Sodium
Also known as: serum sodium, Na+, plasma sodium
The main positively charged ion of the fluid outside cells and the chief determinant of blood osmolality. Its blood level says more about the body's water balance than about salt: a low sodium usually means too much water relative to salt, a high one too little water.
- Also known as
- serum sodium, Na+, plasma sodium
- Units
- mmol/L
- Measured by
- Electrolytes, Kidney function tests
What it is and what it does
Total body sodium sets the volume of the extracellular fluid, but the concentration in blood is regulated by thirst and by antidiuretic hormone (ADH), which tells the kidney how much water to retain. Hyponatraemia, the commonest electrolyte abnormality in hospital, arises when ADH is released inappropriately (illness, pain, some drugs, lung and brain disease), when thiazide diuretics or adrenal failure waste salt, or when heart, liver or kidney failure retain water in excess of salt. A fall that develops quickly swells brain cells and causes confusion, seizures and coma. Hypernatraemia reflects water loss that has not been replaced (fever, diarrhoea, diabetes insipidus) in people who cannot drink enough, typically the very young, the very old and the unwell. Correction in either direction must be gradual to avoid brain injury.
Why it may be higher
- Water loss without replacement: fever, sweating, diarrhoea, osmotic diuresis in uncontrolled diabetes, diabetes insipidus
- Inability to drink: infancy, dementia, unconsciousness
- Excess salt intake or hypertonic fluids (rare)
Why it may be lower
- Syndrome of inappropriate ADH (SIADH): lung and brain disease, cancers, pain, nausea, medicines such as SSRIs, carbamazepine, antipsychotics
- Thiazide diuretics
- Heart failure, cirrhosis, nephrotic syndrome (dilution)
- Adrenal insufficiency, hypothyroidism
- Excess water intake, prolonged endurance exercise with water only
- Spurious (pseudohyponatraemia) with very high lipids or proteins, or a sample drawn near a drip
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- Fluid intake and intravenous fluids at the time of sampling
- Glucose: very high blood glucose lowers the measured sodium
- Medicines, especially diuretics and antidepressants
Medicines that can change it
Each medicine page lists its effect on this marker under "Effects on tests and results", with the source.