Potassium blood test: normal range, high and low results explained

A potassium blood test measures the potassium in your blood, the mineral that carries electrical signals in nerves and muscle, the heart included. Almost all of the body's potassium sits inside cells, so the small amount in blood is tightly controlled, mainly by the kidneys. Small changes in either direction matter, and a high result can also come from how the sample was taken or handled, which is why a surprising figure is usually repeated.

What it measures

About 98% of the body's potassium is inside cells, with roughly 40 times as much inside as outside. You get it from food, and the kidneys pass the extra into the urine to keep the blood level steady. Because the blood holds so little, a small shift out of cells, or a small change in what the kidneys excrete, moves the result noticeably.

Small changes in potassium have a large effect on nerves and muscles, especially the heart. Low levels can lead to an irregular heartbeat, and high levels reduce the heart muscle's activity.

Normal range

Laboratories commonly use a range of about 3.5 to 5.1 mmol/L (the same figures in mEq/L); some use 3.7 to 5.2. Outside the range, guidelines grade the result:

Potassium (mmol/L)	Usually described as
Below 2.5	Severely low
2.5 to 3.4	Mildly to moderately low (hypokalaemia)
3.5 to 5.1	Common reference range
5.5 to 5.9	Mildly high (hyperkalaemia)
6.0 to 6.4	Moderately high
6.5 or above	Severely high

Results between the top of a laboratory's range and 5.5 are flagged as above range but sit below the threshold UK kidney guidance uses to define hyperkalaemia. The sample type matters too: serum, from blood allowed to clot, reads on average about 0.36 mmol/L higher than plasma, because platelets release potassium as blood clots. Read your result against the range on your own report.

What a high result usually points to

The kidneys remove potassium, so the most common cause of a truly high level is reduced kidney function. Other causes include medicines used for blood pressure, heart failure and kidney disease, such as ACE inhibitors, ARBs and spironolactone; potassium supplements and salt substitutes; Addison disease; muscle or tissue injury, burns and red cell breakdown; and poorly controlled type 1 diabetes.

A falsely high result

Potassium can also rise in the tube rather than in the body. UK kidney guidance names a long transit time to the laboratory, or poor storage, as the most common cause of this pseudo-hyperkalaemia. Others include:

• clenching and pumping the fist during the draw, which in one study raised potassium by about 1 mmol/L compared with the other, relaxed hand;
• a tourniquet left on for more than a minute, or a difficult draw that breaks red cells (haemolysis);
• samples kept too cold or too warm before testing, or left too long before processing;
• a very high platelet or white cell count;
• contamination from another tube's additive if the tubes are filled in the wrong order.

So an unexpected high result is usually repeated. UK Kidney Association guidance recommends repeating an unexpected mildly high result within three days, a moderately high one within a day, and treats a severely high result as needing hospital assessment the same day. A doctor typically also looks at kidney function, medicines and supplements, and an ECG. Talk to a doctor about a high result, with the result in front of you.

What a low result usually points to

Low potassium usually comes from losing it: through water pills such as hydrochlorothiazide and furosemide, through vomiting or diarrhoea, heavy sweating, or overuse of laxatives. Hormonal causes such as hyperaldosteronism and Cushing syndrome, a low magnesium level, and occasionally a diet very low in potassium also lower it. Natural liquorice can lower it too.

A mildly low result is common: in outpatients having blood tests, it turns up in almost 14%. A doctor typically checks magnesium, since more than half of clinically significant low potassium comes with low magnesium, and may look at glucose, calcium, thyroid hormone, aldosterone and an ECG, depending on the picture.

What moves it

• The draw and the sample, as above. The same handling problems can also hide a low potassium by pushing it up into the normal range.
• Medicines. Water pills such as hydrochlorothiazide and furosemide tend to lower it; ACE inhibitors, ARBs, potassium-sparing water pills such as spironolactone, and potassium supplements tend to raise it. MedlinePlus notes that certain prescription and over-the-counter medicines and supplements can raise it.
• Kidney function, which sets how much is excreted.
• Fluid losses from vomiting, diarrhoea or heavy sweating.
• Acid-base changes. Acidosis raises it, and anxious over-breathing just before a draw can raise it.
• Diet, which on its own is an uncommon cause of a high or low result.

Preparing for the test

• No special preparation is needed for potassium itself; follow the fasting instructions for the rest of the panel.
• Keep your hand relaxed during the draw rather than clenching and unclenching your fist.
• MedlinePlus advises not eating natural black liquorice for at least two weeks before the test, as it can lower the result.
• Tell the person taking the sample about medicines and supplements, especially water pills, blood pressure medicines and potassium supplements. Keep taking prescribed medicines unless your doctor says otherwise.

How Clare's panel handles it

In Clare's panel, potassium is measured by the lab, with sodium, chloride and bicarbonate, and it sits next to creatinine and eGFR, which show how well the kidneys are clearing it. When Clare opens, members will see it alongside those results and their history.

Price

Potassium is usually sold within a kidney function test rather than on its own. When we read public prices on 11 October 2026, a kidney function test cost from 89 to 900 at six UAE providers. Five of the six were 250 or less, and the top of the range was a specialised package. VAT was rarely stated, some figures were offer or "from" prices, and home collection can add a fee. Not every kidney package lists potassium, so check the contents.

Related tests

Potassium is read with sodium, the electrolyte measured beside it; with magnesium, since low magnesium often travels with low potassium; and with creatinine and eGFR, since the kidneys set the level. For preparing for a full panel, see Comprehensive blood test in Dubai: what to expect.

Why the number means more over time

With potassium a single figure can mislead, because the sample itself can shift it. A high value that is normal on a careful repeat points to the sample; one that stays high on well-handled repeats is the one to discuss with a doctor. People taking medicines that affect potassium, or with reduced kidney function, are checked regularly for this reason: NICE recommends measuring potassium and eGFR before, and one to two weeks after, starting ACE inhibitor or ARB medicines in people with chronic kidney disease.

Clare is opening with a waitlist at clare.health.