Osmolality (Osmolarity)

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Formal name: Osmolality (plasma, urine, stool)
Related tests: Sodium; potassium; urea; glucose; alcohol; antidiuretic hormone (ADH); arginine vasopressin (AVP); water deprivation test

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.