Kidneys and electrolytes
Natrium
The main salt in the blood; an abnormal value usually points to a disturbance of the fluid balance, not of salt intake.
Everything about this testWhat it measures
Sodium is the main salt outside the cells and, together with water, determines the amount of fluid in the blood and tissues. The body keeps the sodium concentration very precisely constant through thirst, the kidneys and hormones such as antidiuretic hormone (ADH). An abnormal value therefore rarely comes from eating too much or too little salt: usually there is too much or too little water in relation to the salt. Sodium is almost always measured together with potassium and creatinine.
- A sodium level that is too low (hyponatremia), usually due to medicines, heart failure, liver disease, kidney disease or disturbed fluid regulation (SIADH)
- A sodium level that is too high (hypernatremia), almost always due to dehydration
- Monitoring when using water pills and blood pressure medicines, according to the Dutch GP guideline (NHG) on cardiovascular risk management
High
A raised sodium almost always means that the body is short of water: drinking too little in hot weather, with fever, diarrhea or vomiting, or in older people who feel less thirsty. As long as someone can drink and feels thirst, a high sodium is rare. A rare cause is diabetes insipidus, in which the kidneys pass too much water because of a shortage of, or insensitivity to, ADH. Symptoms are thirst, a dry mouth, drowsiness, confusion and, at high values, muscle twitches or seizures. A slightly raised value in someone who feels well is often a result of drinking too little before the blood draw.
Low
A low sodium is the most common abnormality of the salts in the blood, especially in older people. Medicines are often the cause: thiazide water pills (for example hydrochlorothiazide), antidepressants (SSRIs), some antiepileptics and painkillers. Other causes are heart failure, liver cirrhosis or kidney disease (the body then retains water), disturbed release of ADH (SIADH) in lung diseases or brain disorders, drinking a lot of water in a short time (for example during endurance sports), an underactive thyroid or an adrenal problem. A markedly raised blood sugar also temporarily lowers sodium. A slightly low sodium usually causes no symptoms; at lower values nausea, headache, confusion, falls and, in severe cases, seizures develop.
If your result is abnormal
With a slightly abnormal sodium without symptoms, the GP repeats the measurement, often together with potassium, creatinine and glucose, and reviews your medicines and drinking habits; a suspected water pill or antidepressant is adjusted if possible. With a clearly low value, the cause is investigated with a urine test and thyroid and adrenal values, and with symptoms or a markedly abnormal value, referral to an internist (specialist in internal medicine) or the emergency department follows.
Reliability
The sodium measurement itself is accurate and differs little between laboratories. The result, however, says little about the cause: a slightly low sodium in someone without symptoms is often temporary and has many possible explanations, from medicines to drinking a lot. Interpretation requires knowledge of medicine use, fluid intake and the other results (potassium, creatinine, glucose) and sometimes a urine test.
Limitations
- Thiazide water pills, SSRIs, some antiepileptics and NSAIDs lower sodium; the NHG therefore advises monitoring after starting these medicines.
- Drinking a lot of water shortly before the blood draw, or drinking very little, hot weather, fever and heavy exercise temporarily shift the value.
- A markedly raised blood sugar lowers sodium; markedly raised fats or proteins in the blood can, depending on the measurement method, give a falsely low value.
How common is this
A slightly low sodium is common: in older people living at home it is estimated to be found in 5 to 10 in 100 people, usually due to medicines, and in hospital even more often. In young healthy adults, an abnormal sodium is unusual. A raised sodium is rare outside hospital and occurs mainly in frail older people with dehydration.
Who is this useful for
Useful when using water pills, antidepressants or other medicines that can lower sodium, with heart failure, liver or kidney disease, in older people with falls, confusion or drowsiness, with dehydration after diarrhea, vomiting or heat, and in endurance athletes who drink a lot. According to the Dutch GP guideline (NHG) on cardiovascular risk management, sodium is checked after starting a thiazide water pill. In healthy adults without medicines or symptoms, the result is almost always normal and the test adds little.
Downsides and risks
- A slightly low sodium is often a side effect of medicines and not a disease; without that context, the result can cause unnecessary worry.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results