Blood potassium: what it is, what it tells you and why we test it

Potassium is a mineral that sits almost entirely inside your cells. Only a little circulates in your blood, and your body holds it within very narrow limits, because it runs your heartbeat and your muscles. A surprise high result is often an effect of the blood draw itself.

Why we test it

Potassium keeps your nerves and muscles working, heart included. That is why your body holds it in a narrow window, and your kidneys clear the extra.

Both too much and too little can upset your heart rhythm. And because the range is narrow, even a value slightly out of range matters.

Common medicines often shift it, such as blood pressure pills and diuretics, and so do kidneys that filter less. It usually causes no symptoms until it is well out of range.

What it tells you, and what it does not

If it is high, the draw is often the reason: red cells break in the tube and release potassium. That is why a first high result is usually repeated.

If it stays high on the repeat test, the kidneys or some medicines are usually involved. If it is low, diuretics, vomiting or diarrhoea are often behind it.

It does not tell you how much potassium you eat: with healthy kidneys, food moves it very little.

That is why it is read together with other tests:

Normal potassium levels

The lab decides what is normal. It is usually 3.5 to 5.1 mmol/L, the same for women and men. Your own report may print different limits.

Some reports print mEq/L instead of mmol/L: for potassium the number is the same. You may also see it written as K.

How the draw went matters a lot: a damaged sample can make it look higher than it is.

In the Rosa report you also see your optimal range, next to creatinine and eGFR.

What your result means

Find the row for your result, in mmol/L. Which values are dangerous? Below 3.5 should always be seen by a doctor, below 3 or above 5.5 today, and above 6 at the emergency department.

If your potassium is high or low

What can skew the result

Potassium is very sensitive to how the draw goes. It helps to know this before you read it.

When to worry

Talk to a doctor if:

With Rosa, a potassium below 3.5 mmol/L is always flagged to a doctor, and below 3 or above 5.5 urgently.

Frequently asked questions

What does K mean on my report? It is the chemical symbol for potassium. Italian reports may also call it potassiemia or kaliemia: all names for the potassium in your blood.

Do I need to fast for a potassium test? Not for potassium. It is usually done with other tests that do need fasting, though, so it is best to arrive fasted. During the draw, keep your hand relaxed.

Does blood potassium show how much potassium I eat? Hardly. With healthy kidneys your body clears the extra, and the level stays steady. Medicines, the kidneys or fluid loss move it more often.

Why are potassium and magnesium read together? Because when magnesium is short, your kidneys lose more potassium and the level struggles to come back up. So with a low potassium, doctors check magnesium too.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. People on some blood pressure medicines or diuretics have it checked more often, on their doctor’s schedule.

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