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.
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.
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.
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.
Can I order a Potassium blood test privately in London?
Yes. WMG Health offers Potassium as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. All panels are custom-built around your specific question; bookings via /contact/ or 020 3239 3378.