Osmolality (Osmolarity)

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 may indicate the presence of a drug or toxin (e.g. 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 toxin present.

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 to 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.

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. It results from 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.

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 sugas 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 (in response of the body ia 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.