Serum Osmolality

Test Code

OSMO

Alias/See Also

Serum Osmolarity

CPT Codes
83930

Preferred Specimen

Serum (gold top tube)

Minimum Volume

0.5 mL

Other Acceptable Specimens

Lithium Heparin Plasma (green top tube)

Instructions

This test is for Osmolality measurement on Plasma or Serum. For Osmolality measurement on Urine, order UROS. For Osmolality measurement on Body Fluids, order MISOSMO.

Transport Container

Serum (gold top) or Lithium Heparin Plasma (green top) Tube

Transport Temperature

Room Temperature or Refrigeration

Specimen Stability

Room Temperature - 2 hours; Refrigerated - 24 hours

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Quantity Not Sufficient, EDTA (purple top tube) contamination, IV contamination

Methodology
Freezing Point Depression

Setup Schedule

Daily upon receipt

Report Available

Upon completion of analysis

Limitations

Gross viscosity or particulate matter may interfere with testing.

Reference Range

280 - 300 mOsm/kg

Clinical Significance

Osmolality is a measure of the number of dissolved solute particles in solution. It is determined by the number and not by the nature of the particles in solution. Dissolved solutes change the physical properties of solutions, increasing the osmotic pressure and boiling point and decreasing the vapor pressure and freezing point. The osmolality of serum increases with dehydration and decreases with overhydration. The patient receiving intravenous fluids should have a normal osmolality. If the osmolality rises, the fluids contain relatively more electrolytes than water. If the osmolality falls, relatively more water than electrolytes is being administered. Normally, the ratio of serum sodium, in mEq/L, to serum osmolality, in mOsm/kg, is between 0.43 and 0.5. The ratio may be distorted in drug intoxication. Generally, the same conditions that decrease or increase the serum sodium concentration affect the osmolality. A comparison of measured and calculated serum osmolality produces a delta-osmolality. If this is >40 mOsm/kg a H2O in a critically ill patient, the prognosis is poor.

This test is for Osmolality measurement on Plasma or Serum. For Osmolality measurement on Urine, order UROS. For Osmolality measurement on Body Fluids, order MISOSMO.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.