Other names: Osmolarity

Osmolality is a measurement, which measures how many particles are dissolved in a kilogram of fluid; osmolarity is very similar, but it measures how many particles are dissolved in a litre of fluid. Osmolality is usually associated with sodium and potassium salts and glucose and urea; a simple calculation is carried out to determine whether the osmolality of the blood is within the target range.

Why is the test used?

The test is done to assess whether the osmolality is within the normal, healthy range. The test may be ordered when a patient is having problems producing concentrated urine (usually following a head injury) or when a patient has abnormal levels of sodium. The test may also be used when a patient is using diuretic medication or when the doctor suspects a patient has diabetes insipidus (this occurs when the blood becomes too concentrated because too much water is lost from the blood).

How is the test done?

The test is done by taking a sample of blood from a vein in the arm; the blood is drawn out using a needle and a syringe. Once the doctor or nurse has a sufficient sample, the blood will be placed in a bottle, labelled with the patient’s name and sent away for analysis.

The patient may also be requested to provide a urine sample.

What do the test results show?

In a healthy person, the osmolality is usually maintained within a narrow target range by carefully controlling the dilution of the blood; if levels are higher than this range, this may indicate that there is a problem with the components which are controlling the system. Damage to the head or brain may affect the hypothalamus, which can result in dilute plasma which causes low osmolality. Diabetes insipidus can also cause the urine to be very dilute.

Diabetes mellitus (the more common form, which affects blood sugar levels) can cause high osmolality if it is untreated or poorly controlled.

Specific Blood Tests

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