eGFR and creatinine blood test: what they measure and what your result means

What are creatinine and eGFR?

Your muscles make creatinine all the time as they work. Healthy kidneys filter it out into your urine, so the level in your blood stays fairly steady. If the kidneys filter less well, creatinine builds up.

eGFR turns your creatinine result into an estimate of kidney filtering. A higher eGFR means better filtering. eGFR naturally falls a little with age.

Why are they tested?

• You have diabetes or high blood pressure, the two most common causes of kidney disease
• You have heart disease, or a family history of kidney disease
• You take medicines that can affect the kidneys, or need a check before starting them
• You have symptoms such as swollen ankles or feet, tiredness or itchy skin
• You want a routine health check

What does a high creatinine or low eGFR mean?

It means less creatinine is being cleared, which can point to reduced kidney function. Common reasons are:

• Long-term kidney disease, most often linked to diabetes or high blood pressure
• Dehydration, or a recent illness with vomiting or diarrhoea
• Some medicines, such as anti-inflammatory painkillers and some blood pressure tablets
• Having a lot of muscle, or taking creatine supplements
• Eating a large amount of meat, especially cooked meat, shortly before the test
• A sudden kidney problem (acute kidney injury), which needs prompt care

Kidney disease is only diagnosed when eGFR stays low for at least three months, so a GP will repeat the test. They'll usually also check your urine for protein, which gives a fuller picture.

What does a low creatinine or high eGFR mean?

This is usually nothing to worry about. Low creatinine is common in people with less muscle, older people, during pregnancy and with a diet low in meat.

What can affect your result?

• Don't eat meat for 12 hours before the test, as this can raise creatinine
• Avoid hard exercise for a day or two before
• Drink normally: being dehydrated can push creatinine up
• Creatine supplements raise creatinine. Tell the doctor reviewing your result if you take them.
• Some medicines affect creatinine without harming the kidneys

When to see a GP

Your results report shows the laboratory's normal range and flags any result outside it. Speak to your GP if:

• Your eGFR is flagged low, or creatinine is flagged high
• Your result has dropped since a previous test
• You have diabetes or high blood pressure and haven't had your kidneys checked in the past year
• You have swollen ankles, feet or hands, or are peeing more at night

Get urgent medical help if you are peeing much less than usual or not at all, or feel confused, drowsy or very unwell. These can be signs of acute kidney injury.