Osmolality (Osmolarity)

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Formal name: Osmolality (plasma, urine, stool)
Related tests: Sodium; potassium; urea; glucose; alcohol; antidiuretic hormone (ADH); arginine vasopressin (AVP); water deprivation test

The Test Sample

What is being tested?

Osmolality is a measure of the number of particles dissolved in a kilogram of fluid. Osmolarity is the number of particles in a litre of fluid. In a dilute fluid osmolality and osmolarity values are approximately the same. An osmotically active substance is any substance that increases the osmolality of a fluid.

Measurement of osmolality in serum and urine is used to assess water balance, and whether the body is reacting appropriately to changes in water balance. Water balance in the body is a dynamic process that is regulated by controlling the amount of water excreted in the urine and by regulating the sensation of "thirst."

Osmotic sensors in the body sense and react to changes in the amount of water and particles in the bloodstream (i.e. factors that alter the osmolality). In health, the osmolality of blood is very closely regulated. When blood osmolality increases, for example in dehydration, the hypothalamus secretes the hormone “antidiuretic hormone” (ADH). ADH signals for the kidneys to conserve water, resulting in formation of concentrated urine (which has a high osmolality). This retention of water dilutes the blood causing a decrease in osmolality back to normal levels. If, on the other hand, blood osmolality decreases (for example following a large drink of water) then ADH secretion is suppressed and the kidneys excrete increased amounts of dilute urine. This results in a decrease in the amount of water in the body, and so blood osmolality rises to normal.

Osmolality can be directly measured in the laboratory using an osmometer.

Normally, the major particles contributing to osmolality in serum are sodium and potassium salts, glucose and urea. Therefore osmolality can be approximated by the following equation:

Calculated Serum Osmolality = 2 (sodium + potassium) + glucose + urea

When all values are in mmol/L. Note, there are several variations of this calculation in use.

The calculated and measured osmolality can be compared to determine if there are other particles present (in addition to sodium, potassium, glucose and urea) that contribute to the measured serum osmolality. The difference between measured and calculated (estimated) osmolality is called the "osmotic gap" or "osmolal gap." An increase in the osmotic gap (greater than 10) indicates the presence of other substances such as toxic alcohols, aspirin, or mannitol.

Osmolality of the urine is used as a measure of the kidneys ability to concentrate urine. Urine osmolality is largely due to the presence of urea and creatinine. The more concentrated the urine is, the higher its osmolality.


How is the sample collected for testing?

A blood sample is taken by needle from a vein in the arm. You may be asked to provide a urine sample.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation may be needed or you may be instructed to fast (nothing to eat or drink except water) for 6 hours before the test. Follow any instructions you are given. Because some medications can interfere with this testing, inform your health care provider of all of the medications you are taking, especially if you are taking mannitol.