At a glance

Also known as

Osmolarity

Why get tested?

To determine whether the osmolality of the blood (or urine) is within normal limits

When to get tested?

When a patient cannot produce concentrated urine, particularly following a head injury; when Diabetes Insipidus is suspected; when symptoms of inappropriate anti-diuretic hormone secretion are present; when an explanation of a low serum sodium is being sought

Sample required?

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

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. Osmolality and osmolarity values are approximately the same. Normally, the major particles contributing to osmolality are sodium and potassium salts, glucose and urea and the osmolatity is approximately:

2(sodium + potassium) + glucose + urea (all measured in mmol/L).

This simple calculation is useful because sodium, potassium, glucose and urea are more frequently and more easily measured than osmolality. Osmolality is measured in the laboratory using an osmometer.

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 as well.

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

No test preparation is needed.

The Test

How is it used?

Osmolality is often measured when the doctor finds an abnormally low sodium (hyponatraemia) which is difficult to interpret. Osmolality may also be measured if you are drinking lots of fluids, when diabetes insipidus or other disorders are being considered. Measurement of osmolality can also help in patients treated with the osmotic diuretic, mannitol.

A high plasma osmolality can rarely be due to large amounts of alcohol in the blood. The difference between the osmolality measured in the laboratory and the value calculated from the sodium, potassium, glucose and urea (above) can give an indication of the amount of alcohol present.

When is it requested?

Your doctor may request the test if other results (eg a high blood sodium) suggest a diagnosis of diabetes insipidus or the syndrome of inappropriate antidiuretic hormone (SIADH) secretion. Diabetes insipidus is a condition very different from diabetes mellitus (sugar diabetes) in which an excessive and inappropriate volume of water is lost in the urine and the blood becomes too concentrated.

Osmolality may also be measured in a Water Deprivation test, if the doctor needs to distinguish between two different types of diabetes insipidus or a condition called psychogenic polydipsia in which a patient drinks water compulsively.

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

What does the test result mean?

Looking for reference ranges?

Physiological mechanisms normally maintain plasma osmolality within a tight range.

If the plasma osmolality increases, meaning that the blood is too concentrated, water is retained by the kidneys so that a concentrated urine is produced and the blood is diluted to normalise the plasma osmolality. Conversely, if plasma osmolality decreases, meaning that the blood is too dilute, water is eliminated by producing a dilute urine and the plasma osmolality then increases.

Plasma osmolality may be abnormal if the mechanisms controlling it are disturbed.

A head injury, or disease of part of the brain, may result in overproduction of anti-diuretic hormone (ADH) by the hypothalamus resulting in a dilute plasma (low osmolality). This is normally detected by finding a persistently low plasma sodium in the presence of an inappropriately concentrated urine (syndrome of inappropriate ADH secretion (SIADH)).

Disease of the adrenal cortex can also result in a low osmolality due to decreased production of other hormones (aldosterone, cortisol) involved in water (osmolality) regulation.

Diabetes Insipidus (‘water diabetes’) is a condition in which the urine is always very dilute, leading to water loss from the body and possible dehydration. Too little antidiuretic hormone or failure of the kidneys to respond to it, results in a high plasma osmolality in the presence of an inappropriately dilute urine.

In uncontrolled Diabetes Mellitus (the more common ‘sugar diabetes’), a high plasma glucose and the water loss associated with glucose excretion by the kidneys, causes a high plasma osmolality.

Water intoxication may also decrease plasma osmolality and dehydration or intake of toxins will increase it.

Is there anything else I should know?

An abnormally high plasma osmolality may result from water depletion (dehydration). Toxins, including alcohol, can increase plasma osmolality because they contribute to the number of particles of solute in the plasma. Similarly sugar diabetes (diabetes mellitus), if uncontrolled, will increase plasma osmolality, both from high plasma glucose concentration and from the water loss resulting from excretion of excess water with the glucose excreted via the kidneys.

Classical diabetes insipidus, in which there is a deficiency of antidiuretic hormone, and nephrogenic diabetes insipidus, where the kidneys do not respond to antidiuretic hormone, also cause a rise in plasma osmolality as a result of excess water loss via the kidneys. These diseases may require a water deprivation test (the response of the body is studied) to distinguish them from each other and from excess water intake (psychogenic polydipsia).

Kidney disease may result in an abnormally high or low plasma osmolality, depending on the stage or type of disease.

Interpretation of an abnormal plasma osmolality often requires measurement of urine osmolality to ascertain whether the body is producing an appropriately concentrated or dilute urine. The diseases causing abnormalities of osmolality are usually treatable.

Common Questions

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.