What does it measure?
This blood test measures sodium concentration in the fluid around your cells. Your kidneys and hormones work together to keep that concentration steady. The result reflects the relationship between sodium and water, rather than simply how much salt you ate yesterday. [1]
Before the test
- Sample
- A blood sample from a vein.
- Preparation
- Confirm any fasting instructions and mention water tablets or recent fluid loss.
What can the result mean?
A high result may mean your body has lost more water than sodium. A low result can happen with fluid retention, vomiting, some medicines, or hormonal conditions. The same number can have different explanations, so other electrolytes and your symptoms matter. [1]
When is it useful, and how is it read?
This test helps when investigating fluid loss, fluid retention, or a medicine effect. A low result with vomiting needs a different assessment from a low result with swelling. We read the trend, other electrolytes, and kidney findings together before deciding what the change means. [1]
What can affect it?
Tell your care team about recent illness, heavy sweating, changes in drinking, and medicines such as water tablets. Your usual routine is helpful context; drinking unusually large amounts of water before testing can make interpretation harder. [1]
Blood sodium is not a salt-intake score
A sodium result within range does not prove your diet is low in salt. The concentration reflects how sodium and water are balanced, so it answers a different question from a dietary assessment. [3]
Questions you might have
Can sodium be low even if I eat salt?
Yes. Extra water can dilute sodium, and medicines or illness can change the balance. The result is not a direct measure of salt intake. [1]
Why compare sodium with other electrolytes?
Together they show whether the change is part of a wider fluid or blood-chemistry problem. Sodium alone does not identify the cause. [1]
What comes next?
Review an unexpected result with your clinician before trying to correct it with extra salt or water. They may repeat the test or check blood and urine concentration. New confusion or seizures need urgent medical attention. [1]
Could a change in fluid balance or one of my medicines explain this result?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
- Moisio MA, Moisio EW. Understanding Laboratory and Diagnostic Tests. Delmar Publishers; 1998.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
