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Laboratory: Clinical Biochemistry
Test Name:
Test Code: OS
Clinical Information:
Osmolality is a measure of the concentration of solute particles that contribute to the osmotic pressure of a solution and reflects plasma tonicity. However, it is more effective to measure electrolytes and glucose to assess tonicity since compounds such as urea or ethanol which diffuse freely across cell membranes affect osmolality measurements but do not affect tonicity.
Serum osmolality determination is used to assess electrolyte balance and acid-base disorders. An increased gap between measured and calculated serum osmolality may indicate injestion of poison, ethylene glycol, methanol or isopropanol. comparison to urine osmolality aids in evaluating the appropriateness and status of water balance in conditions such as diabetes insipidus and psychogenic polyuria.
Collection Devices:
Specimen Required:

Plasma: 0.3 mL

Serum is also acceptable.
Stability 2 days at room temperature, 8 days refrigerated, >14 days frozen

Samples must be transferred to an aliquot tube and stored frozen if analysis will not be complete within 48 hours (#110-10-05 Serum / Plasma Separation Procedure & Transport)
Reference Values:
285-305 mmol/kg                    (mmol/kg = mOsm/kg)
Stat or Routine
See Also:
More Information: