Laboratory Medicine Program

Osmolality, fluids

Clinical Decription:
The osmolality of body fluids controls the distribution of water between various compartments. Water diffuses from a region of lower osmolal concentration to a region of higher concentration so that osmolal concentrations are equalized. Such movement can take place only if the fluid compartment with the higher osmolality can be expanded; however, if the volume of the compartment is fixed, it will undergo an increase in hydraulic pressure. The concentrating ability of the kidney tubules can be measured by simultaneous determination of serum and urine osmolalities. The ratio expresses the actual degree to which the kidney has concentrated the glomerular filtrate. The osmolality ratio of urine/serum is: normal individuals between 1.0-3.0 after fluid restriction 3.0-4.7 in renal tubular deficiency below normal in polyuria of diabetes insipidus 0.2-0.7, even after fluid restriction With average fluid intake, urine osmolality is usually 300-900 mmol/kg. Renal concentration ability is normal if: random urine osmolality is > 600 mmol/kg after 12 h fluid restriction, urine osmolality > 850 mmol/kg. In chronic progressive renal failure the concentrating ability of the tubules is diminished. In acute tubular necrosis the urine osmolality approaches the osmolality of the glomerular filtrate. In polyuria due to diabetes insipidus the urine osmolality is very low; this is used to differentiate this condition from polyuria due to neurogenic origin, where the osmolality is above 850 mmol/kg. Osmolal Gap Measured serum osmolality is related to calculated serum osmolality by the following relationship: Calculated serum osmolality = 1.86 (Na + K) + glucose + urea + 9 This formula is useful to identify toxic substances; e.g., if the measured osmolality is > 20 mmol/kg than the calculated osmolality, there may be an unidentified substance present.
<br/><em>Tietz NW. Textbook of Clinical Chemistry 2nd ed. W.B. Sanders Co. Philadelphia, 1986.</em>

Method: Osmometer; Advanced Instrument OsmoPRO

Component Tests Used: n/a

Reference Ranges Used:
<1200 mmol/kg

Specimen Type: Body Fluid
Volume: 3 mL (minimum: 3 mL)

Shipping: 4C

Special Instructions: n/a

Testing Schedule(s): Daily.

Turnaround Time: 1 day

For more information, call 416.340.5227 or 1.866.865.5227