Miscellaneous Osmolality

Body fluid specimen, including but not limited to: Cerebrospinal Fluid, pleural fluid, pericardial fluid, peritoneal fluid, or synovial fluid. Note fluid type in order comments or on patient specimen label.

Collect body fluid in a body fluid collection bag or other sterile container and transport to lab. Refrigerate if transport will be delayed. Note fluid type in order comments or on patient specimen label.

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

Body fluid collection bag or sterile container

Daily upon receipt

Body Fluid Osmolality can be useful in determining the source and type of fluid

Osmolality is an index of the solute concentration. It is determined by the number and not by the nature of the particles in solution. Dissolved solutes change the physical properties of solutions; they increase the osmotic pressure and decrease freezing point.

Body fluids have the same osmolality as a corresponding serum specimen taken at the same time. "True body fluids" include: ascitic, cerebrospinal, hydrocele, edema, pericardial, pleural, spermatocele and synovial fluids. Secretions not in equilibrium with the extra-cellular fluids of the body include gastric juice, saliva and sweat.

Serum osmolality is normally between 280 to 300 mOsm/kg; it increases with dehydration and decreases with overhydration. Urine osmolality reflects the ability of the kidney to maintain tonicity and water balance. The normal kidney can concentrate urine to 800 to 1,400 mOsm/kg, and with excess fluid intake, a minimal osmolality of 40 to 80 mOsm/kg can be obtained.