Sodium and Potassium Levels Explained: Your U&E Results

Sodium and potassium are electrolytes, salts that carry a tiny electrical charge in your blood. Sodium is the main salt in the fluid around your cells and is closely linked to how much water your body holds. Potassium sits mostly inside cells and keeps your heart, muscles and nerves working with a steady rhythm. Your kidneys and hormones keep both within a narrow range.

Both are reported in a standard U&E (urea and electrolytes) blood test, together with chloride, urea, creatinine and eGFR. This guide focuses on sodium and potassium: the usual ranges, what low and high results can mean, the medicines that commonly affect them, why potassium can be falsely high, and which results need urgent care. For eGFR and creatinine, see our kidney function results guide.

Test it in Chiswick: our Kidney and Electrolytes (U&E) blood test costs £49, with your result explained in person by a practitioner. No GP referral needed. Book this test or call 020 4515 0000.

Quick answer

Many UK laboratories use an adult range of 133 to 146 mmol/L for sodium and 3.5 to 5.3 mmol/L for potassium. Mildly low sodium often causes no symptoms and is commonly linked to diuretics, some antidepressants and other medicines. Sodium below 125 mmol/L, or any low sodium with confusion, vomiting or a seizure, needs urgent care. A potassium of 6.0 mmol/L or above needs same-day review, and 6.5 mmol/L or above is an emergency. A high potassium can also be false, caused by a damaged sample or a delay in processing. Ranges vary, so read the range on your report.

What sodium and potassium do

• Sodium: controls the balance of water inside and outside your cells and helps set blood pressure. A sodium result often tells you as much about water balance as about salt.
• Potassium: keeps your heartbeat regular and lets muscles and nerves work. Small changes matter, because both low and high potassium can upset the heart's rhythm.

The ranges in this guide come from one NHS laboratory. Ranges vary, so compare your result with the range printed on your own report.

What your sodium result means

• Very low (below 125 mmol/L): NHS guidance treats this as severe or needing careful urgent assessment. One NHS guideline advises hospital admission if sodium is below 120 mmol/L, or if there are any symptoms. Seek same-day medical advice, or emergency care if either applies.
• Low (125 to 132 mmol/L): often mild and without symptoms, and usually managed by your GP, who will look for the cause. NHS guidance suggests investigating if sodium stays below 130 mmol/L.
• Within range (133 to 146 mmol/L): your sodium is where it is expected to be.
• High (above 146 mmol/L): most often a sign of dehydration. NHS guidance notes that a stable 146 to 148 mmol/L in someone who is well may not be significant, while 155 mmol/L or above, or a level rising quickly, may need hospital care.

Causes of low sodium (hyponatraemia)

• Medicines: thiazide and loop diuretics, SSRI antidepressants, carbamazepine and some other epilepsy medicines, antipsychotics, proton pump inhibitors (PPIs) and ACE inhibitors.
• Losing fluid: vomiting and diarrhoea.
• Drinking very large amounts: of water or of beer, especially with a poor diet.
• Long-term conditions: heart failure, liver cirrhosis and some kidney conditions.
• Hormone causes: an underactive thyroid, low cortisol (as in Addison's disease) or SIADH, where the body holds on to too much water.

How fast sodium falls matters: a drop of more than 10 mmol/L in under 48 hours is significant even without symptoms.

Symptoms of low sodium

Mild low sodium often causes no obvious symptoms, although levels below 130 mmol/L can affect thinking and concentration. As sodium falls further, symptoms can include headache, tiredness, poor appetite, nausea, dizziness and confusion. Severe low sodium can cause vomiting, drowsiness, seizures and coma, which is a medical emergency.

Causes of high sodium (hypernatraemia)

The most common cause is dehydration, from not drinking enough or losing fluid. Older people and care home residents are at particular risk. Passing large amounts of urine, for example with poorly controlled diabetes, diabetes insipidus or long-term lithium treatment, can also raise sodium.

What your potassium result means

• Very low (below 2.5 mmol/L): severe and needs urgent hospital care.
• Low (2.5 to 3.4 mmol/L): usually managed by your GP, who will look for the cause, check magnesium and may suggest potassium replacement. NHS guidance advises a heart tracing (ECG) if potassium is below 3.0 mmol/L.
• Within range (3.5 to 5.3 mmol/L): your potassium is where it is expected to be.
• Mildly high (up to 5.9 mmol/L): NHS laboratory guidance advises repeating the test within a week and reviewing medicines and diet if potassium is 5.5 to 5.9 mmol/L.
• High (6.0 to 6.4 mmol/L): needs urgent same-day review, a repeat test and an ECG.
• Very high (6.5 mmol/L or above): potentially life threatening and needs emergency hospital treatment.

Why potassium can be falsely high

Red blood cells are packed with potassium. If cells are damaged when the blood is taken (haemolysis), or if the sample waits too long before the laboratory separates the cells from the liquid, potassium leaks out of the cells into the sample. The result then looks high even though the level in your body is normal. This is called pseudohyperkalaemia.

NHS laboratory guidance lists these common causes: a delay in reaching the laboratory, especially beyond about 6 hours; a sample kept in a fridge before it is separated; contamination from another type of blood tube; a difficult blood draw or a tourniquet left on too long; and a very high platelet or white cell count.

Because a truly high potassium is dangerous, an unexpected high result is never simply ignored. If the laboratory reports haemolysis or the result does not fit, a fresh sample is taken promptly to confirm it.

Causes of low potassium (hypokalaemia)

• Diuretics: loop and thiazide diuretics, such as furosemide and indapamide.
• Losses from the gut: vomiting, diarrhoea and overuse of laxatives.
• Low magnesium: NHS guidance notes that low potassium is unlikely to correct until magnesium is corrected.
• Other medicines: steroids, salbutamol and similar inhalers, theophylline and insulin.
• Less common causes: eating large amounts of liquorice and some rare hormone conditions.

Mildly low potassium often has no symptoms of its own. Lower levels can cause muscle cramps, weakness, tiredness, palpitations and constipation.

Causes of high potassium (hyperkalaemia)

• Reduced kidney function: the kidneys clear less potassium.
• Medicines: ACE inhibitors (such as ramipril), angiotensin receptor blockers or ARBs (such as losartan), spironolactone, anti-inflammatory painkillers, potassium supplements and some laxatives that contain potassium. Beta blockers and digoxin can also play a part.
• Diet: high-potassium foods mainly matter when kidney function is reduced.

High potassium often causes no obvious symptoms, which is why it is found on blood tests. When symptoms occur they can include tiredness, weakness, nausea, muscle pain, breathlessness, an unusual heartbeat or chest pain.

When to get urgent help

Call 999 or go to A&E if someone has a seizure, collapses, is very drowsy or newly confused, has chest pain, or has a fast, pounding or irregular heartbeat with dizziness, fainting or breathlessness. Also seek emergency care straight away for a potassium below 2.5 mmol/L or of 6.5 mmol/L or above, or a sodium below 120 mmol/L.

Get same-day advice from your GP or NHS 111 if your potassium is 6.0 to 6.4 mmol/L, your sodium is 120 to 124 mmol/L, or you have any abnormal result with symptoms such as headache, vomiting, marked weakness or confusion.

Book a routine GP appointment for a mildly abnormal result without symptoms, especially if you take one of the medicines above. Do not stop a prescribed medicine because of a single result, as the dose can often be adjusted safely.

Who should consider a U&E test

• People taking diuretics, ACE inhibitors, ARBs, spironolactone, lithium or SSRIs
• People with kidney disease, diabetes, high blood pressure or heart failure
• Anyone recovering from prolonged vomiting or diarrhoea, or who uses laxatives regularly
• People with cramps, weakness or palpitations without an obvious cause

Preparing for the test

You do not need to fast. Drink water as normal, but avoid drinking unusually large amounts beforehand. Because the U&E also measures creatinine, avoid eating meat in the 12 hours before the test, as NICE advises for kidney function tests. Take your medicines as usual unless told otherwise, and bring a list of them. We recommend a clinic sample rather than a home finger-prick kit for potassium, because the result is so sensitive to how the sample is taken and handled.

How often to retest

A potassium of 5.5 to 5.9 mmol/L is usually repeated within a week, and an unexpected high result is repeated promptly. If you start or change a medicine that affects sodium or potassium, your GP will plan your checks. Otherwise, a normal result does not usually need regular repeating.

Frequently asked questions

Is a potassium level of 5.6 high?

It is slightly high. NHS laboratory guidance advises repeating the test within a week and reviewing medicines and diet. It is worth checking whether the sample was haemolysed or delayed.

Can a blood test show falsely high potassium?

Yes. Damaged red cells or a delay before the sample is processed can release potassium into the sample. A fresh sample confirms the true level.

Is a sodium level of 132 dangerous?

It is slightly low but rarely dangerous on its own without symptoms. It is worth looking for a cause, such as a diuretic or antidepressant, and repeating the test.

What does U&E stand for?

Urea and electrolytes. It usually includes sodium, potassium, chloride, urea, creatinine and eGFR.

Sodium and potassium testing in Chiswick

We take your sample at our clinic at 8 to 9 Bedford Corner, The Avenue, Chiswick, London W4 1LZ. We are a short walk from Turnham Green station (District line, with Piccadilly line trains stopping there early morning and late evening only) and close to the 94, E3 and 272 bus routes. Limited parking is available on request when you book. Your sample is analysed at the Randox accredited laboratory, no GP referral is needed, and most results are ready within a few working days. A practitioner explains your result with you in person, and a GP feedback report is available on request.

Book the Kidney and Electrolytes (U&E) test (£49), or the Kidney Health Check on the same page (also £49), which adds calcium, magnesium, phosphate and albumin. Single Sodium and Potassium tests cost £49 each, so the U&E is usually better value. You can also compare every blood test and price, or read our guides to kidney function test results and magnesium levels.

This article is general information, not medical advice. Discuss your results with your GP or practitioner.