Osmolality

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Also known as: Osmolarity
Formal name: Osmolality
Related tests: Sodium, potassium, urea, glucose, alcohol, antidiuretic hormone (ADH), arginine vasopressin (AVP), water deprivation test

At a Glance

Why Get Tested?

To determine whether the osmolality of the blood (or urine) is within normal limits

When to Get Tested?

When a patient cannot produce concentrated urine, particularly following a head injury; when Diabetes Insipidus is suspected; when symptoms of inappropriate anti-diuretic hormone secretion are present; when an explanation of a low serum sodium is being sought

Sample Required?

A blood sample drawn from a vein in your arm; a urine sample taken at the same time usually helps the doctor to interpret the results

The Test Sample

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 osmolatity is approximately:

2(sodium + potassium) + 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.

How is the sample collected for testing?

A blood sample is taken by needle from a vein in the arm. You may be asked to provide a urine sample as well.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.