Home › Biomarkers › Calculated osmolality
Osmolality is the concentration of particles dissolved in your blood. A high value can point to too little water in your blood, a low value to too much water.[1]
On this page you can read what calculated osmolality is, what the guidelines say about a value that is too low or too high, what affects your value and which values are best looked at alongside it.
Also known as: serum osmolality, plasma osmolality. At the lab: n/a.
Osmolality is the concentration of particles dissolved in your body fluids. That concentration depends on how many particles there are in a given amount of fluid. In your blood, these are glucose (sugar), urea (a waste product your liver makes) and electrolytes such as sodium, potassium and chloride.[1]
Electrolytes are minerals that take on an electrical charge when they dissolve in water or body fluids. They help regulate the fluid balance in your body. They also help keep the balance between acids and bases steady.[1]
With calculated osmolality, the lab does not measure the value directly. The lab works out the value using other values from your blood.
When the osmolality in your blood rises, your kidneys hold on to water. Your urine then becomes more concentrated. When the value falls, your urine becomes more diluted instead. This is how your body helps keep a healthy fluid balance.[1]
Sometimes that balance gets disturbed. This can be caused by too much salt, a kidney condition, a heart condition or some types of poisoning.[1]
A doctor can use the test to look at the balance between water and substances in your blood. The test can also help to see whether you are losing or holding on to too much fluid, whether you have swallowed something poisonous such as antifreeze or rubbing alcohol, and whether a kidney condition that makes you urinate often is involved.[1]
Symptoms of a disturbed fluid balance can include: great thirst, nausea and vomiting, headache, confusion, tiredness and seizures.[1]
| Value | Can point to | Who it is for |
|---|---|---|
| Higher than normal[1] | Less water in your blood, or more sodium, chloride or glucose; also losing too much fluid | Anyone whose blood is tested for osmolality |
| Lower than normal[1] | More water in your blood; also a body that holds on to too much fluid | Anyone whose blood is tested for osmolality |
| Out of balance[1] | Too much salt, a kidney or heart condition, or some types of poisoning | Anyone whose blood is tested for osmolality |
Source: MedlinePlus, osmolality tests. A plain-language explanation of osmolality in blood, without reference limits.[1]
A high value can point to too little water in your blood, or to more sodium, chloride or glucose. That can fit with losing too much fluid. Your kidneys then hold on to water and your urine becomes more concentrated.[1]
A low value can point to more water in your blood. That can fit with a body that holds on to too much fluid. Your urine then becomes more diluted.[1]
BioProfile is working on its own zones for each value, from risk to optimal. A doctor reviews those zones before they appear in your result.
If there is less water in your blood, the value rises. If there is more water in your blood, the value falls.[1]
More sodium, chloride or glucose in your blood makes the value rise. Taking in too much salt can disturb your fluid balance.[1]
An overdose of medicines containing salicylates, which are often found in aspirin, can affect the value. The same goes for swallowed poisonous substances such as antifreeze or rubbing alcohol.[1]
Sometimes you need to avoid eating or drinking for 6 hours before the test, or drink less for 12 to 14 hours beforehand. Your healthcare provider will tell you whether this applies to you.[1]
Calculated osmolality is included in BioProfile 77, 87, 121 and 150.[2]
The BioProfile editorial team wrote this page on 10 October 2026 from the sources below, which we read on 10 October 2026. Every number on this page appears in one of those sources, with that source's number next to it. An expert has yet to check this page; after that check, the expert's name and the date of the check will appear at the top. This page offers explanation, not a diagnosis. Discuss a value outside the range with your GP.
