Kidney function blood test: what your eGFR and creatinine really mean

What a kidney function blood test includes

In the UK, kidney function is usually checked as part of the urea and electrolytes (U&Es): creatinine in µmol/L, urea in mmol/L, and sodium and potassium, the salts the kidneys keep in balance. NICE asks laboratories to print an estimated filtration rate, the eGFR, every time creatinine is measured, so the two always arrive together.

Alongside the blood test, your GP will often ask for a urine sample to measure the albumin:creatinine ratio, or ACR, in mg/mmol. The NHS explains that the urine result, read with the eGFR, gives a more accurate picture of how well your kidneys are working than either alone.

Those lines are read as a group, never one at a time. Creatinine and eGFR describe filtration, urea is read against creatinine, the salts describe the balance the kidneys maintain, and the ACR shows whether protein is leaking through.

Creatinine measures your muscle as much as your kidneys

Creatinine is a waste product of muscle, cleared by the kidneys. The amount in your blood therefore depends on two things: how well the kidneys are clearing it, and how much muscle is producing it.

That second half explains results that otherwise look contradictory. NICE asks doctors to interpret eGFR with caution in bodybuilders, in people with muscle-wasting conditions or an amputation, and in people who take protein supplements: more muscle makes the eGFR look lower than it really is, less muscle makes it look higher.

Even a single meal counts. In a UK study, median creatinine rose from 80.5 to 101 µmol/L one to two hours after a cooked-meat meal, and median eGFR fell from 84 to 59.5, enough to cross the line used to talk about kidney disease. A meal without meat made little difference. NICE therefore advises not eating any meat in the 12 hours before an eGFR test.

It is also why ranges differ between men and women, and why a creatinine at the top of its range in someone with little muscle deserves more attention than the same figure in someone with a lot.

eGFR is a calculation, not a measurement

The eGFR printed on your results was not measured. It is worked out from your creatinine, your age and your sex, using an equation, and it carries the uncertainty of that equation. NICE recommends the CKD-EPI creatinine equation for adults, and asks that it be read with caution above 60, where estimates become less accurate.

In 2021 NICE also removed the adjustment that some laboratories applied for people of African-Caribbean or African family origin, finding that it may not be valid. An older result calculated with that adjustment may not be directly comparable with a recent one.

NICE asks laboratories to print anything above 90 simply as ‘greater than 90’, and says that in that zone a rise in creatinine of more than 20% is a better sign of a real fall. Two values from different laboratories, or from different equations, are not directly comparable either.

What is a normal eGFR for your age?

The NHS says healthy kidneys should filter more than 90 ml/min. But eGFR also declines with age, even without kidney disease: the US National Kidney Foundation gives an average of about 116 for people in their twenties and about 75 for people aged 70 and over, so an eGFR of 85 at seventy is a different object from the same figure at twenty-five.

An eGFR between 60 and 89 is not, on its own, chronic kidney disease. Under the NICE definition, it only counts if there are other signs of kidney damage, such as protein in the urine. Each range also leaves about 1 in 20 healthy people outside it on any given test.

One low result is never chronic kidney disease

A single reduced eGFR is a finding, not a diagnosis. NICE asks GPs to confirm a first eGFR below 60 by repeating the test within two weeks, and defines chronic kidney disease as an eGFR below 60 on at least two occasions 90 days or more apart, or signs of kidney damage such as an ACR above 3 mg/mmol, lasting more than three months.

The reverse is just as true: early kidney disease is common and silent. Kidney Research UK estimates that 7.2 million people in the UK, more than 10% of the population, live with chronic kidney disease, 3.25 million of them at stages 3 to 5, and that people in the early stages often feel well and have no symptoms.

That is why NICE recommends testing both eGFR and urine ACR in adults with diabetes, high blood pressure, cardiovascular disease, gout, a previous acute kidney injury or a family history of kidney failure. If any of those apply to you, ask whether your kidney checks include the urine test.

Urea, salts and the rest of the sheet

Urea is a waste product of protein. Lab Tests Online UK lists the reasons it rises: reduced kidney function, but also not drinking enough fluid or anything else that cuts blood flow to the kidneys, and a blockage to the flow of urine, such as kidney stones or an enlarged prostate. It is read alongside creatinine rather than on its own.

The salts on the same sheet move for many reasons, including several common medicines. NICE recommends checking kidney function at least once a year in people taking medicines that can affect it, such as lithium or long-term anti-inflammatory painkillers.