A blood test or kidney-function test is required whenever protein in the urine (proteinuria) is found to be persistent or significant. While a single positive urine test can be caused by temporary factors, a follow-up blood test is essential to measure how effectively your kidneys are filtering waste from your blood. In the UK, this is primarily done using the eGFR (estimated Glomerular Filtration Rate) and creatinine tests. These assessments help your GP determine if the protein leak is a sign of early-stage kidney disease or if it is a secondary effect of conditions like diabetes or high blood pressure.
What We’ll Discuss in This Article
- The transition from urine screening to blood-based kidney function tests
- Understanding the eGFR (estimated Glomerular Filtration Rate)
- The role of creatinine and urea in assessing renal health
- When ‘asymptomatic’ proteinuria requires immediate blood work
- Identifying the underlying causes of a reduced filtration rate
- How blood tests help differentiate between temporary stress and chronic disease
- Monitoring protocols for patients with known risk factors
The Necessity of Kidney Function Tests
When protein is detected in the urine, it suggests that the ‘sieve’ of the kidney (the glomeruli) is allowing substances through that should stay in the blood. However, the urine test only tells half the story. A blood test is required to see if this ‘leak’ is accompanied by a decrease in the kidney’s overall ability to clean the blood.
According to the National Institute for Health and Care Excellence (NICE), a blood test is mandatory if:
- A repeat urine ACR (Albumin-to-Creatinine Ratio) remains elevated.
- The patient has known risk factors such as diabetes or hypertension.
- There are physical symptoms like persistent swelling (oedema) or foamy urine.
- The protein leak is at a ‘macro’ level (significant amounts found on a dipstick).
Understanding the eGFR and Creatinine
The primary blood test used to check kidney function is the eGFR. This is not a direct measurement, but a calculation based on the level of creatinine in your blood, along with your age, sex, and sometimes ethnicity.
- Creatinine: This is a waste product produced by muscle breakdown. Healthy kidneys filter almost all of it out. If your blood creatinine levels rise, it suggests the kidneys are not filtering efficiently.
- eGFR: This score represents your kidney function as a percentage. For example, an eGFR of 90 means your kidneys are working at roughly 90% capacity. In the UK, an eGFR consistently below 60 is one of the main criteria for a diagnosis of chronic kidney disease (CKD).
Causes of a Reduced Filtration Rate
If your blood tests show a reduced eGFR alongside proteinuria, it confirms that the kidneys are struggling. Your GP will then look for the primary driver behind this decline.
Common causes include:
- Diabetic Nephropathy: High blood sugar damaging the small vessels in the kidney.
- Hypertensive Nephrosclerosis: High blood pressure ‘scarring’ the filtration units.
- Glomerulonephritis: An inflammatory or autoimmune attack on the kidney filters.
- Medication Stress: Long-term use of certain drugs, such as high-dose NSAIDs (ibuprofen), affecting blood flow to the kidneys.
Triggers for Urgent Blood Monitoring
In some cases, the discovery of protein in the urine warrants more immediate blood testing rather than a ‘wait and see’ approach.
- ‘When proteinuria is found alongside a sudden rise in blood pressure or new-onset swelling, immediate blood tests are vital to rule out acute kidney injury or rapid inflammatory conditions.’
Research from the UK Kidney Association suggests that early intervention when both the urine ACR and blood eGFR are abnormal can significantly slow the progression of kidney disease through the use of protective medications like ACE inhibitors.
Differentiation: Urine ACR vs. Blood eGFR
It is important to understand that these two tests measure different things, though they are used together to provide a complete ‘Kidney Health’ profile.
| Feature | Urine ACR Test | Blood eGFR Test |
| Focus | Measures ‘Leakiness’ of the filter | Measures ‘Efficiency’ of the filter |
| Early Warning | Often the first sign of damage | May stay normal while protein is leaking |
| Interpretation | High = More damage to the barrier | Low = Reduced waste clearance |
| NHS Review | Done at least annually for risk groups | Done alongside the urine test |
To Summarise
A blood test or kidney-function test is an essential follow-up to proteinuria because it provides a clear measurement of your kidneys’ actual performance. While the urine test identifies a leak, the blood eGFR and creatinine levels tell your GP if that leak is affecting your body’s ability to filter waste. In the UK, these tests are the cornerstone of managing kidney health, allowing for early diagnosis and the implementation of treatments that protect your renal function for the long term.
If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Can I have a normal eGFR but still have protein in my urine?
Yes, this is common in the early stages of kidney stress where the ‘leak’ starts before the overall filtration speed drops.
How often will my blood be checked if I have protein in my urine?
If the finding is persistent, you will typically have blood and urine checks every 6 to 12 months, depending on the severity.
Do I need to fast before a kidney blood test?
Usually, no, but you should avoid eating cooked meat for 24 hours before the test, as this can temporarily raise creatinine levels.
What is a ‘normal’ creatinine level?
This varies depending on your muscle mass and sex, which is why doctors use the eGFR calculation to make the result more accurate.
Can dehydration affect my eGFR?
Yes, severe dehydration can cause a temporary drop in eGFR, which usually recovers once you are properly hydrated.
What happens if my eGFR is below 60?
If it stays below 60 for more than three months, your GP will likely discuss a diagnosis of chronic kidney disease (CKD) and how to manage it.
Is a blood test better than a urine test?
Neither is ‘better’; they provide different information. You need both to get a full picture of your kidney health.
Authority Snapshot
This article was reviewed by Dr. Stefan Petrov, a UK-trained physician with experience in general medicine, surgery, and emergency care. Dr. Petrov has hands-on experience in hospital wards and intensive care units, performing diagnostic procedures and contributing to medical education. The content follows the clinical pathways set by the NHS and NICE for the management of proteinuria and renal function monitoring, ensuring that the information provided is accurate, safe, and trustworthy.
