Kidney function blood test: what your results mean

Your kidneys work quietly in the background every day. They filter waste from your blood, help balance fluid levels, support blood pressure control and keep key salts in the right range.

The difficulty is that early kidney problems often don’t cause obvious symptoms. The NHS says kidney disease does not tend to cause symptoms in the early stages and is often only found when a routine blood or urine test detects a possible problem.

That’s why kidney function blood tests can be useful. They don’t diagnose everything by themselves, and one result rarely tells the whole story. But they can help show whether your kidneys appear to be filtering as expected, or whether the result needs repeating or discussing with your GP.

This guide explains the main kidney markers in plain English: creatinine, eGFR, urea and electrolytes.

What does a kidney function blood test measure?

A kidney function blood test is usually not one single number. It is a small group of markers that help build a picture of how your kidneys are working.

The exact panel can vary, but it commonly includes:

• Creatinine
• eGFR, or estimated glomerular filtration rate
• Urea
• Electrolytes such as sodium, potassium, chloride and bicarbonate

These markers are useful because your kidneys are involved in filtering waste, balancing fluids and keeping blood chemistry stable. If the kidneys are under strain, some of these values may move outside the expected range.

A kidney blood test is often interpreted alongside other information, such as your blood pressure, blood sugar, medication history, hydration, muscle mass and whether you’ve had any recent illness.

Creatinine explained

Creatinine is a waste product made by your muscles. It is produced naturally as part of everyday muscle activity and is normally filtered out of the blood by the kidneys.

If creatinine is raised, it can suggest that the kidneys aren’t filtering as well as expected. But creatinine needs context. A raised result doesn’t automatically mean chronic kidney disease.

Creatinine can be affected by factors such as:

• Muscle mass
• Recent intense exercise
• Hydration
• Diet
• Certain medicines
• Recent illness
• Temporary kidney strain

Someone with more muscle may naturally have a higher creatinine than someone with less muscle. A dehydrated person may also show a temporary change. This is why a single creatinine result should be interpreted carefully rather than treated as a diagnosis.

Creatinine is especially useful because it is used to calculate eGFR, which gives a broader estimate of kidney filtering.

What is eGFR?

eGFR stands for estimated glomerular filtration rate. It is an estimate of how well your kidneys are filtering blood.

The NHS explains that the main test for kidney disease is a blood test measuring creatinine, and that the result is used with factors such as age, size and gender to calculate eGFR.

eGFR is reported in mL/min/1.73m². In plain English, a higher eGFR usually means better filtering function, while a lower eGFR can suggest reduced kidney filtering.

It is important to remember that eGFR is an estimate. It can vary from test to test and can be less accurate in some people, including those at extremes of muscle mass. It shouldn’t be judged in isolation.

What do eGFR numbers mean?

Kidney function is often described using CKD stages. CKD stands for chronic kidney disease.

NICE classifies CKD using both eGFR and albumin:creatinine ratio, known as ACR, and says CKD includes people with kidney damage markers or a GFR below 60 mL/min/1.73m² on at least two occasions separated by at least 90 days.

The eGFR categories are:

These stages are useful, but they can be misunderstood. A single eGFR result isn’t the same as a confirmed diagnosis. Results may need repeating, and GP interpretation matters.

For example, an eGFR above 60 may be reassuring in many cases, but it still needs context if there are other signs of kidney damage. An eGFR below 60 may need confirmation with repeat testing and urine testing.

Urea explained

Urea is another waste product in the blood. It is made when the body breaks down protein and is normally removed by the kidneys.

Urea can rise when kidney filtering is reduced, but it is also affected by other factors. Hydration, diet, recent illness and how much protein someone has eaten can all influence the result.

That means urea is useful, but it is rarely interpreted alone. It is usually considered alongside creatinine, eGFR, electrolytes and the wider clinical picture.

A raised urea result may suggest that something is worth checking, but it doesn’t tell you the cause by itself.

Electrolytes explained

Electrolytes are minerals in the blood that help the body function properly. A kidney function panel may include sodium, potassium, chloride and bicarbonate.

Your kidneys help keep these in balance. If kidney function changes, or if someone is dehydrated, unwell or taking certain medicines, electrolyte levels can shift.

Potassium is particularly important because levels that are too high or too low can matter for heart and muscle function. But electrolyte interpretation belongs in context. A slightly abnormal result may be temporary, while a more significant result may need prompt medical review.

This is one reason kidney markers should be reviewed carefully and not treated as just a set of numbers.

What about urine albumin and ACR?

Kidney assessment isn’t only about blood tests.

A urine test can check for albumin, a type of protein, and creatinine in the urine. This is called the albumin:creatinine ratio, or ACR. The NHS says urine tests may check for albumin and creatinine, as well as blood or protein in urine, and that urine tests alongside eGFR can give a more accurate picture of kidney function.

This matters because some kidney problems show up through urine changes before blood markers change much.

A standard blood panel may include creatinine, eGFR, urea and electrolytes, but urine ACR is a separate urine marker. If your GP is investigating possible kidney disease, they may arrange both blood and urine testing.

Why kidney function matters

Kidneys are easy to overlook because you usually don’t feel them working. But they are central to several important systems.

They help:

• Filter waste products from the blood
• Balance fluid levels
• Keep electrolytes such as sodium and potassium in range
• Support blood pressure regulation
• Play a role in red blood cell production through kidney-related hormone signalling

When kidney function changes, the effects can be wider than the kidneys themselves. Kidney health overlaps with blood pressure, diabetes risk, cardiovascular risk and medication safety.

That’s why kidney function is a useful part of a broader health check. It helps answer a simple question: are the kidneys filtering as expected, or is there something worth following up?

Who should consider a kidney function blood test?

A kidney function blood test can be useful for adults who want a baseline, especially if they have risk factors that can affect kidney health.

You may want to consider checking kidney function if you:

• Have high blood pressure
• Have diabetes or raised blood sugar
• Have a family history of kidney disease
• Have previously had abnormal kidney markers
• Take medicines that may affect kidney function
• Have cardiovascular risk factors
• Want a broader health baseline
• Are making significant lifestyle changes and want a starting point

You don’t need symptoms to check kidney function. In fact, one of the reasons these tests are useful is that early kidney changes may not cause symptoms.

If you already have symptoms or have been told your kidney markers are abnormal, your GP is the right person to advise on follow-up.

What a raised creatinine or low eGFR may mean

A raised creatinine or low eGFR can suggest that your kidneys aren’t filtering as well as expected. But there are several possible explanations.

These can include:

• Dehydration
• Recent intense exercise
• Higher muscle mass
• Certain medicines
• Recent illness
• Temporary kidney strain
• Reduced kidney function that needs further assessment

This is why repeat testing is often important. A single result may be temporary. A persistent pattern is more meaningful.

A private screening result can flag that something needs attention, but diagnosis happens through your GP. Your GP can review symptoms, medication, urine results, previous blood tests and any other relevant history.

What kidney function tests can’t tell you

Kidney function blood tests are useful, but they have limits.

They don’t show kidney structure. If a doctor needs to look at the size, shape or structure of the kidneys, imaging such as an ultrasound may be needed.

They don’t explain every cause. A low eGFR or raised creatinine shows a pattern, not the full reason behind it.

They can vary. Hydration, illness, exercise, muscle mass and medicines can all affect results.

They don’t rule out every kidney problem. Some kidney issues may need urine testing, imaging or specialist review.

The result is a starting point. It helps decide whether follow-up is needed, but it isn’t the whole answer.

Kidney function in a private health check

Kidney function fits naturally into a wider health screen because kidney health overlaps with several other markers.

For example, high blood pressure and raised blood sugar can both affect kidney health. That’s why kidney markers are useful alongside blood pressure, cholesterol, HbA1c and body composition.

If you are comparing common screening markers more broadly, our guide to 5 essential blood tests for adults explains how kidney function fits alongside cholesterol, blood sugar, liver function and full blood count checks.

You can also read more on our dedicated kidney function blood test page.

The Essential Health Check is a sensible starting point if you want a focused baseline. For a broader review, the Advanced Health Check is the related package for this article because it looks at kidney markers alongside a wider health check structure and in-clinic checks.

If you want the fullest general baseline, you can compare the Optimal Health Check or view all private health check packages.

Because kidney function and blood sugar often overlap, our HbA1c blood test guide may also be useful if you are checking diabetes risk.

When to speak to your GP

Speak to your GP if your kidney markers are abnormal, if your eGFR is low, or if you have symptoms that could suggest a kidney or urinary problem.

Symptoms worth taking seriously include:

• Reduced urine output
• Blood in your urine
• Foamy urine
• Swelling in your ankles, feet, hands or face
• Persistent tiredness
• Shortness of breath
• Persistent high blood pressure
• Needing to pass urine more often, especially at night
• Unexplained weight loss or poor appetite

The NHS lists symptoms of later-stage chronic kidney disease including tiredness, swollen ankles, feet or hands, shortness of breath, blood in urine and needing to pee more often, particularly at night.

A private health check is designed for adults who currently feel well and want a clearer picture of their health markers. It isn’t a substitute for medical care if you have symptoms.

In a life-threatening emergency, call 999. For urgent advice that isn’t an emergency, NHS 111 is available online or by phone.

A useful marker, not the full story

Kidney function blood tests are useful because they can show whether your kidneys appear to be filtering as expected. Creatinine, eGFR, urea and electrolytes can all add useful context.

But they are still markers. They make most sense when interpreted alongside your wider health picture: blood pressure, blood sugar, cholesterol, medication history, hydration, symptoms and any previous results.

If you want to check kidney function as part of a broader review, the Advanced Health Check is a strong option. If you are unsure which package fits, contact us and we can help you choose the most appropriate starting point. Appointments are available from our Kingston upon Thames and Crawley clinics.