What does low osmolality indicate?

From the Guidelines

Low osmolality indicates a decreased concentration of solutes in the blood or other body fluids compared to normal levels, typically below 275 mOsm/kg. Normal serum osmolality typically ranges from 275-295 mOsm/kg [ 1 ]. When osmolality falls below 275 mOsm/kg, it is considered low. This condition can result from several causes, including excess water intake, syndrome of inappropriate antidiuretic hormone secretion (SIADH), heart failure, liver cirrhosis, kidney disease, or certain medications like diuretics. Low osmolality often correlates with hyponatremia (low sodium levels) since sodium is a major contributor to serum osmolality [ 1 ]. Symptoms of low osmolality may include headache, nausea, confusion, seizures, and in severe cases, coma. Treatment depends on the underlying cause but may involve fluid restriction, sodium supplementation, or medications like tolvaptan that help increase water excretion [ 1 ]. The body normally maintains osmolality within a narrow range through complex mechanisms involving the kidneys, antidiuretic hormone, and thirst regulation, so disruptions to this balance can indicate significant physiological disturbances. Some key points to consider in managing low osmolality include:

• Monitoring serum osmolality and electrolyte levels closely
• Avoiding hypotonic solutions that can exacerbate cerebral edema in patients with acute ischemic stroke [ 1 ]
• Using isotonic solutions like 0.9% saline for fluid replacement in certain cases [ 1 ]
• Considering the use of medications that help increase water excretion, such as tolvaptan, in specific situations [ 1 ]
• Being cautious with fluid management in patients with renal or cardiac compromise to avoid iatrogenic fluid overload [ 1 ].

From the FDA Drug Label

Removal of excess free body water increases serum osmolality and serum sodium concentrations.

Low osmolality indicates a state of hyponatremia or excess free body water.

• Key points:
  • Low osmolality is associated with diluted serum sodium concentrations.
  • Removal of excess free body water can help increase serum osmolality and serum sodium concentrations.
  • Monitoring of serum sodium levels is necessary to prevent hypernatremia during treatment with tolvaptan 2.

From the Research

Definition of Low Osmolality

• Low osmolality is often associated with hyponatremia, a condition where the sodium levels in the blood are lower than normal 3, 4.
• It can be caused by various factors, including heart failure, liver cirrhosis, nephrotic syndrome, and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) 3, 4.

Clinical Implications of Low Osmolality

• Low discharge serum osmolality has been found to be independently predictive of worse postdischarge mortality and readmission in patients with heart failure 5.
• Patients with low osmolality are more likely to have lower serum sodium, creatinine, systolic blood pressure, and ejection fraction (EF) 5.
• Low osmolality can also be an indicator of chronic kidney disease, which can affect the response to certain treatments, such as tolvaptan 6.

Measurement and Calculation of Osmolality

• Osmolality can be measured directly or calculated using formulas, such as the Smithline-Gardner formula [2(Na) + Glu + Urea] 7.
• The osmolal gap, which is the difference between the measured osmolality and the calculated osmolality, can be used to screen for the presence of unmeasured osmotically active particles 7.

Treatment Options for Low Osmolality

• Treatment for low osmolality depends on the underlying cause and severity of the condition 3, 4.
• Options may include fluid restriction, extracellular rehydration, and the use of aquaretics, such as vaptans, which act through antidiuretic hormone receptor antagonism 3, 4.