This kidney function blood test measures creatinine and urea, two waste products cleared by the kidneys, together with sodium, and reports an estimated glomerular filtration rate (eGFR). eGFR is the figure most often used in UK practice to describe how well the kidneys are filtering the blood.
Before you book
- Price: £50. This covers laboratory analysis and your laboratory report.
- Where: Your blood sample is taken from a vein by our team at 82 King Street, Maidstone, or at a partner clinic near you, anywhere in the UK.
- Preparation: Don't eat meat for 12 hours beforehand.
- Results: the laboratory's report shows each result beside its reference range. We confirm the expected turnaround and any collection fee before you book.
- Optional: a written interpretation (£50) or a results consultation with Marina (£100).
After you order, we contact you to arrange your blood draw. A postal finger-prick kit may be possible for this test: please ask before ordering.
A blood test gives part of the picture. A full kidney assessment also uses a urine test for protein (the albumin to creatinine ratio, or ACR), which is not included here.
What is measured
- Creatinine
- eGFR (calculated from creatinine)
- Urea
- Sodium
Useful for
- Current figures to discuss with your GP if you have high blood pressure, diabetes or another risk factor for kidney disease
- Following up a previous creatinine or eGFR result
- A baseline before discussing medicines or supplements that affect the kidneys with your clinician
Sample: venous blood sample. Preparation: avoid eating meat for 12 hours beforehand. Biomarkers: 4 results (3 measured, eGFR calculated). Price includes: laboratory analysis and the laboratory's results report. Interpretation is optional and priced separately.
Helpful links: Compare health checks · How to prepare for a blood test · Understanding your report · All blood tests
Kidney Function Blood Test
- Creatinine (measured): a muscle waste product cleared by the kidneys; the basis for eGFR.
- eGFR (calculated): an estimate of filtration in mL/min/1.73 m², calculated from creatinine, age and sex.
- Urea (measured): a protein waste product, also influenced by hydration and diet.
- Sodium (measured): the main electrolyte in blood, reflecting fluid and salt balance.
The laboratory counts this panel as 4 biomarkers. It does not include potassium, which is often part of a standard NHS urea and electrolytes test, a urine albumin to creatinine ratio, or cystatin C.
Each kidney contains around a million tiny filtering units called nephrons. Blood is filtered in the glomerulus at the start of each nephron, and the rest of the nephron fine-tunes water, salt and acid balance. The total amount filtered per minute is the glomerular filtration rate (GFR), the standard measure of kidney function.
Why creatinine is used
Measuring GFR directly is complex, so it is estimated from creatinine, a waste product made at a fairly steady rate from creatine in muscle and cleared almost entirely by filtration. When filtration falls, creatinine rises. Because creatinine also depends on muscle mass and diet, an equation adjusts for age and sex to produce eGFR. NICE recommends the CKD-EPI creatinine equation for adults.
Urea and sodium
Urea is made in the liver when protein is broken down and is cleared by the kidneys. It is influenced by hydration, protein intake and bleeding in the gut as well as kidney function, so it is less specific than creatinine. Sodium is the main salt in the fluid outside cells. Its level mostly reflects the balance between water and salt, regulated by hormones and the kidneys.
How kidney disease is defined
NICE defines chronic kidney disease (CKD) as abnormalities of kidney function or structure present for more than 3 months, with implications for health. CKD is classified using both GFR category (G1 to G5) and urine ACR category (A1 to A3), because protein in the urine carries important information about risk. A single blood test cannot diagnose CKD.
Your report shows each result against the laboratory's reference range. The NHS describes an eGFR above 90 as typical of healthy kidneys, though many people with an eGFR of 60 to 89 have no kidney disease, particularly if they have no protein in the urine and no other kidney abnormality.
Why eGFR can mislead
- Muscle mass: NICE notes that high muscle mass leads to underestimation of GFR and low muscle mass to overestimation. Bodybuilders may show a low eGFR with healthy kidneys; frail older adults may show a reassuring eGFR that overstates function.
- Diet and supplements: cooked meat eaten within 12 hours can raise creatinine. The KDIGO 2024 guideline lists creatine supplements and high-protein diets among causes of a raised creatinine that do not reflect filtration.
- Rapid change: eGFR assumes kidney function is stable and is unreliable during acute illness, dehydration or acute kidney injury.
- Pregnancy and age: standard eGFR is not used to judge kidney function in pregnancy, and different methods are used for children.
When creatinine-based eGFR is less accurate and the result matters for a decision, KDIGO recommends combining creatinine with cystatin C, a separate test not included here.
Urea and sodium
Urea can be raised by dehydration, a high-protein diet or bleeding in the gut, and is naturally lower in pregnancy and with very low protein intake. Low sodium can result from drinking large volumes of water, from some medicines (thiazide diuretics and some antidepressants are recognised causes), and from heart, liver, kidney and hormonal conditions. High sodium usually reflects water loss.
What happens next
NICE advises that an eGFR below 60 in someone not tested before is confirmed by repeating the test within 2 weeks, and that urine ACR is measured as well. If your results need medical attention you will be advised to see your GP, and with your consent Juvenology can write to them. A markedly abnormal sodium, especially with confusion, drowsiness or seizures, needs urgent medical care.
Please do not stop or change any medicine because of this result. Some medicines that affect kidney blood tests are prescribed to protect the kidneys or heart, so any change should be made by your prescriber.
Early kidney disease usually causes no symptoms, and it is often found through blood and urine tests. NICE recommends offering CKD testing with eGFR and urine ACR to adults with risk factors including diabetes, high blood pressure, previous acute kidney injury, cardiovascular disease and gout. People in these groups would normally be tested through their GP.
People also choose a private kidney blood test to follow up a previous borderline result, or before discussing regular use of anti-inflammatory painkillers, creatine or other supplements with their clinician.
This test suits adults who want current kidney blood results to discuss with their GP, including people with high blood pressure, diabetes or a family history of kidney disease, and athletes who want context for a creatinine result.
May not be the right choice
- Feeling acutely unwell: a big drop in urine output, severe vomiting or diarrhoea, or confusion need same-day medical assessment through NHS 111 or your GP, not a booked blood test.
- Existing CKD or medicine monitoring: continue the monitoring arranged by your GP or kidney team.
- Potassium needed: if a clinician has asked for potassium, for example because of a medicine you take, this panel does not include it.
- Pregnancy: eGFR is not valid in pregnancy; kidney checks are arranged by your maternity team.
Established clinical use: eGFR and urine ACR together are the basis for detecting, staging and monitoring chronic kidney disease in UK and international guidance.
Population patterns: average eGFR tends to be lower in older age groups. NICE advises that age, gender and ethnicity alone should not be used as reasons to test for CKD, and a lower eGFR in an older adult still needs interpreting alongside ACR, muscle mass and overall health.
Emerging practice: KDIGO's 2024 guideline gives cystatin C a larger role in confirming kidney function when creatinine may mislead, for example with very high or low muscle mass. Its routine use is still developing in UK practice.
What this test cannot tell you: it cannot diagnose CKD on its own, assess kidney structure, show protein in the urine, predict lifespan or give a "biological age".
- Meat: NICE advises not eating any meat in the 12 hours before an eGFR blood test, as cooked meat can raise creatinine.
- Creatine: if you take creatine or protein supplements, tell us when booking so the result can be read in context.
- Fluids: drink normally. Avoid being dehydrated, and also avoid drinking unusually large volumes beforehand, which can lower sodium.
- Exercise: keep to light activity on the day before and the morning of the test.
- Medicines: keep taking prescribed medicines and list everything you take, including anti-inflammatory painkillers and supplements.
- Illness: if you have recently had vomiting, diarrhoea or a fever, wait until you have recovered unless a clinician advises otherwise.
Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.
- Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
- Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
- Postal home finger-prick kit: Ask before ordering. We offer a home kit only where the laboratory confirms this test is suitable for a finger-prick sample.
Sample: Venous blood sample (serum); sample tube type confirmed by the laboratory
Collection notes: Avoid eating meat for 12 hours before the sample (NICE NG203). Drink normally and avoid heavy exercise beforehand. NICE advises that samples for eGFR are received and processed by the laboratory within 12 hours of collection, so a postal finger-prick kit is offered only if the laboratory confirms it is suitable for this panel.
The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.
Beyond the blood test: optional support, priced separately
- Personalised written interpretation report: £50
- General consultation: £50
- Results consultation (results discussion and follow-up advice): £100
- Personalised programme: from £150, after we assess whether a programme suits you
Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.
If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.
Does an eGFR below 90 mean I have kidney disease?
Not on its own. Many healthy people have an eGFR between 60 and 89. CKD is diagnosed when abnormalities persist for more than 3 months, using eGFR and a urine ACR test together.
I lift weights and take creatine. Will that affect my result?
It can. Higher muscle mass and creatine supplements can raise creatinine and lower eGFR without any kidney problem. Mention them when you book; a clinician may suggest cystatin C if the result needs clarifying.
Why is there no urine test?
This product is a blood test only. Urine ACR is an important part of a full kidney assessment, and your GP can arrange it if your blood results or risk factors suggest it is needed.
Why is potassium not included?
This panel contains sodium, urea, creatinine and eGFR only. Potassium is sensitive to sample handling and delays, and if you need it measured your clinician can advise on the most suitable test.
Do I need to fast?
No, but avoid meat for 12 hours beforehand and drink normally.
Is interpretation included in the price?
The price covers laboratory analysis and the laboratory's report with reference ranges. You can add an optional personalised written interpretation report (£50) or a results discussion consultation (£100).
- Chronic kidney disease: assessment and management (NG203). National Institute for Health and Care Excellence, 2021. https://www.nice.org.uk/guidance/ng203
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International, 2024;105(4S). https://kdigo.org/wp-content/uploads/2024/03/KDIGO-2024-CKD-Guideline.pdf
- Chronic kidney disease: diagnosis. NHS. https://www.nhs.uk/conditions/kidney-disease/diagnosis/
- eGFR (estimated glomerular filtration rate). Lab Tests Online UK. https://labtestsonline.org.uk/tests/egfr-calc-creatinine-clearance
- Urea test. Lab Tests Online UK. https://labtestsonline.org.uk/tests/urea-test
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Nephrology Dialysis Transplantation, 2014;29(Suppl 2). https://doi.org/10.1093/ndt/gfu040
