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Determination of osmolality of blood serum
Last reviewed: 23.04.2024
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The osmolality of blood serum (P osm ) and urine osmolality (U ocm ) are considered to be a direct and accurate indicator of the osmoregulatory function of the kidneys with the subsequent calculation of derived values derived from the principle of clearance.
Osmolality of blood and urine creates osmotically active electrolytes (sodium, potassium, chlorides), as well as glucose and urea. Normally, the osmolality of serum is 275-295 mOsm / l. The share of electrolytes accounts for the bulk of osmolality (approximately twice the osmotic concentration of sodium - 2x140 mOsm / l = 280 mOsm / l), glucose and urea for about 10 mOsm / l (of which glucose is 5.5 mOsm / l, and for urea - 4.5 mOsm / l). In urine osmolality, along with electrolytes, urea and ammonium contribute significantly.
The method has become widespread in clinical practice, but its accessibility is significantly inferior to the determination of the relative density of urine. To determine blood and urine osmolality, a cryoscopic method is used in clinical practice, i.e. Determine the freezing point of the solutions under study. It is proved 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 calculation of derived indicators is performed.
The clearance of osmotically active substances (C osm ) is the conditional volume of plasma (in ml / min), which is purified by the kidney from osmotically active substances in 1 min. It is calculated by the formula:
With osm = (U osm xV): P osm
Where V is a minute diuresis.
If we assume that the osmotic concentration of urine is equal to the osmotic concentration of the plasma, then C osm = V. Under such conditions it is obvious that the kidney does not concentrate and does not dissolve the urine.
Under conditions of allocation of hypotonic urine, the ratio U osm / P osm <1, i.e. To the urine is added a fraction of water, free of osmotic substances. This water is called osmotically free water (С Н 2 0). In this situation, the equalities are valid: V = C ocm + CH 2 0 and, respectively, C H 2 0 = VC ocm. Consequently, the clearance of osmotically free water in this situation characterizes the ability of the renal tubules to separate the diluted hypotonic urine. Under these conditions, the value of С Н 2 0 is always a positive value. If the value of C H 2 0 is negative, this indicates a process of concentration in the kidneys. In this situation, it is obvious that, in addition to the reabsorption of water in the osmotically active substance, an osmotically free liquid is additionally reabsorbed. Reabsorption of osmotically free water (Т Н 2 0) in numerical expression is equal to С Н 2 0, but opposite in sign.
Thus, the clearance and reabsorption of osmotically free water are quantitative indicators that reflect 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 osmolar clearance to creatinine clearance.
Along with laboratory methods for determining blood and urine osmolality, computational methods for calculating the osmolality of blood and urine have become widespread. Blood osmolality is calculated as the sum of osmolality of osmotically active substances in blood serum (sodium and predominantly chlorine) and osmolality of glucose and urea. Since the osmolality of chlorine and sodium is the same, a factor of 2 is introduced into the formula. Several formulas are used to calculate the osmolality of the blood.
P osm = 2x (Na + K) + (serum glucose concentration: 18) + (serum urea nitrogen concentration: 2.8),
Where the concentration of glucose and urea nitrogen in blood serum is expressed in mg / dL. For example, at a sodium concentration of 138 mmol / L, potassium 4.0 mmol / L, glucose and nitrogen, blood serum urea 120 mg / dl (6.66 mmol / L) and 10 mg / dl (3.6 mmol / l), respectively osmolality of the plasma 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 value of osmolality of blood usually does not exceed 10 Osm / L. This difference is the osmolar interval (interval). A gap of more than 10 Osm / L is detected with a high concentration of lipids or blood proteins, as well as in conditions of metabolic acidosis due to an increase in the concentration of lactic acid in the blood.
The parameters of the osmoregulatory function of the kidneys are normal: R osm - 275-295 Osm / L, and press (at diuresis about 1,5) - 600-800 Osm / L, C does not exceed 3 l / min, EF does not exceed 3.5% , С Н 2 О from -0,5 to -1,2 l / min, Т Н 2 О from 0,5 to 1,2 l / min.
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