Serum osmolality test explained: what it measures and why it is done

Osmolality describes how concentrated a fluid is. Lab Tests Online UK explains that it is the number of osmotically active particles dissolved in a kilogram of water in biological systems, and that these particles draw water across cell membranes. For clinical purposes, osmolality and osmolarity are approximately the same and are used interchangeably.

The particles that contribute most to serum osmolality are sodium, chloride, potassium, bicarbonate, glucose and urea. Because sodium and its accompanying ions dominate, a blood sodium result gives a good first impression of osmolality, which is why the two tests are so closely linked.

The body keeps blood osmolality within a tight range. Osmotic sensors detect changes, and when osmolality rises, for example in dehydration, the hypothalamus promotes release of antidiuretic hormone (ADH) from the pituitary, which tells the kidneys to conserve water and produce concentrated urine. Rising osmolality also increases thirst. When osmolality falls, ADH release is suppressed and the kidneys excrete more dilute urine. The NHS describes the same hormone, AVP, as helping the body retain water by reducing the amount lost through the kidneys.

It is not a routine test. Lab Tests Online UK describes the situations in which serum and urine osmolality may be measured.

• A low blood sodium (hyponatraemia), to help work out the cause.

• A high blood sodium (hypernatraemia).

• An unusually high or low urine output.

• Excessive thirst.

• Suspected ingestion of toxic alcohols such as methanol or ethylene glycol, when serum osmolality should be tested.

• Very rarely, stool osmolality in unexplained diarrhoea.

A low sodium result does not always mean the blood is truly dilute. Lab Tests Online UK explains that serum osmolality is used to separate low sodium with low osmolality (hypotonic hyponatraemia) from pseudohyponatraemia, where sodium is low but osmolality is normal, for example in people with high protein or triglyceride levels, and from low sodium with high osmolality, for example with high glucose, ethylene glycol, urea or mannitol. This helps direct further investigation and treatment.

Urine osmolality then helps show how the kidneys are responding. When blood sodium is low, the kidneys would normally excrete dilute urine. In syndrome of inappropriate antidiuresis (SIAD), there is overproduction of ADH, giving a dilute serum with low sodium and low osmolality alongside inappropriately concentrated urine. Lab Tests Online UK describes this as a diagnosis of exclusion in people with low sodium.

Lab Tests Online UK explains that serum osmolality can be measured directly in the laboratory, and can also be calculated from sodium, potassium, glucose and urea using one of several equations, with no clear consensus on the best equation. The osmolar (or osmolal) gap is the difference between the measured and calculated values.

An increase in the gap, greater than 10 mOsm/kg, indicates the presence of other osmotically active substances, such as toxic alcohols or mannitol, that contribute to osmolality but are not routinely or easily measured. Differences between measured and calculated osmolality are therefore particularly useful when poisoning is suspected. A raised gap is a prompt for urgent clinical assessment and is not something to interpret at home.

Diabetes insipidus, which the NHS describes as caused by problems with the hormone arginine vasopressin (AVP, also called ADH), involves passing large volumes of dilute urine. Lab Tests Online UK says it is diagnosed using a water deprivation test, which should be considered in people with proven high urine volume and a spot urine osmolality that is inappropriately low compared with a high serum osmolality.

During the test, fluid intake is restricted and weight, serum and urine osmolality and sodium are measured frequently. Failure to concentrate urine despite fluid deprivation supports the diagnosis. Synthetic ADH may be given part-way through to distinguish a failure to produce ADH from a failure of the kidneys to respond. Lab Tests Online UK stresses that the test has risks and should only be performed carefully by specialist medical teams.

A blood sample is taken from a vein in the arm, and a urine sample taken at the same time usually helps the doctor interpret the result. Lab Tests Online UK advises following any instructions provided, for example paired serum and urine samples before treating low sodium, and telling your healthcare provider about all medications you take, for example mannitol.

Reference ranges differ between laboratories, so your result should be read against the range on your own report. If a clinician has requested osmolality, ask which question it is trying to answer, as the interpretation depends on your sodium, glucose, urea, urine output and symptoms. Lab Tests Online UK adds that abnormal osmolality is treatable, and that people may sometimes be referred to an endocrinologist, because abnormal hormone production may be responsible.