Osmolality (Blood)

Serum osmolality, osmolality serum, plasma osmolality, tonicity.

This test measures the concentration of dissolved particles (osmolality) in the blood.

Plasma osmolality measures the body’s electrolyte/water balance.

While osmolality and osmolarity are measures that are technically different, they functionally are the same for normal use. 

Whereas osmolality is defined as the number of osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg), osmolarity is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L). 

This test can help diagnose a fluid or electrolyte imbalance, including dehydration.

Electrolytes are mineral salts that help move nutrients into cells and waste products out of cells.

Electrolytes also control your acidity and pH levels.

The more diluted the blood and urine are, the lower the concentration of particles.

When there is less water in the blood, the concentration of particles is greater.

Osmolality increases with dehydrated and decreases with too much fluid in the blood.

Effective osmolality, tonicity, refers to the contribution of osmolality by solutes with low cell membrane permeability (sodium, its anions, and glucose), causing transcellular water shifts.

Tonicity is calculated from sodium and glucose, and for patients without hypoglycemia, tonicity is approximated by doubling the serum sodium level.

Normal tonicity is 270 mOsm per kilogram to 285 mOsm/kilogram.

The body has a unique way to control osmolality.

When osmolality increases, it triggers the body to make antidiuretic hormone (ADH).

This hormone tells your kidneys to keep more water inside your blood vessels and the urine becomes more concentrated.

When osmolality decreases, the body doesn’t make as much ADH, and blood and urine become more diluted.

Serum osmolality helpful in evaluating seizures, ADH levels, dehydration, possible diabetes insipidus, frequent urination, diluted urine, postural hypotension, thirst, hypotension, edema, and coma.

Low blood pressure, which can lead to shock and organ failure

Symptoms of hyponatremia include:

Nausea and vomiting


Mental confusion




Muscle weakness

Spasms or cramps

Seizures or passing out

The results of both urine osmolality and blood osmolality tests may be beneficial.

Results are given in milliosmoles per kilogram (mOsm/kg).

Normal results are:

275 to 295 mOsm/kg for adults and older adults

275 to 290 mOsm/kg for children

Levels are higher than normal:




Diabetes insipidus

Kidney problems












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