to 420 mOsm/kg are detected, grand mal seizures can occur. Values greater than 420 mOsm/kg can be lethal. The simultaneous use of urine osmolality helps in the interpretation and evaluation of problems with osmolality.

Interfering factors

 • Such diseases as cerebrovascular accident (stroke) or brain tumors may interfere with test results through inappropriate secretion of ADH.

 Procedure and patient care

• See inside front cover for Routine Blood Testing.

• Fasting: no

 • Blood tube commonly used: red

• For pediatric patients, draw blood from a heel stick.

Abnormal findings

 Increased levels

- Hypernatremia

- Dehydration

- Hyperglycemia

- Mannitol therapy

- Azotemia

- Uremia

- Ingestion of ethanol, methanol, or ethylene glycol

- Hyperosmolar nonketotic hyperglycemia

- Diabetes insipidus

- Hypercalcemia

- Renal tubular necrosis

- Severe pyelonephritis

- Ketosis

- Shock

Decreased levels

- Hyponatremia

- Overhydration

- Syndrome of inappropriate antidiuretic hormone (SIADH) secretion

- Paraneoplastic syndromes associated with lung carcinoma

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