Osmolality Test

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Osmolality Test


The osmolality test—sometimes called serum osmolality test or blood osmolality test—is used to measure the osmolality of chemical particles dissolved in serum. Osmolality is defined as the osmotic pressure(or concentration of substances) of a solution. Chemicals that primarily affect serum osmolality include bicarbonate, chloride, potassium, proteins, sodium, glucose, and blood urea nitrogen. Other chemical particles affect serum osmolality in lesser degrees.


When chemicals are dissolved in body fluids, the number of distinct particles in solution is measured as osmolality; that is, as a concentration. The quantity is measured in osmoles (Osm) of dissolved substance per kilogram (kg) of solvent, which is equal to the number of moles of osmotically active particles in the solution. For example, one mole of sodium (Na) is equal to one osmole, while one mole of sodium chloride (NaCl) is equal to two osmoles (one mole of sodium plus one mole of chloride).

Water is constantly entering (through drinking and breathing) and exiting (through urination and sweating) the body. If this water balance is not maintained, then the concentration of chemicals in the blood can change, either increasing with too little water or decreasing with too much water.

The osmolality test is performed to measure the concentration of particles in solution, which then determines the balance between water and the chemicals dissolved in blood. An osmolality urine test is used specifically to measure the osmolality of urine. Osmolality is controlled primarily by the hormone called antidiuretic hormone (ADH). ADH is produced by the hypothalamus within the brain. It is then released by the pituitary gland into the bloodstream and affects the amount of urine the body produces.

When not enough water enters the body, osmolality is increased. To counterbalance this imbalance, ADH is secreted in larger amounts, which reduces the amount of water excreted by the kidneys. Because of this action, increased water reabsorption, more concentrated urine, and less concentrated blood plasma occurs to restore normal water levels.

On the other hand, when too much water enters the body, osmolality is decreased. ADH is then released in smaller amounts to solve the imbalance, which increases the amount of water excreted by the kidneys. As a direct result, decreased water reabsorption, less concentrated urine, and more concentrated blood plasma occurs.

The osmolality test helps to determine if the hypothalamus is producing ADH in a normal way. It is also used to determine the cause of comas and seizures, to evaluate the kidney's ability to expel water and chemicals from the blood and to discard them in urine, and to determine the percentage of ingested poisons. When the blood osmolality test is used with the osmolality urine test, the condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH) can be diagnosed. A positive result of SIADH can indicate the presence of cancer, diseases involving the central nervous system, lung disease, and the excessive use of some medications.


During the osmolality test, when a needle is inserted through the skin to draw blood, some people feel moderate pain. Others feel only a slight stinging or pinching sensation when the needle first pricks the skin. Some people feel a stinging sensation while the needle is inside the vein. Others feel no pain. Afterward, there may be throbbing or bruising around the puncture area. Healthcare workers should take universal precautions when drawing blood.


The osmolality test begins when blood is drawn from a vein. The health professional finds a vein usually on the back of the hand or on the inside of the elbow. The area of needle insertion is cleansed with antiseptic. An elastic pressure band (tourniquet) is tightly wrapped around the upper arm, which forces blood to collect in the vein and, thus, makes it easier to insert the needle into the vein. After the needle has been inserted into the vein, blood is extracted from the vein and collected into airtight collection tubes such as vials or, alternatively, may be collected into a syringe. After the required amount of blood is drawn, the needle is carefully removed and the site is covered with a gauze pad or cotton ball. Pressure is applied to the site for several minutes to stop any bleeding that may occur. A bandage is placed over the gauze pad/ cotton ball.


The patient should fast for six hours before the test. In order to prevent a false reading, alcohol should not be consumed before the osmolality test. If a blood transfusion was recently given to the patient, the health professional administrating the test should be informed as the test may need to be delayed.


No aftercare is necessary.


There is little serious risk from complications due to the test. Excessive bleeding, feeling light-headed or fainting, hematoma, bruising, and puncture marks are all minor complications that could result. The vein may become inflamed and/or infected afterwards, but this occurrence is rare. Continued bleeding is possible for those with bleeding disorders. Such conditions should be communicated to the health professional before the test.


When the results from the blood draw are returned from the laboratory, a normal osmolality reading for adults should lie between 275 and 300 milli-osmoles per kilogram (mOsm/kg) of water. However the normal range varies somewhat among different medical facilities and laboratories.

If the reading is above 300 mOsm/kg or another predetermined maximum value, it may indicate one of the following:

  • Addison's disease,
  • congestive heart failure,
  • dehydration,
  • diabetes insipidus,
  • head trauma,
  • hyperglycemia,
  • hypernatremia,
  • poisoning by alcohol,
  • pyelonephritis,
  • renal tubular necrosis,
  • shock,
  • stroke, or
  • uremia.

If the reading is below 275 mOsm/kg or another predetermined minimum value, it may indicate one of the following:

  • aldosteronism,
  • diabetes insipidus,
  • excess fluid intake,
  • hyponatremia,
  • overhydration, or
  • paraneoplastic syndromes.

Health care team roles

The health professional will administer the blood draw, while the patient's physician will explain the test results to the patient after it is returned from the laboratory.


Addison's disease— Partial or total failure of adrenocortical function.

Aldosteronism— Excessive secretion of aldosterone by adrenal cortex.

Dehydration— Too little water in the body.

Diabetes insipidus— Rare disease in which kidneys produce excessive amounts of urine.

Fibrinogen— Protein important for the clotting of blood.

Glucose— Sugar.

Hematoma— Blood accumulating under the skin.

Hyperglycemia— Excessive sugar in the blood.

Hypernatremia— Excess level of sodium in the blood.

Hyponatremia— Deficient level of sodium in the blood.

Overhydration— Excessive amount of water in the body.

Paraneoplastic syndromes Complications that develop when cancer cells release certain substances into the blood stream that cause abnormal function of cells and tissues.

Pyelonephritis— Inflammation of kidneys.

Renal tubular necrosis— Kidney disorder characterized by damage to renal tubule cells and eventually to failure of the kidneys.

Serum— Liquid part of blood.

Solvent— Substance that dissolves something else.

Uremia— Blood poisoning.



Grossman, Ashley, editor. Clinical Endocrinology. Oxford, UK: Blackwell Publishing, 1998.


AllRefer Health. "Osmolality Test." 〈http://health.allrefer.com/pictures-images/osmolality-test.html〉 (November 28, 2005).

Payne, Kattie. "Serum Osmolality." 〈http://www.webmd.com/hw/health_guide_atoz/hw203418.asp〉 (November 28, 2005).