Potassium

This is the harmonised UK range for serum; plasma samples read about 0.2 lower, and some labs print 3.5 to 5.0.

Potassium is the main salt inside the body's cells, and only a small fraction is in the blood. That fraction is tightly controlled by the kidneys and by hormones, and it matters for the electrical activity of the heart, nerves and muscles. The test measures the concentration in a blood sample.

It is part of the routine 'U&E' test and is checked closely in anyone with kidney disease, heart disease, or taking medicines that affect potassium, such as water tablets and blood-pressure tablets.

• Reduced kidney filtering
• Blood-pressure medicines of the ACE inhibitor, ARB and spironolactone type
• Potassium supplements or salt substitutes
• Uncontrolled diabetes and acid build-up in the blood
• Tissue breakdown after injury or very hard exercise
• Adrenal gland underactivity

• Water tablets (loop and thiazide diuretics)
• Vomiting, diarrhoea or laxative use
• Poor intake over weeks
• Some asthma inhalers and steroid tablets
• Adrenal gland overactivity
• Low magnesium, which makes potassium hard to hold on to

• The commonest cause of a high reading in the general population is a sample effect: a tight tourniquet, fist clenching, delay before the lab, or cells breaking down in the tube (the lab may print 'haemolysed')
• A sample kept cold or left overnight reads higher
• Plasma samples read slightly lower than serum

No fasting needed. If the lab reports the sample as haemolysed or delayed, the value is not reliable and is usually repeated.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.