Osmolarity (blood) | Laboratory test

Serum osmolality is the concentration of particles (molecules) of substances dissolved in it. This is an indicator of how many substances are dissolved in the serum (solvent).

Serum (or plasma) osmolarity is determined by the concentration of substances dissolved in it, such as: sodium salts, glucose, urea and others.

Particles that affect osmolarity are small molecules with high concentrations in serum.

The unit of osmolality is milliosmole per liter of solvent.

The number of particles dissolved in a solution (plasma or serum) determines the number of osmoles. Each particle has a unit value. That is, 1 millimole of glucose is produced per 1 mosm/kg of liquid.

The ability of a particle to participate in plasma or serum osmolarity is determined by its ability to cross the cell membrane.

In patients with normal glycemia and adequate kidney function, the change in serum osmolarity is determined by the change in the sodium index (participation of glucose and urea particles in the change in osmolarity is relevant only when the concentration of these substances in the blood is increased). Substances that pass freely through the cell membrane are "safe" because they do not create an osmotic gradient (difference) between different sides of the membrane.

Serum osmolarity is controlled by two main mechanisms that ensure adequate distribution of fluid between the intracellular and extracellular spaces: antidiuretic hormone (ADH) release and thirst sensation. The antidiuretic hormone released at the level of the neurohypophysis increases serum osmolarity by 1%. This mechanism ensures the strengthening of water reabsorption.

Thirst mechanism is activated when plasma osmolality exceeds 290 mosm/kg H ₂ O, there is a need to regulate water scarcity. The body protects itself from dehydration with the mechanism of thirst.

A change in osmolarity, as a result of excessive intake or loss of water, causes a violation of the intracellular and extracellular fluid distribution, which is manifested by swelling or dehydration at the cellular level. The clinical manifestation of these changes depends on the etiology of the change in osmolality, the severity of the change, and the nature of the electrolytes causing it.

with a slow rate of osmolality of 60-80 mosmol/kg H ₂ O decrease is not dangerous, while 40-60 mosmol/kg H ₂ An increase in osmolarity with O, as a result of a water deficit, or an increase in some particles (glucose, sodium) that do not pass through the cell membrane, can lead to coma and even death.

Water-electrolyte imbalance is clinically manifested by varying degrees of altered consciousness, lethargy, lack of strength, although it may be asymptomatic.

When is the survey conducted?

Osmolality research is necessary in the following cases:

• Diagnosis of water-electrolyte and alkaline-alkaline imbalance
• Antidiuretic hormone release monitoring
• Epileptic seizures
• Diabetes insipidus
• Primary polydipsia (increased thirst)
• Water intoxication
• Obtaining ethylene glycol and methanol
• Liver diseases
• Detection of pseudo and hypernatremia

Preliminary preparation: It is necessary to abstain from drinking alcohol

Study material: Venous blood

Interpretation of results

Serum osmolarity is growing in the following cases:

• Hypernatremia
• dehydration
• Hypercalcemia
• brain damage
• Mannitol treatment
• Azotemia
• Receiving ethanol, methanol, ethylene glycol
• Kidney damage
• Coma

Serum osmolarity is decreasing in the following cases:

• Hyperhydration
• Hyponatremia
• Decreased release of antidiuretic hormone (often accompanied by paraneoplastic syndrome in lung tumors)