Biomarker . Patient guide
Potassium Blood Test
What is Potassium
Potassium is the main electrolyte inside your cells and is essential for the electrical activity of the heart and muscles. Both high and low levels can affect the heart rhythm, so it is one of the most important routine electrolytes.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in mmol/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Adults | 3.5 to 5.3 | mmol/L |
What it is
Most of the body's potassium sits inside cells, with only a small amount in the blood, kept in a tight range by the kidneys and hormones. Small shifts matter because potassium controls the electrical gradients that make nerves fire and the heart beat.
Why a clinician would order it
Potassium is checked as part of a U&E, to monitor medicines that affect it (ACE inhibitors, diuretics), in kidney disease, and to investigate muscle weakness, palpitations or an abnormal ECG.
If your level is outside the range
Symptoms of low Potassium
- Muscle weakness or cramps
- Fatigue
- Constipation
- Palpitations
What low can indicate. Low potassium (hypokalaemia) from diuretics, vomiting or diarrhoea, or hormone problems.
Symptoms of high Potassium
- Often none until high
- Muscle weakness
- Palpitations or, when severe, dangerous heart rhythm changes
What high can indicate. High potassium (hyperkalaemia) from reduced kidney function or medicines such as ACE inhibitors and potassium-sparing diuretics. A raised potassium can also be a false result if red cells break down in the sample (haemolysis), so a genuinely high value is confirmed on a fresh, cleanly drawn sample. A very high potassium, or a high result with palpitations, chest pain, marked weakness or feeling faint, needs urgent medical care: call NHS 111, or 999 or go to A&E if you feel very unwell. A mildly raised result without symptoms should still be repeated and discussed with your GP promptly.
Testing tips
No fasting needed. Clenching the fist or a difficult draw can falsely raise potassium, and a delay before the sample reaches the lab can too, so an unexpectedly high result is usually repeated.
If you book Potassium as part of General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
Where you can get this tested
Potassium is included in the following WMG Health panels. Same-day appointments are usually available at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Looking to book a test?
Comprehensive private health screening
Full blood screening at 134 Harley Street, London, reviewed by GMC-registered doctors with a written interpretation.
Symptoms often investigated with Potassium
Potassium is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Related markers
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about Potassium
What is a normal Potassium range?
Adults: 3.5 to 5.3 (mmol/L). Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low Potassium result mean?
Low potassium (hypokalaemia) from diuretics, vomiting or diarrhoea, or hormone problems.
What does a high Potassium result mean?
High potassium (hyperkalaemia) from reduced kidney function or medicines such as ACE inhibitors and potassium-sparing diuretics. A raised potassium can also be a false result if red cells break down in the sample (haemolysis), so a genuinely high value is confirmed on a fresh, cleanly drawn sample. A very high potassium, or a high result with palpitations, chest pain, marked weakness or feeling faint, needs urgent medical care: call NHS 111, or 999 or go to A&E if you feel very unwell. A mildly raised result without symptoms should still be repeated and discussed with your GP promptly.
Do I need to fast or prepare for the Potassium blood test?
No fasting needed. Clenching the fist or a difficult draw can falsely raise potassium, and a delay before the sample reaches the lab can too, so an unexpectedly high result is usually repeated. If you book Potassium as part of General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
Can I order a Potassium blood test privately in London?
Yes. WMG Health offers Potassium in several standard panels and as part of a custom panel at our 134 Harley Street clinic. Results are released as each marker is reviewed, and the completion date is set by the slowest marker in your panel. We confirm the expected timeframe at booking. Book a standard panel online, or ask for a custom panel via /contact/ or 020 3239 3378.