ORGAN HEALTH
Kidney Function Test UK: What It Measures, Results Mean & When to Worry
Kidney function tests — often called U&Es (urea and electrolytes) or renal profile — are among the most commonly ordered blood tests in UK general practice. The NICE chronic kidney disease guideline (NG203) estimates that 1 in 10 UK adults has some degree of chronic kidney disease (CKD) — yet up to 50% of those with stage 3 CKD are undiagnosed because the kidneys can lose significant function before symptoms appear.
This guide explains every marker in a standard kidney function panel, what abnormal results actually mean, how CKD is staged, and when you should see your GP.
1. What Are Kidney Function Tests?
Kidney function tests are a group of blood (and sometimes urine) tests that measure how well your kidneys are filtering waste from your blood. Your kidneys process roughly 180 litres of blood per day, filtering out toxins, excess fluid and electrolytes to produce about 1–2 litres of urine.
Unlike many organs, the kidneys have enormous functional reserve. You can lose up to 50% of kidney function before blood tests start to show abnormalities, and up to 90% before symptoms become obvious. This is why Kidney Research UK calls CKD a “silent disease” — proactive blood testing is often the only way to catch it early.
The NHS estimates that chronic kidney disease affects around 4.2 million people in England alone. Early detection through routine blood testing can slow or halt progression in the majority of cases — particularly when the underlying cause is diabetes or hypertension.
2. What Gets Tested: The Full Kidney Panel
A standard UK kidney function panel (U&Es) typically includes five to seven markers. Each tells a different part of the story:
Creatinine
A waste product from normal muscle metabolism. Creatinine is produced at a fairly constant rate and filtered almost entirely by the kidneys, making it the most widely used marker for kidney filtration. A rising creatinine usually means the kidneys are clearing waste less efficiently. However, very muscular individuals naturally produce more creatinine, so the raw number must always be interpreted alongside eGFR.
NHS range: 59–104 μmol/L (male), 45–84 μmol/L (female) · Optimal: lower end of range for your age and sex
eGFR (Estimated Glomerular Filtration Rate)
Not directly measured — eGFR is calculated from your creatinine level, age, sex and (in the 2021 CKD-EPI equation) body surface area. It estimates how many millilitres of blood your kidneys filter per minute. The higher the number, the better. eGFR is the gold standard for CKD classification (NICE NG203).
Normal: ≥90 mL/min/1.73m² · Mildly reduced: 60–89 · CKD stage 3: 30–59 · Stage 4: 15–29 · Stage 5: <15
Urea (Blood Urea Nitrogen)
Produced when the liver breaks down protein. Urea is filtered by the kidneys, so rising levels can indicate reduced kidney function. However, urea is more variable than creatinine — it rises with high-protein diets, dehydration, gastrointestinal bleeding and catabolic states. It is most useful when interpreted alongside creatinine and eGFR.
NHS range: 2.5–7.8 mmol/L · Optimal: 3.0–6.5 mmol/L
Sodium (Na+)
The primary electrolyte controlling fluid balance and blood pressure. The kidneys regulate sodium with remarkable precision. Low sodium (hyponatraemia) is the most common electrolyte abnormality in hospitalised patients and can indicate kidney disease, heart failure, or certain medications. High sodium (hypernatraemia) typically signals dehydration.
NHS range: 133–146 mmol/L · Optimal: 136–142 mmol/L
Potassium (K+)
Critical for heart rhythm, nerve conduction and muscle function. The kidneys are the primary regulators of potassium balance. High potassium (hyperkalaemia) is a hallmark of advancing kidney disease and can be dangerous — NICE NG203 flags it as a medical emergency above 6.5 mmol/L because it can trigger cardiac arrhythmias.
NHS range: 3.5–5.3 mmol/L · Optimal: 3.8–4.8 mmol/L
Chloride (Cl−)
Works closely with sodium to maintain fluid balance and acid–base equilibrium. Chloride abnormalities often mirror sodium changes. Low chloride can indicate metabolic alkalosis (common with persistent vomiting), while high chloride may suggest kidney tubular acidosis or excessive saline intake.
NHS range: 95–108 mmol/L · Optimal: 98–106 mmol/L
Albumin:Creatinine Ratio (ACR) — urine test
Not a blood marker but often requested alongside U&Es. ACR detects tiny amounts of albumin (protein) leaking into urine — the earliest sign of kidney damage, often appearing years before eGFR drops. NICE recommends annual ACR testing for everyone with diabetes, hypertension or established CKD.
Normal: <3 mg/mmol · Moderately increased (A2): 3–30 mg/mmol · Severely increased (A3): >30 mg/mmol
3. NHS Reference Ranges vs Optimal Levels
NHS reference ranges tell you whether a result is statistically “normal” for the general population. Optimal ranges are narrower targets associated with the lowest risk of kidney disease progression and cardiovascular events.
| Marker | NHS Range | Optimal | Why It Matters |
|---|---|---|---|
| Creatinine | 59–104 μmol/L (M) 45–84 μmol/L (F) | Lower end of range | Rising trend matters more than single reading |
| eGFR | ≥90 mL/min | ≥90 mL/min | Below 60 = CKD stage 3+ |
| Urea | 2.5–7.8 mmol/L | 3.0–6.5 mmol/L | High = dehydration, high protein intake or reduced kidney function |
| Sodium | 133–146 mmol/L | 136–142 mmol/L | Low sodium is most common electrolyte disorder |
| Potassium | 3.5–5.3 mmol/L | 3.8–4.8 mmol/L | >6.5 = cardiac emergency |
| ACR (urine) | <3 mg/mmol | <3 mg/mmol | Earliest sign of kidney damage |
Creatinine
Trend matters more than single value
eGFR
Below 60 = CKD stage 3+
Urea
Affected by diet and hydration
Sodium
Low = most common electrolyte disorder
Potassium
>6.5 = cardiac emergency
ACR (urine)
Earliest sign of kidney damage
Source: NHS kidney disease diagnosis, NICE NG203, KDIGO 2024 CKD guidelines. Individual laboratory ranges may vary.
4. eGFR Explained — The Single Most Important Number
If you only look at one number on your kidney function results, make it eGFR. It estimates the volume of blood your kidneys filter per minute, adjusted for body surface area. A healthy young adult typically has an eGFR of 100–120 mL/min/1.73m².
eGFR naturally declines with age — roughly 1 mL/min per year after age 40, according to longitudinal data from the BMJ. An eGFR of 75 in a 70-year-old is very different from an eGFR of 75 in a 30-year-old. Context and trend matter enormously.
The 2021 CKD-EPI equation (NEJM) removed the race coefficient that was previously included in eGFR calculations, following evidence that race-based adjustments introduced bias and delayed CKD diagnosis in Black patients. UK laboratories have adopted the race-free equation.
Key principle: a single eGFR reading has limited value. What matters is the trajectory. A decline of more than 5 mL/min/1.73m² over 12 months is considered a significant decline by NICE and warrants further investigation.
5. CKD Staging: What Each Stage Means
Chronic kidney disease is classified into five stages based on eGFR, plus albuminuria categories (A1–A3) based on ACR. The combination of both determines your overall risk:
| Stage | eGFR | Description | What Happens |
|---|---|---|---|
| 1 | ≥90 | Normal or high | Kidney damage present (e.g. protein in urine) but normal filtration. No symptoms. |
| 2 | 60–89 | Mildly decreased | Common in older adults. Usually monitored, not treated. No symptoms. |
| 3a | 45–59 | Mild–moderate decrease | NICE recommends annual monitoring. Cardiovascular risk rises. |
| 3b | 30–44 | Moderate–severe decrease | Anaemia, bone disease risk. Medication dose adjustments needed. |
| 4 | 15–29 | Severely decreased | Nephrology referral. Preparation for dialysis or transplant may begin. |
| 5 | <15 | Kidney failure | Dialysis or transplant typically required. Symptoms include fatigue, nausea, swelling. |
Normal filtration with kidney damage present. No symptoms.
Mildly decreased. Common in older adults. Monitored, not treated.
Mild–moderate decrease. Annual monitoring. CV risk rises.
Moderate–severe. Anaemia and bone disease risk.
Severe. Nephrology referral. Dialysis preparation may begin.
Kidney failure. Dialysis or transplant typically required.
Source: NICE NG203 CKD classification, KDIGO 2024.
6. Common Causes of Abnormal Kidney Markers
An abnormal kidney function result does not automatically mean kidney disease. The most common causes in UK primary care include:
Dehydration
The single most common reason for a mildly elevated creatinine and urea in otherwise healthy people. Blood becomes more concentrated, kidney markers rise. Rehydrate and retest before drawing conclusions.
Type 2 diabetes
Diabetes is the leading cause of CKD worldwide. High blood sugar damages the tiny blood vessels in the kidneys (diabetic nephropathy). NICE NG28 recommends annual kidney function checks for everyone with diabetes.
Hypertension
The second most common cause of CKD. Sustained high blood pressure damages the kidneys' filtering units (glomeruli). The relationship is bidirectional — kidney disease alsocauses hypertension, creating a vicious cycle.
NSAIDs (ibuprofen, naproxen)
Regular NSAID use reduces blood flow to the kidneys. The NHS advises limiting NSAID use, particularly in people over 65 or with existing kidney disease. Even short courses can cause acute kidney injury in vulnerable individuals.
High muscle mass / intense exercise
Heavy weight training or endurance exercise can raise creatinine without any kidney problem. Very muscular individuals may have creatinine above the NHS range while having perfectly healthy kidneys. Cystatin C can be tested as an alternative filtration marker that is not affected by muscle mass.
Urinary tract obstruction
Kidney stones, an enlarged prostate, or other obstructions can back up urine and acutely impair kidney function. This is usually reversible once the obstruction is relieved.
7. GP Kidney Test vs Helvy
Your GP can request kidney function tests on the NHS, but access varies considerably:
| NHS GP | Helvy | |
|---|---|---|
| Cost | Free (if clinically indicated) | Included in Essential panel (from £129) |
| Who decides | GP must agree a test is warranted | You decide when to test |
| Markers | U&Es (creatinine, eGFR, urea, Na, K) | U&Es + broader metabolic panel (liver, lipids, HbA1c, vitamins) |
| Wait time | 2–4 weeks for appointment + 1–2 weeks for results | Kit arrives in 1–2 days, results in 5 working days |
| Explanation | Brief summary, often by letter or phone | Detailed report reviewed by GMC-registered doctor |
| Tracking | Results in NHS App (basic) | Historical trends, colour-coded ranges, change alerts |
Cost
Who decides
Markers
Wait time
Explanation
Tracking
If your GP has identified a kidney concern, the NHS pathway is appropriate and free. Helvy is designed for proactive adults who want a comprehensive baseline — kidney markers are included in every panel alongside liver, lipid, thyroid and metabolic markers, giving you the full picture in one test.
8. Interpreting Your Results: 4 Common Patterns
Rather than looking at each marker in isolation, patterns across multiple markers are far more informative:
PATTERN 1
High creatinine + low eGFR, normal electrolytes
The classic CKD pattern. Filtration is reduced but the kidneys are still managing electrolyte balance. If consistent across two tests ≥3 months apart, this confirms CKD and staging depends on the eGFR value.
PATTERN 2
High creatinine + high urea, normal eGFR
Often dehydration or a high-protein diet. Both creatinine and urea rise because blood is more concentrated. Rehydrate for 48 hours and retest. If it normalises, there is no kidney problem.
PATTERN 3
Normal creatinine + high potassium
Isolated hyperkalaemia can be caused by medications (ACE inhibitors, ARBs, spironolactone), potassium supplements, or a haemolysed blood sample (cells burst during collection, releasing potassium). If the result is borderline, your GP will usually retest before investigating further.
PATTERN 4
Rapidly rising creatinine over days to weeks
This suggests acute kidney injury (AKI), not chronic kidney disease. AKI is a medical emergency. NICE NG148 defines AKI as a creatinine rise of ≥26 μmol/L within 48 hours or ≥50% within 7 days. Common triggers: dehydration, sepsis, NSAIDs, urinary obstruction. Requires urgent medical attention.
9. Who Should Get Tested
NICE NG203 recommends kidney function testing for anyone with:
- •Diabetes (type 1 or type 2) — annual testing mandatory
- •Hypertension — annual U&Es
- •Cardiovascular disease or family history of early heart disease
- •Family history of CKD or polycystic kidney disease (PKD)
- •Regular NSAID use (ibuprofen, naproxen, diclofenac)
- •Age over 60 — kidney function naturally declines
- •Previous acute kidney injury (AKI)
- •Urinary tract abnormalities (recurrent UTIs, kidney stones)
- •Autoimmune conditions (lupus, vasculitis)
- •Long-term use of ACE inhibitors, ARBs, or lithium
If none of the above apply to you, a baseline kidney function test as part of a comprehensive blood panel is still worthwhile — especially if you have never been tested. CKD is silent in its early stages, and catching a declining eGFR trend early gives you the widest range of interventions.
10. Evidence-Based Ways to Protect Your Kidneys
The majority of CKD cases are preventable or manageable with lifestyle changes:
Stay hydrated
Aim for 1.5–2 litres of water daily. Chronic mild dehydration concentrates waste products and forces the kidneys to work harder. The NHS recommends 6–8 glasses per day.
Control blood pressure
Target <140/90 mmHg (or <130/80 if you have CKD with significant proteinuria). Reducing dietary sodium to <6g/day, regular exercise and maintaining a healthy weight are first-line interventions. NICE NG136 (hypertension) covers pharmacological management when lifestyle changes alone are insufficient.
Manage blood sugar
If you have prediabetes or type 2 diabetes, maintaining HbA1c below 48 mmol/mol (or a personalised target set by your GP) dramatically slows diabetic nephropathy. The UKPDS trial (Lancet) showed that each 1% reduction in HbA1c reduced microvascular complications (including nephropathy) by 37%.
Limit NSAIDs
Avoid ibuprofen and naproxen for chronic pain if possible. Use paracetamol as a first-line alternative. If NSAIDs are necessary, use the lowest effective dose for the shortest time. Anyone with eGFR <60 should generally avoid NSAIDs entirely.
Moderate protein intake
Very high protein diets (>2g/kg/day) increase the kidneys' workload. For people with established CKD, the KDIGO guidelines recommend 0.8g/kg/day. For healthy adults without CKD, moderate protein intake (1.2–1.6g/kg/day) is perfectly safe.
Stop smoking
Smoking accelerates kidney function decline and doubles the risk of CKD progression. A BMJ meta-analysis found that current smokers had a 34% higher risk of developing CKD compared to never-smokers.
11. Medications and Your Kidneys
Several commonly prescribed medications require kidney function monitoring — and some are actively nephroprotective:
ACE inhibitors & ARBs
Lisinopril, ramipril, losartan and similar drugs reduce blood pressure and protect the kidneys by reducing intraglomerular pressure. They may cause a small initial rise in creatinine (up to 30% is acceptable per NICE NG203). Monitoring kidney function 1–2 weeks after starting or dose changes is essential.
SGLT2 inhibitors (dapagliflozin, empagliflozin)
Originally developed for diabetes, these drugs are now recommended by NICE for CKD even in people without diabetes. The DAPA-CKD trial (NEJM) showed dapagliflozin reduced the risk of kidney failure or sustained eGFR decline by 39%. This is one of the most significant advances in nephrology in decades.
NSAIDs and over-the-counter painkillers
Ibuprofen, naproxen and diclofenac reduce blood flow to the kidneys. The “triple whammy” combination (NSAID + ACE inhibitor/ARB + diuretic) is particularly dangerous and significantly increases AKI risk. Always check with a pharmacist before combining these medications.
Metformin
Safe and beneficial in most people with diabetes, but dose adjustment is required as eGFR falls. Below eGFR 30, metformin should generally be stopped due to the risk of lactic acidosis. Your prescriber should review your kidney function at least annually if you take metformin.
12. The Diabetes and Blood Pressure Connection
Diabetes and hypertension together account for roughly two-thirds of all CKD cases in the UK. Understanding this relationship is crucial because both conditions are modifiable:
Diabetic nephropathy develops when chronically elevated blood sugar damages the tiny blood vessels in the kidney's filtering units (glomeruli). The earliest detectable sign is microalbuminuria (ACR 3–30 mg/mmol) — protein leaking into urine because the filters are becoming “leaky.” This can appear 5–15 years before eGFR drops.
Hypertensive nephrosclerosis occurs when sustained high blood pressure hardens and narrows the renal arteries. The kidneys receive less blood, and the filtering units scar over time.
The good news: the UKPDS trial and the RENAAL trial (NEJM) both demonstrated that tight blood pressure and blood sugar control dramatically slows kidney function decline. Early detection through blood testing (HbA1c, kidney function, and ACR) is the critical first step.
13. When to See Your GP
Book an appointment promptly if your results show any of the following:
RED FLAGS
- ■eGFR below 60 mL/min/1.73m² on two tests ≥3 months apart
- ■eGFR decline of more than 5 mL/min in 12 months
- ■Potassium above 5.5 mmol/L (urgent if >6.5)
- ■ACR ≥3 mg/mmol (confirmed on repeat testing)
- ■Creatinine rising significantly between tests
- ■Blood in your urine (haematuria) alongside abnormal kidney function
- ■Unexplained ankle swelling, persistent fatigue, or foamy urine
- ■Any kidney function abnormality combined with diabetes or hypertension
A private blood test is a screening tool, not a diagnosis. If anything looks abnormal, share the results with your GP. They can arrange repeat tests, urine albumin testing, imaging if needed, and referral to a nephrologist for stages 4–5 or rapidly declining function.
14. Frequently Asked Questions
Do I need to fast before a kidney function test?
No — a standard U&E panel does not require fasting. However, if your blood test also includes fasting insulin, glucose or a lipid profile, you may be asked to fast for 8–12 hours. Drinking water is always fine. Stay well hydrated before your test to avoid dehydration artefacts.
Can exercise affect my kidney function results?
Yes. Intense exercise in the 24–48 hours before a test can temporarily raise creatinine and, in extreme cases, cause transient proteinuria. For the most accurate results, avoid heavy training the day before your blood test.
What is the difference between acute kidney injury (AKI) and chronic kidney disease (CKD)?
AKI develops over hours to days (e.g. severe dehydration, infection, medication reaction) and is often reversible with treatment. CKD develops over months to years and is defined as kidney damage or eGFR <60 persisting for at least three months. CKD is usually not reversible but progression can be slowed or halted.
My eGFR is 65 — should I be worried?
It depends on your age and the trend. An eGFR of 65 in a healthy 70-year-old may represent normal age-related decline. An eGFR of 65 in a 30-year-old is more concerning. A single reading means little — what matters is whether the number is stable or falling. Retest in 3 months and share both results with your GP.
Can I reverse kidney damage?
Early-stage damage (stages 1–2) can often be stabilised or partially reversed by treating the underlying cause — controlling blood pressure, managing diabetes, stopping nephrotoxic drugs. Later stages (3b+) are generally not reversible, but progression can be significantly slowed with medications like SGLT2 inhibitors and ACE inhibitors.
How often should I retest my kidney function?
For healthy adults with normal results: annually as part of a comprehensive blood panel is a sensible baseline. For people with known risk factors (diabetes, hypertension, family history of CKD): at least annually, or more frequently as directed by your GP. For established CKD: NICE recommends monitoring frequency based on stage (from annual for stable 3a to monthly for stage 5).
Related Biomarkers and Guides
CHECK YOUR KIDNEY FUNCTION
Kidney markers are included in every Helvy panel.
Creatinine, eGFR, urea and electrolytes are part of the Essential panel (from £129), alongside liver, lipid, thyroid and vitamin markers. One test. The full picture.
View blood test panels