Potassium (urine)

The test measures how much potassium the kidneys excrete in urine. It is most often performed when blood potassium is low or high without a clear cause, to distinguish whether potassium is being lost through the kidneys or another route. Results are always interpreted together with blood potassium.

Who is this test for

• people with low blood potassium whose cause is unclear;
• people with high blood pressure and low potassium when a doctor is investigating a cause, for example excess aldosterone;
• people with high blood potassium when kidney excretion needs assessment;
• sometimes people whose doctor wants to assess dietary potassium intake.

What the test shows

We obtain potassium from food, and the kidneys excrete most of the excess. A healthy person excretes approximately as much potassium in urine as they consume in food, so urine potassium varies from day to day depending on diet and fluid intake.

Urine potassium therefore has no single “normal” value that is good or bad in isolation. It becomes meaningful alongside blood potassium: it shows whether the kidneys are doing what they should in that situation, conserving potassium when blood levels are low and excreting it when they are too high.

Panels including this test

Test Potassium (urine) is also included in these panels.

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All panels including Potassium (urine) (14) ›

Preparation and sample

The doctor will say whether a spot urine sample is sufficient or a 24-hour collection is required. With a spot sample, doctors often also request creatinine from the same urine to account for urine concentration. Blood potassium is usually sampled the same day.

Bring the urine sample as soon as possible, preferably within 2 hours. Eat and drink as usual before the test. Diuretics and laxatives change potassium excretion, but do not stop taking them without discussing it with your doctor. When submitting the sample, state which medicines and supplements you take.

How results are interpreted

For 24-hour urine, the result is expressed in mmol per day; for a spot sample, in mmol/L. If creatinine is also measured in the same sample, the doctor calculates and interprets the potassium-to-creatinine ratio. In adults on a usual diet, 24-hour urine contains approximately 25 to 125 mmol of potassium, but this range depends mainly on diet; the cut-offs are printed on the report. A doctor interprets the result alongside blood potassium, medicines and symptoms.

When blood potassium is low

If urine potassium excretion is low alongside low blood potassium, the kidneys are conserving it and the cause is sought outside the kidneys. This is most commonly gastrointestinal loss, for example prolonged diarrhoea or excessive laxative use, or insufficient intake.

If potassium continues to be excreted in larger amounts, it is being lost through the kidneys. Diuretics are the most common reason, followed by excess aldosterone and other adrenal hormones, and rare inherited renal tubule disorders.

When blood potassium is high

With high blood potassium, low urine potassium suggests the kidneys are not excreting enough. This may be due to impaired kidney function, medicines such as ACE inhibitors, sartans and spironolactone, or adrenal hormone deficiency. This is why the test is accompanied by creatininetesting.

High urine potassium with normal blood potassium most often simply reflects a diet rich in fruit and vegetables.

How to arrange testing

The test is performed without a referral or appointment: every branch accepts urine samples, and you can also have blood taken there for blood potassium (branches are listed on the Contactpage). Prices for all tests and panels are listed in the service price list.

Frequently asked questions

What is the difference between blood and urine potassium?

Blood potassium shows the amount currently in the blood and is important for heart and muscle function. Urine potassium shows how much the kidneys excrete. The urine test alone does not show whether there is enough potassium; it helps identify why blood potassium is low or high.

Is a spot urine sample sufficient?

Often yes, particularly when creatinine is measured from the same sample alongside potassium. A 24-hour collection gives a more accurate total amount, so a doctor requests it when this matters for a decision. The referring doctor decides which sample is needed.

Should I stop taking diuretics before the test?

Not on your own. Diuretics and laxatives change potassium excretion, which a doctor considers when interpreting results. Change treatment only in consultation with your doctor, and state which medicines you take when submitting the sample.