Synonyms: Na, blood sodium, urine sodium, sodium
Related tests: Potassium, Chloride
Clinical significance
Sodium is the main electrolyte outside cells and helps maintain the volume of fluid inside and outside cells. The sodium concentration outside cells is maintained by an active transport system (the sodium/potassium pump), which moves sodium out across the membrane and brings potassium back in. In the kidneys, most of the sodium filtered by the glomeruli is reabsorbed in the proximal tubule and the loop of Henle and then, under the control of aldosterone, exchanged for potassium in the distal tubule; the sodium excreted in the urine reflects the amount taken in (some is also eliminated in sweat and stools).
Clinical indications
In outpatient practice the test is mainly used to assess excretion by the kidneys.
Sample type
The patient must have a blood sample taken.
The patient must bring a 24-hour urine collection if requested.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.