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Osmolality

Definition:
This test measures the osmolality (concentration of particles) of serum (the fluid portion of blood, without fibrinogen).



How the test is performed:
Adult or child:

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.



Infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site, if there is any continued bleeding.



How to prepare for the test:
Fast for 6 hours before the test.



Infants and children:

The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:



How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.



Why the test is performed:
Osmolality measures the concentration of particles in solution. Osmolality increases with dehydration and decreases with overhydration. In normal people, increased osmolality in the blood will stimulate secretion of ADH (antidiuretic hormone). This will result in increased water reabsorption, more concentrated urine, and less concentrated plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.



Normal Values:
Normal values range from 280 to 303 mOsm/kg.



Note: mOsm/kg = milliosmoles per kilogram



What abnormal results mean:

Abnormal results are based on the following:

Greater than normal levels may indicate:

Lower than normal levels may indicate:
  • Excess fluid intake
  • Hyponatremia
  • Overhydration
  • Paraneoplastic syndromes associated with lung cancer
  • Syndrome of inappropriate ADH secretion
Additional conditions under which the test may be performed:



What the risks are:
  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)
  • Multiple punctures to locate veins




Special considerations:

One interfering factor is mannitol (a diuretic) therapy.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.





Review Date: 1/25/2002

Reviewed By: Debbie Cohen, M.D., Renal and Electrolyte Division, University of Pennsylvania, Philadelphia, PA. Reviewed provided by VeriMed Healthcare Network.

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