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 osmolality can be approximated by the following equation:

(2 x sodium) + 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.

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 sodium, potassium and urea. The more concentrated the urine is, the higher its osmolality.

How is it used?

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.

Serum and urine osmolality can be used to investigate for conditions that affect the secretion of ADH, such as diabetes insipidus, or the syndrome of inappropriate ADH secretion (SIADH).

Serum and urine osmolality are also used together to help investigate causes of hyponatraemia (low sodium concentration). 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.

Measurement of osmolality can also help detect the presence of toxins such as methanol and ethylene glycol, and to monitor osmotically active drug therapies such as 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 (e.g. 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.

What does the result mean?

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.

More information

RCPA Manual: Osmolality

RCPA Manual: Osmolality - urine