Urea and electrolytes (U&Es): what the test is for and what upsets it

Four core lines, one small group

Urea and electrolytes, written U&E or U&Es on most forms, is one of the commonest blood tests in the UK. The core is four lines from one tube: sodium, potassium, urea and creatinine. Most laboratories add an estimated filtration rate, eGFR, which is calculated from the creatinine rather than measured. Chloride and bicarbonate are sometimes added on request.

It is narrower than the “metabolic panel” you may read about on American websites. Blood sugar, calcium and the liver tests are separate boxes on a UK request form, and U&Es are usually requested alongside a full blood count rather than instead of one.

Those four lines are not four independent verdicts. Urea is read next to creatinine, sodium next to how much you had been drinking and what tablets you take, and potassium next to how the sample was handled.

What is a urea and electrolytes test for?

U&Es are requested when someone is unwell without a clear cause, as part of routine tests, to check the kidneys before starting certain medicines, and to monitor people already taking them. Water tablets (diuretics) and ACE inhibitors, both common for high blood pressure, are typical reasons for regular checks.

Unlike a fasting glucose test, U&Es need no fasting. NICE does advise adults not to eat any meat in the 12 hours before a blood test used to estimate kidney function, and asks that samples reach the laboratory within 12 hours.

When to worry about urea levels

A high urea suggests the kidneys are not clearing waste as well as they should, but kidney disease is far from the only cause. Dehydration, reduced blood flow to the kidneys as in heart failure, bleeding into the gut, a recent heart attack, severe stress or a blockage to the flow of urine can all raise it. So can a high-protein diet, and urea rises gently with age.

A low urea is uncommon and rarely a cause for concern. It is seen in severe liver disease and malnutrition, and normally in pregnancy, when the extra blood volume dilutes it.

That is why urea is almost never read alone. A raised urea with a normal creatinine after a hot day and too little to drink is a different story from both lines rising together over several tests.

Potassium is the line most often wrong

Potassium is far more concentrated inside red cells than in the liquid around them. Damage a few of those cells anywhere between your arm and the analyser, or leave the sample too long before it is spun, and potassium leaks out: the result rises without anything having happened to you. Laboratories track this with a haemolysis index, and a verification study confirmed potassium among the values that damaged red cells push upwards.

In the UK, the journey to the laboratory is a known culprit. An audit by an NHS pathology service in the West Midlands found that 82% of critically high potassium results from GP surgeries in November 2024 were normal on a repeat sample, against 41% in August: cold weather makes the leak worse when samples spend a long time on the road. In Grampian, putting centrifuges into GP practices so samples were spun on site cut the number of falsely high potassium results.

Other causes are mechanical and mundane: a tourniquet left on too long, a fist clenched and pumped during the draw, a shaken tube. European sampling guidance asks that the tourniquet come off as soon as blood flows and that pumping the fist be avoided.

So a potassium flagged slightly high on otherwise ordinary results has a dull first explanation, and the usual next step is another sample rather than a treatment.

A low sodium is often a tablet

Mild hyponatraemia, a low sodium, is one of the commonest findings on U&Es, and in people treated for high blood pressure the explanation is frequently on their own prescription. A cohort of 2,613 adults newly treated for hypertension found that among those who stayed on a thiazide diuretic, roughly three in ten developed a sodium at or below 130 mmol/L.

That is not a reason to stop the tablet. It is a reason for the number to be read next to your repeat prescription rather than on its own, and for the GP who prescribed it to decide what follows.

The same logic runs through the whole group. Many of these lines move with the medicines you take, with what you drank, and with how long the sample waited before it was processed.

eGFR is an estimate, not a measurement

The eGFR printed under your creatinine was never measured. It is calculated from the creatinine, your age and your sex using an equation, which NICE asks UK laboratories to standardise on the CKD-EPI creatinine equation.

An equation carries the limits of an equation. NICE notes that eGFR is less reliable in pregnancy, with very high or very low muscle mass, in people who use protein supplements, and that it has not been well validated in some black, Asian and other minority ethnic groups living in the UK. Results from older tests calculated another way may not compare directly.

A first low result is not a diagnosis. NICE recommends repeating it within two weeks to rule out a temporary dip, for example from a short illness, and allowing for a natural variation of about 5% in creatinine when comparing results.

What causes abnormal urea and electrolytes?

Four to six lines, each with a range that leaves about 1 in 20 healthy people outside it, make a flag somewhere unremarkable. The useful questions are whether the line’s partners agree, how far outside it sits, and what else was going on: your medicines, what you ate or drank, how long the sample travelled.

That is why the usual next step for a lone, mild abnormality is a repeat rather than a treatment. A result that stays abnormal on a clean second sample, or several lines moving together, is the one that gets investigated, and your GP will read it against your history and prescriptions.