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Determination of serum osmolarity
Last reviewed: 07.07.2025
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A direct and accurate indicator of the osmoregulatory function of the kidneys is considered to be the osmolality of the blood serum (P osm ) and the osmolality of urine (U osm ), followed by the calculation of derivative values obtained on the basis of the principle of clearance.
Osmolality of blood and urine is created by osmotically active electrolytes (sodium, potassium, chlorides), as well as glucose and urea. Normally, the osmolality concentration of blood serum is 275-295 mOsm/l. Electrolytes account for the bulk of osmolality (approximately double the osmotic concentration of sodium - 2x140 mOsm/l = 280 mOsm/l), glucose and urea account for about 10 mOsm/l (of which glucose - 5.5 mOsm/l, and urea - 4.5 mOsm/l). In addition to electrolytes, urea and ammonium make a significant contribution to the osmolality of urine.
The method has become widespread in clinical practice, but is significantly less accessible than the determination of the relative density of urine. To determine the osmolality of blood and urine in clinical practice, the cryoscopic method is used, i.e. the freezing point of the solutions being studied is determined. It has been proven that the decrease in the freezing point is proportional to the concentration of osmotically active substances. The research method is simple and accessible. Based on the principle of clearance, the derivative indicators are calculated.
Clearance of osmotically active substances (C osm ) is the conditional volume of plasma (in ml/min) that is cleared by the kidney from osmotically active substances in 1 min. It is calculated using the formula:
With osm = (U osm x V):P osm
Where V is the minute diuresis.
If we assume that the osmotic concentration of urine is equal to the osmotic concentration of plasma, then C osm = V. Under such conditions, it is obvious that the kidney does not concentrate or dilute urine.
Under conditions of excretion of hypotonic urine, the ratio U osm /P osm < 1, i.e. a fraction of water free of osmotic substances is added to the urine. This water is called osmotically free water (С Н 2 0). In this situation, the following equalities are valid: V = С ocm + CH 2 0 and, accordingly, С Н 2 0 = VC ocm. Consequently, the clearance of osmotically free water in this situation characterizes the ability of the renal tubules to excrete diluted hypotonic urine. Under these conditions, the value of С Н 2 0 is always a positive value. If the value of С Н 2 0 is negative, this indicates a concentration process in the kidneys. In this situation, it is obvious that, in addition to the reabsorption of water in a state associated with osmotically active substances, osmotically free fluid is additionally reabsorbed. The reabsorption of osmotically free water (TH2O ) is numerically equal to CH2O , but opposite in sign.
Thus, clearance and reabsorption of osmotically free water are quantitative indicators reflecting the intensity of the kidney's work in concentrating and diluting urine.
Excreted fraction of osmotically active substances (EF osm ) is the percentage ratio of osmolal clearance to creatinine clearance.
Along with laboratory methods for determining the osmolality of blood and urine, calculation methods for calculating the osmolality of blood and urine have become widespread. Blood osmolality is calculated as the sum of the osmolalities of the osmotically active substances of the blood serum (sodium and mainly chlorine) and the osmolality of glucose and urea. Since the osmolality of chlorine and sodium is the same, a coefficient of 2 is introduced into the formula. Several formulas are used to calculate blood osmolality.
P ocм = 2x(Na+K) + (serum glucose concentration: 18) + (serum urea nitrogen concentration: 2.8),
Where the concentration of glucose and urea nitrogen in the blood serum is expressed in mg/dL. For example, with a concentration of sodium of 138 mmol/L, potassium of 4.0 mmol/L, glucose and urea nitrogen in the blood serum of 120 mg/dL (6.66 mmol/L) and 10 mg/dL (3.6 mmol/L), respectively, the plasma osmolality will be:
P osm =[2x(138+4.0)]+[120: 18]+[10: 2.8]=284.0+6.7+3.6=294.3 Osm/l.
The difference between the calculated and measured blood osmolality value usually does not exceed 10 Osm/L. This difference is the osmolal gap (interval). A gap of more than 10 Osm/L is detected with a high concentration of lipids or proteins in the blood, as well as in conditions of metabolic acidosis due to an increase in the concentration of lactic acid in the blood.
Normal indicators of the osmoregulatory function of the kidneys: P osm - 275-295 Osm/l, and FM (with diuresis of about 1.5) - 600-800 Osm/l, C does not exceed 3 l/min, EF does not exceed 3.5%, C H 2 O from -0.5 to -1.2 l/min, T H 2 O from 0.5 to 1.2 l/min.
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