Sodium (serum or urine)

The test measures sodium concentration in serum or urine. Blood sodium primarily reflects the body’s water and salt balance: low sodium most often means relatively too much water in blood, and high sodium too little. It is used for nausea, confusion and weakness, with medicines such as diuretics, and in heart, liver and kidney disease.

Who the test is for

• people with nausea, headache, confusion, drowsiness, unsteady walking and falls, particularly older people;
• people taking medicines that can lower sodium: thiazide diuretics, some antidepressants and antiepileptics;
• people with prolonged vomiting, diarrhoea or dehydration, particularly young children and older people;
• people with heart, liver, kidney, thyroid and adrenal disease;
• people before surgery and adults having general check-ups;
• people with low blood sodium whose doctor also requests urine sodium;
• people with high blood pressure or kidney stones, in whom sodium in 24-hour urine reflects salt intake.

What sodium is and what the test indicates

Sodium is the main electrolyte in fluid outside cells. It retains water in blood vessels and tissues and is needed for nerve and muscle function. It appears on the report as the chemical symbol Na.

Blood sodium is regulated by thirst, hormones and the kidneys. Antidiuretic hormone (vasopressin) makes the kidneys conserve water, and aldosterone makes them conserve sodium. Blood sodium therefore says more about water than salt: healthy kidneys excrete excess dietary salt while blood sodium remains normal.

Urine sodium shows how much the kidneys excrete. A single sample helps identify the cause of low blood sodium, while 24-hour urine estimates salt intake well because most dietary sodium is excreted in urine.

Panels including this test

Test Sodium (serum or urine) is also included in this panel.

Energy balance panelvitamin D, vitamin B12, folic acid, calcium, magnesium and 498.50€Details ›

All panels including Sodium (serum or urine) (11) ›

Preparation and sample

Venous blood is used for the blood test. Fasting is not required unless other tests that day need it. Drink as usual before sampling, without unusually large amounts of water. State which medicines you take, but do not stop them without medical agreement.

For urine testing, your doctor will specify a single sample or urine collected over 24 hours. Bring the urine sample in as soon as possible, preferably within 2 hours.

How results are interpreted

Sodium is expressed in mmol/L. The adult serum reference interval is usually approximately 137 to 146 mmol/L, with exact limits depending on laboratory and method and printed on your report. A doctor interprets it alongside symptoms, medicines, glucose, potassium and chloride and indicators of kidney function.

Low sodium (hyponatraemia)

The most common causes are medicines, particularly thiazide diuretics, some antidepressants and antiepileptics, and drinking large amounts of water, for example during prolonged exercise. Sodium also falls when fluid lost through vomiting, diarrhoea or sweating is replaced with water alone, in heart failure, liver cirrhosis and kidney disease, and when the body secretes too much antidiuretic hormone, for example with lung or brain disease and after surgery. Less often, the cause is an underactive thyroid or adrenal gland. With very high blood glucose, sodium falls because glucose draws water out of cells.

Mildly low sodium often causes no symptoms or presents as fatigue, unsteady walking and falls. Sudden or very low sodium, particularly below 125 mmol/L, with headache, vomiting, confusion, drowsiness or seizures requires emergency care. Severe abnormalities are corrected gradually under medical supervision because overly rapid correction can damage the brain.

High sodium (hypernatraemia)

High sodium almost always means water deficiency. It occurs when someone does not drink enough, for example an older person with reduced thirst or someone unable to drink unaided, and with fluid loss through fever, sweating, diarrhoea or vomiting. Less often, the cause is excessive urination in poorly controlled diabetes or diabetes insipidus (now also called vasopressin deficiency or resistance), a rare antidiuretic hormone disorder. Thirst, weakness, irritability and confusion occur; markedly high sodium with confusion or drowsiness requires emergency care.

Urine sodium

With low blood sodium, low urine sodium suggests the kidneys are conserving salt, for example because of fluid loss, heart failure or cirrhosis. Higher quantities suggest other causes such as diuretics or hormonal disorders. Sodium in 24-hour urine has no single “normal” value because it depends on salt intake.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Every branch accepts urine samples. Prices for all tests and panels are available in the service price list.

Frequently asked questions

Does high blood sodium mean I eat too much salt?

Generally not. With healthy kidneys and enough fluid, excess salt is excreted in urine, keeping blood sodium normal. Too much salt still raises blood pressure, and sodium in 24-hour urine better reflects salt intake. The World Health Organization recommends less than 5 g of salt daily for adults. Read more in the article on hypertension.

Can too much water lower sodium?

Yes, if more water is drunk in a short time than the kidneys can excrete, for example during a marathon while drinking only water. Normal drinking according to thirst does not pose this threat to healthy people.

Is low sodium dangerous in older people?

Even mildly low sodium in older people is associated with unsteady walking and falls. A medicine is often responsible, so a doctor reviews treatment and repeats testing. Confusion, drowsiness or vomiting with low sodium requires urgent assessment.