Why Your Kidneys Matter More Than You Think

Your kidneys are remarkable organs, quietly working around the clock to keep you healthy. These two fist-sized organs filter approximately 180 litres of blood every single day, removing waste products, balancing fluid levels, regulating blood pressure, producing essential hormones, and maintaining the delicate chemical balance your body needs to function.

Chronic kidney disease (CKD) is often called a "silent" condition because it typically causes no symptoms until significant damage has occurred, sometimes not until 30-40% of kidney function is lost.

The numbers tell a sobering story. In the UK, over 7.2 million people live with chronic kidney disease, accounting for more than 10% of the population. Among adults aged 35 and over, approximately 22% have some stage of CKD, with 11% having the more severe stages (3 to 5). The condition costs the NHS an estimated £1.4 billion annually, yet around 44% of people living with CKD remain undiagnosed without screening.

This is why kidney function testing is so important, and why it's included in all our health screening packages. Early detection can halt progression, prevent complications, and keeps your kidneys stable long term

Understanding How Your Kidneys Work

Before exploring kidney function tests, it helps to understand what your kidneys actually do. Each kidney contains approximately one million tiny filtering units called nephrons. Contained with each nephron are tiny blood vessels called glomeruli that act as microscopic sieves, filtering waste and excess fluid from your blood while retaining important proteins and blood cells.

The filtered fluid then passes through a series of tubes (tubules) where your body reabsorbs needed substances like sodium, potassium, and water, while concentrating waste products for excretion as urine. This intricate process happens continuously, maintaining the precise chemical balance your body needs.

When kidneys are healthy, they perform these functions efficiently. But when damaged by disease, the glomeruli can't filter properly, allowing protein to leak into urine while waste products accumulate in blood. The tubules may fail to maintain proper salt balance. Over time, if undiagnosed and untreated, this can lead to fluid retention, electrolyte imbalances, anaemia, bone disease, high blood pressure, and ultimately kidney failure requiring dialysis or transplantation.

The Essential Kidney Function Tests

Several blood and urine tests work together to provide a complete picture of kidney health. Understanding what each test measures—and what the results mean—empowers you to take control of your kidney health.

Serum Creatinine: The Foundation Test

What it measures: Creatinine is a waste product produced by normal muscle metabolism. Your muscles continuously break down creatine phosphate during activity, producing creatinine at a relatively constant rate. Healthy kidneys filter creatinine from blood and excrete it in urine.

Why it matters: When kidney function declines, creatinine builds up in your bloodstream. While creatinine itself isn't harmful, its level serves as an excellent marker of kidney filtration capacity. Rising creatinine levels indicate declining kidney function.

Normal ranges: Creatinine levels vary based on muscle mass, age, sex, and ethnicity. Typical ranges are:

Men: 60-110 µmol/L
Women: 45-90 µmol/L

People with greater muscle mass naturally produce more creatinine and may have higher baseline levels without kidney disease. Conversely, elderly individuals or those with low muscle mass may have lower creatinine levels even with reduced kidney function.

Important considerations: NICE guidelines recommend avoiding meat consumption for 12 hours before testing, as cooking converts creatine in meat to creatinine, potentially affecting results. Recent evidence also suggests avoiding fish before testing.

Estimated Glomerular Filtration Rate (eGFR): The Gold Standard

What it measures: eGFR estimates how much blood your kidneys filter per minute, expressed as millilitres per minute per 1.73 square metres of body surface area (ml/min/1.73m²).

Why it matters: eGFR is the most important measure of kidney function and the standard method for detecting and monitoring CKD. It's calculated using your serum creatinine, age, and sex. In the UK, all laboratories automatically calculate and report eGFR alongside creatinine results.

Normal values: Healthy kidneys should filter more than 90 ml/min/1.73m². However, values of 60 ml/min/1.73m² or more are considered normal if there's no other evidence of kidney disease (such as protein in urine or structural abnormalities).

Age-related decline: eGFR naturally declines with age, even in people without kidney disease. This is why eGFR interpretation must consider age and other clinical factors, not just the number itself.

Understanding your eGFR:

Greater than 90: Normal kidney function (if no other kidney abnormalities)
60-89: Normal or mildly reduced
45-59: Mild to moderately reduced (CKD stage 3a)
30-44: Moderately to severely reduced (CKD stage 3b)
15-29: Severely reduced (CKD stage 4)
Less than 15: Kidney failure (CKD stage 5)

Limitations of eGFR: While extremely useful, eGFR is only an estimate and can be inaccurate in people with unusual muscle mass—including bodybuilders, amputees, severely malnourished individuals, and those who are morbidly obese or oedematous. In these situations, more accurate GFR measurement may be needed using specialised tests.

Urea: Additional Context

What it measures: Urea is another waste product, formed when your body breaks down protein from food and from normal cell turnover. Like creatinine, healthy kidneys filter urea and excrete it in urine.

Why it matters: Urea levels provide additional information about kidney function and can be affected by factors beyond kidney health.

Normal range: Typically 2.5-7.8 mmol/L

What affects urea:

Kidney function: Reduced kidney function causes urea accumulation
Hydration status: Dehydration can elevate urea levels
Protein intake: High protein diet increases urea production
Certain medications: Some drugs affect urea levels

Urea is particularly useful alongside creatinine for assessing kidney function trends and for people on dialysis, where urea levels indicate how effectively treatment removes waste products.

Albumin:Creatinine Ratio (ACR): The Protein Test

What it measures: ACR measures the ratio of albumin (a protein) to creatinine in urine. Healthy kidneys prevent albumin from passing into urine, so its presence sometimes indicates kidney damage, however there are other causes.

Why it matters: ACR is crucial because it can detect kidney disease even when eGFR is still normal. The presence of albumin in urine (albuminuria) indicates damaged glomeruli that are "leaking" protein—an early sign of kidney disease that may precede eGFR decline.

How it's measured: A simple urine sample, ideally the first morning sample, is tested for both albumin and creatinine. The ratio accounts for urine concentration, providing more accurate results than measuring albumin alone.

Normal values and staging:

A1 (normal): Less than 3 mg/mmol
A2 (microalbuminuria): 3-30 mg/mmol
A3 (macroalbuminuria): Greater than 30 mg/mmol

Clinical significance: ACR is both a marker of kidney damage and an independent risk factor for cardiovascular disease. Even mildly elevated ACR can increase the risk of heart attack, stroke, and progression to kidney failure. This is why ACR testing is essential alongside eGFR for complete kidney health assessment.

Electrolytes and Minerals

Comprehensive kidney function assessment includes testing for:

Sodium: Essential for fluid balance and nerve function. Kidney disease can impair sodium regulation.

Potassium: Critical for heart rhythm and muscle function. Damaged kidneys may fail to excrete potassium properly, leading to dangerous accumulation.

Calcium and phosphate: Kidneys regulate these minerals, which are vital for bone health. CKD commonly causes imbalances leading to bone disease.

Bicarbonate: Kidneys help maintain acid-base balance. Low bicarbonate sometimes indicates metabolic acidosis, in advanced CKD, but can also be normal.

Chronic Kidney Disease: Stages and Classification

CKD is classified using a combination of eGFR (G stages) and ACR (A stages), creating a comprehensive picture of kidney health and cardiovascular risk.

GFR Stages (G1-G5)

G1: eGFR ≥90 ml/min/1.73m² with other evidence of kidney damage
G2: eGFR 60-89 ml/min/1.73m² with other evidence of kidney damage
G3a: eGFR 45-59 ml/min/1.73m²
G3b: eGFR 30-44 ml/min/1.73m²
G4: eGFR 15-29 ml/min/1.73m²
G5: eGFR <15 ml/min/1.73m² (kidney failure)

Albuminuria Stages (A1-A3)

A1: ACR <3 mg/mmol (normal to mildly increased)
A2: ACR 3-30 mg/mmol (mildly increased)
A3: ACR >30 mg/mmol (mild or greater increase)

Combined Classification

CKD stage combines both measurements. For example:

Someone with eGFR 50 and ACR 5 has CKD G3aA2
Someone with eGFR 25 and ACR 35 has CKD G4A3

This classification helps predict prognosis and guide management. Those with both low eGFR and high ACR have the highest risk of progression and cardiovascular complications.

Important note: People with eGFR above 60 ml/min/1.73m² should only be classified as having CKD if they have other evidence of kidney disease, such as persistent albuminuria, haematuria (blood in urine), structural abnormalities on imaging, or a history of kidney transplantation.

Who Should Have Kidney Function Testing?

While kidney function tests are valuable for everyone as part of comprehensive health screening, certain groups have higher risk and particularly benefit from regular testing.

High-Risk Groups

People with diabetes: Diabetes is a leading cause of kidney disease. All diabetics should have annual kidney function testing as part of routine care.

Those with high blood pressure: Hypertension both causes and results from kidney disease. Regular monitoring is essential for anyone with elevated blood pressure.

People over 60: CKD prevalence increases with age. Among those aged 75 and over, approximately 48% have some stage of CKD, and 36% have the more severe stages (3-5).

Family history of kidney disease: CKD can run in families, particularly conditions like polycystic kidney disease. Having a relative with advanced CKD or kidney failure increases your risk.

People from South Asian and African-Caribbean backgrounds: These ethnic groups have higher CKD risk, partly related to increased rates of diabetes and hypertension.

Cardiovascular disease: Heart disease and kidney disease are closely linked. Those with heart failure, coronary artery disease, or stroke should have regular kidney function testing.

History of urinary tract problems: Recurrent infections, kidney stones, or bladder outlet obstruction can damage kidneys over time.

People taking certain medications: Regular monitoring is essential for those taking:

Non-steroidal anti-inflammatory drugs (NSAIDs)
Lithium
ACE inhibitors or angiotensin receptor blockers (ARBs)
Certain antibiotics (aminoglycosides)
Chemotherapy drugs
Some immunosuppressants

Obesity: Excess weight increases risk of diabetes, hypertension, and direct kidney damage.

Smokers: Smoking accelerates CKD progression and increases cardiovascular risk.

When to Repeat Testing

If your first test shows eGFR less than 60 ml/min/1.73m², your doctor will normally repeat the test within two weeks to confirm the result. CKD diagnosis requires at least two abnormal results spread over at least 90 days, as transient changes can occur.

The frequency of ongoing monitoring depends on your CKD stage:

Normal or stage 1-2: Annual testing if at risk
Stage 3a: Every 6-12 months
Stage 3b: Every 3-6 months
Stage 4-5: Every 3 months or more frequently

Understanding Your Results

Kidney function tests must be interpreted in context, considering the complete clinical picture rather than isolated numbers.

What "Normal" Really Means

Reference ranges provide general guidance, but individual factors matter enormously. A bodybuilder might have higher creatinine than average without kidney disease, while an elderly person with low muscle mass might have normal creatinine despite reduced kidney function. This is precisely why eGFR calculation includes age and sex adjustments.

Trends Matter More Than Single Values

For any individual, identifying trends in kidney function can be more informative than one-off readings. A gradual decline in eGFR over months to years suggests progressive CKD requiring intervention. Stable eGFR over time, even if slightly below 60, is less concerning and can be kept stable with treatment.

Acute vs. Chronic Changes

A sudden drop in eGFR requires urgent assessment to rule out acute kidney injury, which may be reversible if treated promptly. Causes include severe dehydration, infections, medication effects, or urinary tract obstruction. In contrast, gradual decline over months to years indicates chronic kidney disease.

If eGFR falls rapidly or you develop symptoms suggesting kidney problems; decreased urination, swelling, fatigue, nausea, seek medical attention promptly.

What Happens If Tests Show Kidney Disease?

An abnormal kidney function test isn't the end—it's the beginning of a plan to protect your remaining kidney function and prevent progression.

Initial Assessment

If testing reveals CKD, your doctor will:

Repeat tests to confirm the diagnosis
Check for reversible causes (medications, obstruction, dehydration)
Assess for other evidence of kidney disease (urine tests, imaging)
Evaluate cardiovascular risk factors
Screen for complications (anaemia, bone disease, salt abnormalities)

Management Strategies

CKD management focuses on slowing progression and preventing complications:

Blood pressure control: Aggressive blood pressure management is crucial. Target blood pressure is generally below 140/90 mmHg, and lower for those with significant albuminuria. ACE inhibitors or ARBs are often preferred as they protect kidneys.

Diabetes management: Good glucose control reduces kidney damage progression. Target HbA1c is typically 48-53 mmol/mol (6.5-7%).

Lifestyle modifications:

Healthy diet (potentially including reduced protein intake in advanced CKD)
Regular exercise
Weight management
Smoking cessation
Limiting alcohol
Reducing salt intake

Medication review: Stopping drugs which affect the kidneys where possible and adjusting doses of other medications based on kidney function.

Treating complications: Managing anaemia, bone disease, and salt imbalances that accompany advanced CKD.

Cardiovascular risk reduction: Statins for cholesterol management, as cardiovascular disease is the leading cause of death in CKD patients.

When to See a Specialist

Referral to a nephrologist (kidney specialist) is typically recommended for:

eGFR less than 30 ml/min/1.73m² (stage 4-5 CKD)
Rapidly declining kidney function
ACR greater than 70 mg/mmol
Blood in urine (haematuria) of kidney origin which is unexplained
Suspected rare or genetic kidney disease
Difficult-to-control blood pressure despite multiple medications
Persistent salt abnormalities

Protecting Your Kidneys: Prevention Strategies

While some kidney disease risk factors can't be changed (age, family history, ethnicity), many are modifiable. Taking proactive steps can significantly reduce your risk of developing CKD or slow its progression if already present.

Control Blood Pressure and Blood Sugar

These are the two most important modifiable risk factors. Regular monitoring and treatment optimisation can prevent or slow kidney damage.

Maintain a Healthy Weight

Obesity increases diabetes and hypertension risk while also directly damaging kidneys. Even modest weight loss improves kidney health.

Stay Active

Regular physical activity helps control blood pressure, blood sugar, and weight while improving overall cardiovascular health, all benefiting your kidneys.

Eat a Kidney-Friendly Diet

Limit processed foods high in sodium & sugar
Choose fresh fruits and vegetables
Include whole grains
Limit red meat
Reduce added sugars
Stay well-hydrated (unless advised otherwise)

Avoid Nephrotoxic Substances

Limit or avoid NSAIDs (ibuprofen, naproxen) unless specifically recommended
Avoid herbal supplements without medical advice, some affect the kidneys
Limit alcohol consumption
Never smoke

Regular Screening

Early detection through regular kidney function testing allows for timely intervention when it's most effective.

The Importance of Comprehensive Health Screening

Kidney function testing doesn't exist in isolation. Your kidneys interact closely with other body systems—particularly your cardiovascular system. High blood pressure damages kidneys while kidney disease worsens blood pressure. Diabetes affects kidneys while kidney disease complicates diabetes management.

This is why comprehensive health screening that includes kidney function tests alongside blood pressure, glucose, cholesterol, and other markers provides the most complete picture of your health. Our screening packages include kidney function tests (creatinine, eGFR, and urea) as standard, giving you essential information about this vital organ system.

Understanding your kidney health empowers you to make informed decisions, take preventive action, and work with your healthcare team to optimise treatment if needed. Whether your results show healthy kidneys or early changes requiring attention, knowledge is power when it comes to protecting your long-term health.

Take Action Today

Kidney disease is often silent until advanced stages but it doesn't have to be. Regular screening can detect problems early when interventions are most effective. Don't wait for symptoms to appear. By the time you feel unwell from kidney disease, significant damage has often already occurred.

If you have risk factors for kidney disease, diabetes, high blood pressure, family history, age over 60, cardiovascular disease, or use of kidney affecting medications, regular kidney function testing is essential. Even if you have no known risk factors, baseline testing as part of comprehensive health screening provides invaluable information about your current kidney health and establishes a reference point for future comparison.

All our health screening packages include essential kidney function tests with full review and consultation by experienced physicians. Take control of your kidney health today, your future self will thank you.

References

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National Institute for Health and Care Excellence. (2021). Chronic Kidney Disease: Assessment and Management. NICE Guideline NG203. Retrieved from https://www.nice.org.uk/guidance/ng203
UK Kidney Association. (2024). Measurement of Kidney Function. Retrieved from https://www.ukkidney.org/health-professionals/information-resources/uk-eckd-guide/measurement-kidney-function
UK Kidney Association. (2024). CKD Staging. Retrieved from https://www.ukkidney.org/health-professionals/information-resources/uk-eckd-guide/ckd-staging
Kidney Care UK. (2024). Understanding Your CKD Blood and Urine Test Results. Retrieved from https://kidneycareuk.org/kidney-disease-information/treatments/patient-info-understanding-your-ckd-blood-and-urine-test-results/
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NHS. (2024). Chronic Kidney Disease - Diagnosis. Retrieved from https://www.nhs.uk/conditions/kidney-disease/diagnosis/
Hirst JA, et al. (2020). Prevalence of Chronic Kidney Disease in the Community Using Data from OxRen: A UK Population-Based Cohort Study. British Journal of General Practice, 70(693):e285-e293. doi: 10.3399/bjgp20X708245
North West Kidney Network. (2024). Chronic Kidney Disease (CKD). Retrieved from https://www.nwkidneynetwork.nhs.uk/our-workstreams/chronic-kidney-disease-ckd
Hounkpatin HO, et al. (2020). Prevalence of Chronic Kidney Disease in Adults in England: Comparison of Nationally Representative Cross-Sectional Surveys from 2003 to 2016. BMJ Open, 10(8):e038423. doi: 10.1136/bmjopen-2020-038423
Lab Tests Online UK. (2024). eGFR - Estimated Glomerular Filtration Rate. Retrieved from https://labtestsonline.org.uk/tests/egfr-estimated-creatinine-clearance
Kidney Care UK. (2024). Tests for Chronic Kidney Disease. Retrieved from https://kidneycareuk.org/kidney-disease-information/about-kidney-health/tests-for-chronic-kidney-disease/

Understanding Kidney Function Tests: Your Essential Guide to Protecting Your Kidneys