How is kidney function checked? 

Kidney function is primarily checked using a combination of blood tests and urine tests to see how well the organs are filtering waste and managing fluid. In the UK, these tests are standardised according to NHS and NICE protocols to ensure that any decline in function whether sudden or long-term is identified accurately and early. 

The kidneys act as the body’s filtration system, and when they are not working correctly, specific markers begin to appear in higher concentrations in the blood or leak into the urine. For most people, these tests are performed as part of a routine check-up or when they are unwell with an illness that could impact the kidneys, such as an infection or dehydration. In this article, you will learn about the role of serum creatinine, the eGFR filtration rate, and how urine analysis provides vital clues about your renal health. 

What We Will cover in This Article 

• The significance of serum creatinine as a waste product marker 

• Understanding the eGFR calculation for long term kidney health 

• How urine dipstick tests identify blood and protein leaks 

• The importance of monitoring urine output during acute illness 

• Why imaging such as ultrasound may be used to check kidney structure 

• Clinical evidence regarding the accuracy of renal function testing 

The Primary Blood Test: Serum Creatinine 

The most common way to check kidney function is by measuring the level of serum creatinine in your blood. Creatinine is a natural waste product produced by muscle breakdown and is normally filtered out of the blood by the kidneys. If your kidneys are struggling, the level of creatinine in your blood will rise. 

In a clinical setting, doctors don’t just look at the creatinine number in isolation. They compare it to your previous results to see if there is a sudden rise, which could indicate acute kidney injury (AKI), or a slow, persistent elevation, which suggests chronic kidney disease (CKD). 

Key Blood Markers: 

• Serum Creatinine: Measured in micromoles per litre (µmol/L). A higher number generally indicates lower kidney function. 

• eGFR (Estimated Glomerular Filtration Rate): A calculation based on your creatinine, age, and sex that tells doctors roughly what percentage of function your kidneys have. 

• Urea: Another waste product that can rise during dehydration or kidney stress. 

• Electrolytes: Measuring salts like potassium and sodium, which the kidneys must balance to keep the heart and nerves working safely. 

Urine Tests: Analysis and Output 

Urine tests are essential because they provide direct information about the physical state of the kidney’s filtering units. A simple urine dipstick test can be performed in a GP surgery or hospital to check for substances that should not normally be present in urine. 

If the filters (glomeruli) are damaged, they may leak protein or blood cells into the urine. Additionally, monitoring the volume of urine you produce over 24 hours is a critical ‘test’ used in hospitals to detect early signs of kidney failure. 

Common Urine Checks: 

• Urine Dipstick: A quick check for blood (haematuria), protein (proteinuria), and signs of infection like nitrites or white blood cells. 

• ACR (Albumin to Creatinine Ratio): A more precise laboratory test used to measure small amounts of protein leakage, often used to monitor people with diabetes. 

• Urine Output Monitoring: Checking if you are passing at least 0.5ml of urine per kilogram of body weight per hour, which is a key safety metric in the NHS. 

Imaging and Structural Checks 

Sometimes, blood and urine tests show a problem, but they cannot explain why the kidneys are struggling. In these cases, the NHS uses imaging tests to look at the physical structure of the kidneys. This is particularly important if a blockage, such as a kidney stone or an enlarged prostate, is suspected. 

Test Type	What it Checks	Why it is Performed
Ultrasound Scan	Size, shape, and blockages.	To rule out stones, tumours, or swelling (hydronephrosis).
CT Scan	Detailed 3D view of the kidneys.	To look for complex stones or structural abnormalities.
Kidney Biopsy	Microscopic tissue samples.	Reserved for complex cases where internal inflammation is suspected.

 

To Summarise 

Kidney function is checked using blood tests for creatinine and electrolytes, along with urine tests to check for leaks or signs of infection. These tests help doctors determine if the kidneys are filtering waste effectively and if they are under acute or chronic stress. Monitoring these levels is the most reliable way to identify kidney issues early, often before any physical symptoms such as swelling or fatigue develop. 

If you experience severe sudden or worsening symptoms call 999 immediately. 

Authority Snapshot 

This article was written by Dr. Rebecca Fernandez, a UK trained physician with an MBBS and extensive experience in internal medicine, emergency care, and intensive care. It was reviewed by the MyPatientAdvice clinical team to ensure it follows the latest NHS and NICE [NG148] guidelines on the monitoring of renal function. Our goal is to provide the public with clear, factual, and medically safe information to help them understand the importance of routine kidney screening.