Synonym

• Serum osmolality

Tubes

• Red, tiger, or green top tube
• 5 mL venous blood

Additional information

• Patient is instructed to avoid food, alcohol, medications, or smoking for 8-10 hrs before testing
• Muscular exercise is avoided during the test
• Handle sample gently to prevent hemolysis
• Send the sample immediately to lab on an ice slurry

Info

• The serum osmolality test is an indirect measurement of the concentration of particles in blood
• Serum osmolality is contributed by sodium, blood urea nitrogen (BUN), blood glucose, chloride, and bicarbonate, and is regulated by anti-diuretic hormone (ADH)
• This test can be useful in determining the presence of alcohols and glycols through the osmolar gap, which is the difference between the actual measured osmolality and the calculated osmolality (based on the formula below:
  Osmolality (mosm/kg H₂O) = (2 × Na⁺) + (Glucose/18) + (BUN/2.8)
  
  Note: Where Na⁺ is in meq/L and BUN and glucose are in mg/dL
• Serum osmolality can be estimated by three methods:
• Direct measurement by using freezing point depression osmometer
• Direct measurement by using heat of vaporization method
• Indirect estimation by using the formula above with use of the measured levels of sodium, BUN and blood glucose
• The osmolar gap is the difference between the measure and calculated osmolality (+/- 5-10 mosm/kg H₂O is normal)
• Use of the heat of vaporization method is not adviseable if alcohol or glycol ingestion is a possibility (these boil off causing false normal values)
• Estimation of Alcohol or Glycol levels (in mg/dL) from the osmolar gap can be performed using the following conversion factors where: Serum level in mg/dL = Osmolar gap × conversion factor
• Ethanol = 4.6
• Ethylene glycol = 6.2
• Isopropyl alcohol = 6
• Methanol = 3.2

Clinical

• The clinical utility of serum osmolality include:
• Evaluation of antidiuretic hormone (ADH) function
• Evaluate electrolyte and acid-base balance
• Evaluation of hydration status in conditions such as seizures, liver disease, and coma
• Aids in the rapid screening for toxic substances such as ethylene glycol, ethanol, isopropanol, and methanol ingestions (A patient with an anion gap acidosis and an osmolar gap should be considered to have ethylene glycol or methanol ingestion until proven otherwise)
• Assists in the diagnosis of metabolic, renal, and endocrine disorders
• Assists in the calculation of osmolal gap
• To monitor osmotherapy (mannitol, glycerol) used to control intracranial pressure
• An osmolality >350 mosm/L or <250 mosm/L is usually necessary to produce clinical signs and symptoms. However, symptoms may occur at values closer to the normal range if the change in osmolality has developed rapidly
• Critically high levels of serum osmolality may present as:
• Poor skin turgor
• Listlessness
• Acidosis (decreased pH)
• Shock
• Seizures
• Coma
• Cardiopulmonary arrest
• The osmolal gap is the difference between the measured and calculated values and is normally 5 to 10 mOsm/kg. If the difference is >15 mOsm/kg, consider ethylene glycol, isopropanol, methanol, or ethanol toxicity.
• Evaluation of serum osmolality using osmolal gap (measured - calculated serum osmolality). If osmole gap is >10 mosm/kg H₂O, then consider the following differentials:
• Decreased serum water content
• Hyperlipidemia
• Hyperproteinemia (total protein > 10 g/dL)
• Low-molecular-weight (M.Wt.<150) substances in serum:
• Poisoning with or ingestion of ethanol, methanol, isopropanol, ethylene glycol
• Iatrogenic, such as mannitol, glycerol, propylene glycol, sorbital, glycine
• Medical conditions, such as alcoholic ketoacidosis, diabetic ketoacidosis, very low birth weight infants, hemorrhagic shock, trauma, multi-organ system failure, undialyzed chronic renal failure
• Laboratory or calculation error

Additional information

• Normal day to day variation of serum osmolality is 1-2% with peak levels in morning and lower by average of 7.5% in late afternoon or evening
• The simultaneous determination of serum and urine osmolality as in urinary/serum ratio is helpful to compare values between the two fluids. Normal ranges for the urinary/serum ratio are 0.2-4.7 and may be >3 with overnight dehydration
• In the Syndrome of Inappropriate Anti-Diuretic Hormone Secretion (SIADH), sodium and urine osmolalities are higher than appropriate, given the serum osmolality
• Factors interfering with test results include:
• Recent administration of radiographic contrast media
• Recent blood transfusion
• Volatile substances such as ketones, ethanol, isopropanol, and methanol, do not increase osmolality, causing absence of 'osmotic gap' if heat of vaporization method is utilized; It is therefore mandatory to confirm that freezing point depression method is utilized in such cases in order to give an accurate measurement
• Mannitol, sorbitol, or glycine solutions used as irrigation during transurethral resection of the prostate
• Related laboratory tests include:
• Ammonia
• Anion gap
• Arterial blood gas
• Blood and urine creatinine
• Blood and urine electrolytes
• Blood and urine ketones
• Blood and urine urea nitrogen
• Glucose
• Hematocrit
• Isopropyl alcohol level
• Serum proteins
• Urine osmolality
• Osmolal gap

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result

Conditions associated with elevated levels of serum osmolality (hyperosmolality) include:

• Increase in normal constituents of serum
• Azotemia - Uremia
• Cerebral lesions
• Hypernatremia with normal hydration such as hypothalamic disorders, defective osmostat
• Hyperglycemia as in diabetes mellitus, diabetic ketoacidosis
• Hypercalcemia
• Decrease in water content
• Dehydration
• Burns (Severe)
• Diabetes insipidus (Central or nephrogenic)
• Drugs (see below)
• Exercise (Heavy without adequate fluid replacement)
• Fever
• Hyperventilation
• Osmotic diuresis
• Renal disease (Chronic)
• Water intake (Inadequate)
• Increased exogenous substances / drugs
• Acetone
• Citrates (as an anticoagulant)
• Corticosteroids
• Ethanol
• Ethyl Ether
• Ethylene glycol
• Glycerin
• Glycerol
• Inulin
• Ioxithalamic acid
• Isopropanol
• Lorazepam
• Mannitol
• Methanol
• Paraldehyde
• Polyethylene Glycol
• Propylene Glycol
• Trichloroethane

Low Result

Conditions associated with decreased levels of serum osmolality (hypoosmolality) include:

• Hyponatremia with hypovolemia
• Burns (Severe)
• Diarrhea
• GI fistulas or drainage tubes
• Pancreatitis
• Peritonitis
• Renal insufficiency
• Sweating (Excessive)
• Vomiting
• Hyponatremia with hypervolemia
• Cirrhosis
• Congestive heart failure (CHF)
• Nephrotic syndrome
• Postoperative state
• Renal failure
• Adrenocortical insufficiency
• Syndrome of inappropriate antidiuretic hormone (SIADH)
• Inappropriate secretion of ADH as in trauma and lung cancer
• Excessive water replacement
• Overhydration
• Water intoxication
• Consumption of large quantities of beer
• Panhypopituitarism
• Pyelonephritis
• High altitudes
• Pregnancy (third trimester)
• Drug
• Acetazolamide
• Amiloride
• Amiodarone
• Amphotericin
• Angiotensin II receptor blockers
• Angiotensin-converting enzyme inhibitors
• Basiliximab
• Bendroflumethiazide
• Carbamazepine
• Carvedilol
• Chlorpromazine
• Chlorthalidone
• Clofibrate
• Cyclophosphamide
• Desmopressin
• Donepezil
• Eplerenone
• Gabapentin
• Haloperidol
• Heparin
• Hydroxyurea
• Indomethacin
• Ketorolac
• Lorcainide
• MDMA (ecstasy)
• Opiates
• Oxytocin
• Pimozide
• Propafenone
• Proton pump inhibitors
• Sirolimus
• Sulfonylureas
• Thiazides diuretics
• Tolbutamide
• Vincristine
• Zalcitabine
• Zonisamide

References

• ARUP Laboratories®. Osmolality, Serum or Plasma. [Homepage on the internet]©2007. Last accessed on July 11, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0020046.jsp
• Chu CH et al. Prognostic factors of hyperglycemic hyperosmolar nonketotic state. Chang Gung Med J. 2001 Jun;24(6):345-51.
• eMedicine from WebMD®. Hyponatremia. [Homepage on the Internet] ©1996-2007. Last updated on January 18, 2007. Last accessed on July 11, 2007. Available at URL: http://www.emedicine.com/emerg/topic275.htm
• Kapur G et al. Serum osmolal gap in patients with idiopathic nephrotic syndrome and severe edema. Pediatrics. 2007 Jun;119(6):e1404-7. Epub 2007 May 7.
• Laboratory Corporation of America®. Osmolality, Serum. [Homepage on the internet]©2007. Last accessed on July 11, 2007. Available at URL: http://www.labcorp.com/datasets/labcorp/html/chapter/mono/sc016800.htm
• Lab Tests online®. Osmolality. [Homepage on the Internet]©2001-2007. Last reviewed on January 30, 2006. Last accessed on July 11, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/osmolality/glance.html
• Sterns RH et al. Brain volume regulation in response to hypo-osmolality and its correction. Am J Med. 2006 Jul;119(7 Suppl 1):S12-6.
• Stoner GD et al. Hyperosmolar hyperglycemic state. Am Fam Physician. 2005 May 1;71(9):1723-30.
• Vialet R et al. Calculated serum osmolality can lead to a systematic bias compared to direct measurement. J Neurosurg Anesthesiol. 2005 Apr;17(2):106-9.