Potassiummmol/L
Cardiac & muscle electrolyte balance
Typical optimal range
3.5–5.3 mmol/L
A typical optimal range is considered to be 3.5–5.3 mmol/L, though reference intervals vary between laboratories and should always be interpreted alongside your own lab's reported range. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Interpretation and management of potassium abnormalities follows NHS and NICE clinical guidance, including NICE guideline NG203 on chronic kidney disease, which addresses monitoring and management of hyperkalaemia in that context.
What is Potassium?
Potassium is the body's main intracellular electrolyte, meaning the vast majority of it is held inside cells rather than circulating in the bloodstream. It plays a critical role in maintaining normal heart rhythm, enabling muscle contraction, and allowing nerves to send signals properly. Because blood levels are kept within a very narrow range by the kidneys, measuring serum potassium gives a reliable snapshot of how well this balance is being maintained. Even small deviations outside the normal range can have significant effects, particularly on the heart.
What a high Potassium means
A raised potassium level, known as hyperkalaemia, is a result that warrants prompt clinical attention, as high levels can affect the electrical activity of the heart. It is not a diagnosis in itself, but a signal that your GP will want to investigate further and, in many cases, repeat.
- Kidney dysfunction. The kidneys are responsible for excreting excess potassium, so reduced kidney function — whether acute or chronic — is one of the most common reasons for a raised result. Your GP may check kidney function markers such as creatinine and eGFR alongside potassium.
- Certain medications. ACE inhibitors, angiotensin receptor blockers (ARBs), and potassium-sparing diuretics (such as spironolactone) are commonly prescribed in the UK and can raise potassium levels as a known side effect. If you take any of these, regular monitoring may already be part of your care plan.
- Sample haemolysis (lab error). If red blood cells break down during blood collection or handling, potassium leaks out of the cells into the sample, falsely raising the result. This is called haemolysis and is a frequent cause of unexpectedly high readings — your GP will often simply arrange a repeat test.
A single raised reading should never be acted on in isolation; haemolysis is common and a repeat sample often resolves the picture. Your GP will consider your full clinical context before drawing any conclusions.
What a low Potassium means
A potassium level below the normal range, known as hypokalaemia, can cause symptoms such as muscle cramps, weakness, and fatigue, and at lower levels may affect heart rhythm. It is a clinically important finding that your GP will want to explore.
- Diuretic medications. Thiazide and loop diuretics, widely used in the UK to treat high blood pressure and heart failure, increase potassium loss through the urine and are a leading cause of low potassium. If you take a diuretic, your GP may monitor your potassium regularly.
- Gastrointestinal losses. Prolonged vomiting or diarrhoea leads to significant potassium loss from the gut. Conditions such as inflammatory bowel disease or eating disorders affecting purging behaviour can also contribute to persistently low levels.
Low dietary intake alone is rarely the sole cause in otherwise healthy adults, but a diet very low in fruit and vegetables can be a contributing factor alongside other drivers.
Should I be worried?
A single potassium result outside the normal range is worth taking seriously, but it rarely tells the whole story on its own. Because haemolysis can falsely elevate results and because levels can fluctuate with medication, illness, or diet, your GP will often repeat the test before drawing firm conclusions. If your result is significantly abnormal — particularly if it is very high — you may be contacted promptly, as potassium affects heart rhythm. Mildly abnormal results in someone who feels well are more likely to be investigated at a routine appointment. Your GP may also check kidney function, other electrolytes such as sodium and magnesium, and review any medications you are taking. If you experience palpitations, muscle weakness, or unusual fatigue alongside an abnormal result, it is worth mentioning these symptoms when you speak to your GP.
What actually helps
If your GP has identified that your potassium is on the low side, or simply for supporting healthy levels as part of a balanced diet, these lifestyle factors have good supporting evidence.
- Eat more potassium-rich whole foods. A diet rich in vegetables, fruit, and legumes is the most practical and evidence-backed way to support adequate potassium intake. Most people in the UK do not meet the recommended daily intake of 3,500 mg, and increasing whole plant foods is the most straightforward way to close that gap.
- Reduce ultra-processed food intake. Highly processed foods tend to be low in potassium and high in sodium, a combination associated with less favourable cardiovascular and kidney health. Shifting towards less processed options naturally improves the potassium-to-sodium ratio in the diet.
- Stay well hydrated. Adequate fluid intake supports kidney function, which in turn helps regulate potassium balance. This is particularly relevant during hot weather or illness when fluid and electrolyte losses increase.
- Review medications with your GP. If you are taking diuretics or other medications known to affect potassium, speak to your GP before making significant dietary changes — particularly if you are considering potassium supplements, as these can interact with certain drugs and cause potassium to rise to unsafe levels.
The following foods have good supporting evidence as practical sources of potassium within a varied, balanced diet.
Potatoes (especially baked or boiled with skin)
One of the richest everyday sources of potassium available in the UK diet, and widely accessible and affordable.
Bananas
A convenient and well-recognised source of potassium, practical as a daily snack for most people.
Leafy green vegetables (spinach, kale, Swiss chard)
Good sources of potassium that also provide magnesium, which works alongside potassium in muscle and nerve function.
Legumes (lentils, kidney beans, chickpeas)
Excellent plant-based potassium sources that also provide fibre and protein, supporting overall dietary quality.
Tomatoes and tomato-based products
Concentrated sources of potassium; tinned tomatoes and tomato purée are particularly potassium-dense and widely used in UK cooking.
Dried fruit (apricots, prunes, raisins)
Calorie-for-calorie very rich in potassium; useful in small quantities as part of a varied diet, though portion awareness is sensible given sugar content.
Track your own Potassium
Upload a lab PDF and Better Days trends Potassium over time, flags it when it drifts, and suggests foods that help.
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Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.