Osmolality

Osmolality is the sum of the molar concentration of all osmotically active particles per kilogram of solvent. These osmotically active substances include primarily calcium, chloride, glucose, urea, potassium, magnesium, and sodium. Thus, physiological serum osmolality depends almost exclusively on sodium concentration. Osmotic changes in the other electrolytes are not compatible with life.

The unit is osm/kg or osmol/kg.

The determination of osmolality is used to control the water and electrolyte (blood salt) balance of the body. A distinction is made between:

  • Hyperosmolality (hyperosmolal) – in this case, there is a greater number of dissolved particles per kilogram of fluid than in the reference fluid
  • Isoosmolality (isoosmolal) – here is the same number of dissolved particles.
  • Hypoosmolality (hypoosmolal) – here the number of dissolved particles is less than in the reference liquid per kilogram of liquid.

The method

Material needed

  • Blood serum
  • Urine

Preparation of the patient

  • Not necessary

Disruptive factors

  • None known

Normal value – blood serum

Normal value in mosmol/kg
Child, 1st day of life 276-305
Child, 7th day of life 274-305
Child, 28th day of life 275-300
Adults 280-300

Normal value – urine

Normal value in mosmol/kg 50-1.200

Indications

Interpretation

Interpretation of elevated values (osmolality/sodium elevated).

  • Dehydration (lack of fluid) – decrease in the amount of blood circulating in the circulation; caused by decreased fluid intake or increased fluid losses:
    • Diarrhea (diarrhea; especially in children and the elderly).
    • Severe vomiting
    • Heavy sweating (hyperhidrosis)
    • Polyuria (increased urine output; > 1.5-3 l / day)
  • Hyperhydration (hypervolemia with hypernatremia (excess sodium), hematocrit (share of all cellular components in the volume of blood/erythrocytes make up about 95% of it) ↓):
    • Excessive saline intake:
      • Iatrogenic (caused by a physician).
      • Primary hyperaldosteronism (Conn syndrome)
    • Increased sodium reabsorption:
      • Impaired renal function
  • Renal diabetes insipidus (synonym: ADH– or vasopressin-resistant diabetes insipidus).
  • Central diabetes insipidus (synonym: diabetes insipidus neurohormonalis) – the cause is an absence or insufficient production of antidiuretic hormone (ADH) – the cause is a defect in the kidney, which despite the presence of the hormone ADH can not form a normally concentrated urine

Interpretation of decreased values (osmolality / sodium normal / decreased).

  • Ethanol intoxication (alcohol intoxication).
  • Hyperglycemia (high blood sugar)
  • Lactic acidosis – form of metabolic acidosis/metabolic acidosis in which a drop in blood pH is due to the accumulation of acidic lactate.
  • Uremia (occurrence of urinary substances in the blood above normal values).

Note

  • Plasma osmolality should always be assessed in combination with sodium concentration.
  • Urine osmolality should be assessed in conjunction with sodium and glucose and, if necessary, other osmotically active substances in the urine.
  • For functional test (thirst test) withdrawal time with specify.