Clotted blood tube
(Serum gel separator clot activator tube)
Minimum blood volume: 5 mL
Plain urine bottle
Volume: 10 mL
Plain bottle for fluid
Volume: 10 mL
Paediatric clotted blood tube
Number of vials: 2
Minimum blood volume: 1.6 mL
Serum osmolality: 275 - 295 mOsm/kg
(Source: Tietz NW Textbook of Clinical Chemistry 2nd Edition)
Serum osmolality:
Investigation of hyponatremia
Identification of an osmolal gap
(Suspected poisoning with alcohol, methanol, ethylene glycol, acetone, isopropanol, diethyl ether or paraldehyde)
Urine osmolality:
An important test of renal concentrating ability, for identifying disorders of the ADH mechanism, and identifying causes of hyper or hyponatraemia
Serum osmolality:
Osmolal gap is a mathematical difference between the measured and calculated serum osmolality. The mean should be approximately zero with a SD of ~ 6 mOsm/kg.
Calculated osmolality = 1.86 x [ Na ] + [ Glu ] + Urea + 9
An increased osmolar gap indicates the presence of alcohol or other osmotically active substances.
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Moderate increase (up to 10) is associated with ketoacidosis, renal acidosis and lactic acidosis.
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More severe increase (above 15) is associated with poisonings with non-electrolytes such as acetone, ethanol, methanol, isopropanol, ethylene glycol and etc.
Urine osmolality:
Physiological range is 50 - 1,200 mmol/kg.
The kidneys can normally concentrate urine to an osmolality of > 800 mmol/kg within 12 hours of fluid restriction (water deprivation test).
Comparison of serum and urine osmolalities may help to determine the cause of polyuria, to diagnose SIADH and to distinguish pre-renal from renal causes of impaired renal function.
Daily