Your kidney results: creatinine, eGFR and urea explained

Your kidneys filter your whole blood volume many times a day, removing waste and balancing water and salts. Kidney disease is common, often silent until it is advanced, and much easier to slow down when it is found early. A simple blood test is the first step.

Creatinine: a waste product from muscle

Creatinine is made at a steady rate by your muscles and cleared by your kidneys. When the kidneys filter less well, creatinine builds up in the blood. The catch is that the amount you make depends on how much muscle you have. A muscular man can have a creatinine above the lab range with perfectly healthy kidneys, while a small, older woman can have a normal looking creatinine even when her kidneys are working less well.

A few everyday things can nudge creatinine up for a day or two:

• A large meal of cooked meat in the hours before the test.
• Creatine supplements, popular for gym training.
• Hard exercise in the day or two before the draw.
• Being short of fluids.
• Some medicines, including anti-inflammatory painkillers and certain blood pressure tablets.

eGFR: the best single guide

Because creatinine alone is hard to interpret, labs use it to calculate an estimated glomerular filtration rate, or eGFR. The formula takes your creatinine, age and sex into account and gives a figure in mL/min/1.73m², roughly the volume of blood your kidneys clean each minute. Current formulas no longer use race, which was a correction that has been dropped.

LifePrint uses the KDIGO 2024 guideline categories, which are used by kidney specialists worldwide:

• G1, 90 and above: normal.
• G2, 60 to 89: mildly decreased. This is normal for many adults, especially as we get older.
• G3a, 45 to 59: mildly to moderately decreased.
• G3b, 30 to 44: moderately to severely decreased.
• G4, 15 to 29: severely decreased.
• G5, below 15: kidney failure.

An eGFR below 60 is the chronic kidney disease range. An eGFR of 60 to 89 appears in LifePrint's grey zone. It is not kidney disease on its own, but it is worth tracking over time. An eGFR below 30 is marked as urgent, because it needs a doctor's input soon.

Urea: useful, but easily swayed

Urea is made in the liver when protein is broken down, and is cleared by the kidneys. It rises when kidney function falls, but it also moves with things that have nothing to do with your kidneys. A high protein diet, dehydration, steroid medicines and bleeding in the stomach or gut can all raise it. A low urea is common and usually harmless, seen with a low protein diet, pregnancy, or drinking a lot of water.

For these reasons urea is read alongside creatinine and eGFR, not on its own. A raised urea with a normal creatinine most often points to being short of fluids.

What the blood test cannot show

Kidneys can leak protein into the urine long before the eGFR falls. The blood test does not measure this. If you have diabetes, high blood pressure, a family history of kidney disease, or HIV, your doctor may want a urine albumin test as well. Diabetes and high blood pressure are the two leading causes of kidney disease, which is why the Essentials panel also checks your blood sugar.

Looking after your kidneys

• Keep blood pressure and blood sugar in a healthy range, with your doctor's help if needed.
• Use anti-inflammatory painkillers sparingly and only as directed, especially if your eGFR is reduced.
• Drink enough water for your climate and activity, without forcing large volumes.
• Do not smoke.
• Tell any doctor or pharmacist your eGFR, because many medicines need a dose change when kidney function is reduced.

If your eGFR is below 60, or has dropped noticeably since a previous test, speak to a registered healthcare practitioner. Your LifePrint report explains your results in context, but it is health information, not a diagnosis.