Serum osmolality

Some labs print 280 to 300; a calculated osmolality is usually a few units lower than a measured one.

Osmolality is a measure of how concentrated the blood is, counting all the dissolved particles together, mostly sodium and its partner salts, glucose and urea. It reflects how much water the body is holding relative to those particles. The test measures it directly on a blood sample using a freezing-point instrument.

Doctors request it when sodium is unexpectedly low or high, to help work out whether the problem is with water, salt or something else in the blood. It is also used, together with urine osmolality, when looking into excessive thirst or urine output.

• Dehydration
• High blood sugar
• Reduced kidney filtering, which raises urea
• Alcohol or certain poisons such as ethylene glycol or methanol
• Some hormone conditions where the kidneys lose water

• Drinking a very large amount of water
• Water tablets
• Some antidepressants, epilepsy medicines and painkillers that make the body hold water
• Hormone conditions affecting the adrenal, pituitary or thyroid glands
• Heart, liver and kidney conditions where fluid builds up

• Alcohol the night before raises it
• It moves in step with sodium, so hydration on the day matters
• A calculated osmolality and a measured one are different numbers and should not be compared with each other

No fasting needed, though alcohol beforehand raises it. Usually paired with a urine osmolality taken at the same time.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.