Sodium

What is being tested?
Sodium, a mineral essential for normal body functions, is a positively charged electrolyte that, together with other electrolytes such as potassium, chloride and bicarbonate, helps maintain the body’s fluid and salt balance. Sodium is present in all body fluids, but in higher concentrations in the extracellular fluids with which the plasma is in equilibrium. Sodium is taken in through the diet as table salt (sodium chloride) and partly also in food; a balanced diet provides enough sodium: the amount needed is absorbed in the intestine, while the excess is eliminated in the urine so that blood levels are kept within a narrow range. Regulation of the body’s sodium content is closely linked to control of extracellular fluid volume and involves several mechanisms:

• the secretion of hormones that can increase (natriuretic peptides) or reduce (aldosterone) the elimination of sodium in the urine
• the secretion of ADH, the antidiuretic hormone, whose biological function is to prevent water loss in the urine
• activation of the thirst reflex, which stimulates water intake

Changes in any of these regulatory mechanisms lead to changes in blood sodium concentration, which are reflected in changes in the body’s water content, commonly dehydration or oedema.

How is the sample collected for testing?
The sample is collected by drawing blood from a vein in the arm. In some cases a urine sample may be requested.

How is it used?
When is it requested?
What does the test result mean?

How is it used?
Plasma sodium is measured in a wide range of conditions, for example when dehydration or oedema is found. The doctor may order this test on its own or together with other electrolytes to check for an electrolyte imbalance. In patients with a known electrolyte imbalance, serum sodium may be requested at regular intervals to monitor the effectiveness of treatment; the test may also be ordered for patients taking medicines that can change plasma sodium levels, such as diuretics. In this case it is also useful to know how much sodium the kidneys are excreting.
The conditions in which urine sodium is most often requested are high blood pressure and kidney disease, where sodium measurement can help the doctor identify the cause of the damage; the test may also be ordered in conditions affecting the central nervous system, lungs, heart and adrenal glands.

When is it requested?
Plasma sodium is usually measured together with other electrolytes such as chloride and potassium; it may be performed as part of a regular routine check-up or when monitoring patients with kidney disease or heart failure, or patients with high blood pressure taking medicines, such as diuretics, that can interfere with the regulation of body fluid volume and osmolality. These conditions may be associated with changes in the body’s water and sodium composition, whose symptoms are generally rather non-specific and more severe the faster the changes develop: in particular, hyponatraemia causes neurological disturbances such as weakness, confusion and lethargy; the symptoms of hypernatraemia include thirst, reduced urine output, neuromuscular irritability and/or agitation.

What does the test result mean?
Both hyponatraemia and hypernatraemia reflect changes in the ratio of sodium to water in the extracellular compartment. The symptoms of hyponatraemia, whose severity depends on how quickly it develops, include – more in acute than in chronic forms – neuromuscular irritability, lethargy, seizures and even coma; the causes vary and can generally be traced to excessive salt loss or excessive fluid intake or retention; hyponatraemia is rarely due to low dietary intake. In outline, the most common causes can be divided into:

1) excessive sodium loss, as occurs in Addison’s disease, diarrhoea, heavy sweating, diuretic use or kidney failure;
2) fluid retention, due for example to heart failure, cirrhosis, nephrotic syndrome, inappropriate secretion of ADH (antidiuretic hormone) caused by tumours, or certain medicines.

The symptoms of hypernatraemia include dry mucous membranes, thirst, agitation, restlessness, muscle tremors and seizures when hypernatraemia develops very suddenly. The causes, which can be traced to fluid loss or sodium retention, include Cushing’s syndrome and diabetes insipidus, which is characterised by deficient ADH (antidiuretic hormone) secretion or reduced kidney sensitivity to it.

Urine sodium can help identify the cause of a fluid and salt imbalance and, in particular, makes it possible to distinguish between kidney and non-kidney causes. The body normally eliminates excess sodium, so a high urine concentration reflects high blood sodium; a high urine concentration may also indicate increased excretion by the kidneys, in which case the plasma concentration will be normal or low. With insufficient dietary intake, both plasma and urine sodium will be low.

1. What is the daily sodium requirement? Are there differences between men and women?
2. Are there particular conditions in which people are more exposed to changes in blood sodium?

1. What is the daily sodium requirement? Are there differences between men and women?
Most dietary sodium comes from table salt and food, whether naturally present or added during artisanal or industrial processing. The LARN (Italian Recommended Levels of Energy and Nutrient Intake – 1996 revision) state that daily sodium intake, for both men and women, should be between 575 mg/day and 3,500 mg/day.

2. Are there particular conditions in which people are more exposed to changes in blood sodium?
Yes. For example, changes in blood sodium may be seen with diarrhoea, heavy sweating, heart failure, and acute and chronic kidney failure.