Formal Name
Osmolality (plasma, urine, stool)
This article was last reviewed on
This article waslast modified on 14 November 2018.
At a Glance
Why Get Tested?

Urine and serum osmolality may be measured together to investigate the cause of a low sodium concentration in the blood (hyponatraemia), or a high or low urine output. Serum osmolality may also be measured when ingestion of toxic alcohols such as methanol and ethylene glycol are suspected. Occasionally, stool osmolality is measured to help determine the cause of chronic diarrhoea.

When To Get Tested?

Urine and serum osmolality should be tested when an explanation is sought for either i) a low serum sodium concentration ii) an unusually high urine output or iii) an unusually low urine output.

Serum osmolality should be tested if poisoning with toxic alcohols is suspected.

Stool osmolality may be tested when an explanation for chronic diarrhoea is being sought.

Sample Required?

A blood sample taken from a vein in your arm; a urine sample taken at the same time usually helps the doctor to interpret the results

Test Preparation Needed?

None may be required or you may be instructed to fast for 6 hours before the test; you may be requested to collect a sample of the first urine passed in the morning; follow any instructions provided.

Inform your health care provider of all medications you are taking, especially mannitol.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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.

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.

Accordion Title
Common Questions
  • How is it used?

    The osmolality test is requested to help evaluate the body's water balance or its ability to produce and concentrate urine. It is used to help investigate low sodium levels (hyponatraemia), to detect the presence of toxins such as methanol and ethylene glycol, and to monitor osmotically active drug therapies such as mannitol. It is also requested to help monitor the effectiveness of treatment for any conditions found to be affecting a person's osmolality.

    Serum and urine osmolality are often used together to help investigate causes of hyponatraemia. Hyponatraemia can occur either due to loss of sodium (e.g. in the urine) or increased fluid volume in the bloodstream. Increased fluid may be due to either increased intake of fluids (e.g. excessive drinking) or retention of fluid by the kidneys (observed as decreased urine output). People who chronically drink excessive amounts of water either by choice or due to a psychological condition may have chronic hyponatraemia.

    Serum and urine osmolality are also often used together to help investigate causes of either a high or low urine output.

    Increased urine output may be due to:

    1) A high fluid intake

    2) Elevated levels of an osmotically active agent in the urine (e.g. glucose in diabetes mellitus)

    3) Lack, or inappropriate action, of ADH (Diabetes Insipidus)


    Decreased urine output may also be due to a variety of causes, including:

    1) An appropriate response to dehydration

    2) Decreased blood flow to the kidneys

    3) Damage to tubular cells in the kidneys.

    Urine sodium is often requested along with urine osmolality to help determine the cause of hyponatraemia. 


    Serum osmolality and calculation of the osmolar gap (the difference between measured and calculated osmolality) is used to help detect the presence of ingested toxins such as methanol. The two calculations required are as follows:

    Calculated osmolality = 2 (sodium + potassium) + glucose + urea

    Osmolar gap = measured osmolality – calculated osmolality

    An osmolal gap of greater than 10 is considered abnormal and represents the presence of an osmotically active substance (such as methanol) in the blood. 

    Stool osmolality may sometimes be requested to help evaluate chronic diarrhoea, for which a cause cannot be found. People with watery chronic diarrhoea may have an osmotically active substance that is inhibiting the reabsorption of water by the intestines. Sometimes a stool osmotic gap is calculated.


  • When is it requested?

    Your doctor may request a urine and serum osmolality test in the following circumstances:

    1) If hyponatraemia (low sodium) has been found on a previous blood test

    2) To investigate a low urine output

    3) To investigate a high urine output

    4) To investigate increased thirst

    5) As part of a “water deprivation test”, a test used to help confirm a diagnosis of Diabetes Insipidus.

    Serum osmolality may also be measured when ingestion of a toxin is suspected and to assess how much toxin is present in the blood.

  • What does the test result mean?

    Physiological mechanisms normally maintain plasma osmolality within a tight range. 

    A high plasma osmolality may be observed in the following conditions

    1. Water depletion (dehydration). This would be accompanied by a reduced urine output.

    2. An elevated glucose concentration in the blood (due to uncontrolled Diabetes Mellitus). Urine output also increases in this disorder.

    3. Following ingestion of toxins, including alcohol, because they contribute to the number of particles of solute in the plasma.

    4. Diabetes Insipidus (“water diabetes”), a condition in which the urine is always very dilute, leading to water loss from the body and possible dehydration. It results from too little ADH or failure of the kidneys to respond to it. Persons with Diabetes Insipidus can have a high plasma osmolality in the presence of inappropriately dilute urine. High concentrations of calcium or low concentrations of potassium in the blood can also impair the kidneys response to ADH, causing symptoms of Diabetes Insipidus.

    A decreased plasma osmolality may be seen in water intoxication or any disorder that causes water retention by the kidneys. For example, in a disorder called “Syndrome of Inappropriate ADH Secretion” (SIADH) there is overproduction of anti-diuretic hormone (ADH) by the hypothalamus resulting in a dilute serum (low osmolality). This is normally detected by finding persistently low plasma sodium in the presence of inappropriately concentrated urine.

  • Is there anything else I should know?

    Calculation of "free water clearance" is sometimes used to help evaluate the ability of the kidney tubules to appropriately concentrate and dilute urine. When urine osmolality is about the same as plasma osmolality, then free water clearance is zero. When blood volume decreases and urine is concentrated, then free water clearance will be negative. When fluid levels are increased and urine is dilute, then free water clearance will be positive..

  • Are diseases that cause abnormal osmolality treatable?

    Yes. Sometimes people need to be referred to an endocrinologist - a doctor who deals with abnormalities of the hormone producing glands, which may be responsible for a low or high plasma osmolality.