Nephrology

Kidney Function Tests: Understanding Creatinine, eGFR & More

Your kidneys filter approximately 180 litres of blood every day, producing about 1.5 litres of urine. These bean-shaped organs are among the hardest-working in your body — and blood tests are the primary way to monitor their health.

What Your Kidneys Do

Far more than simple filters, your kidneys perform at least four critical functions that keep your entire body in balance.

Filter Waste Products

Remove creatinine, urea, and other metabolic waste from the blood. Each kidney contains roughly 1 million nephrons — microscopic filtering units.

Regulate Fluid & Blood Pressure

Control the volume of fluid in your body and release renin to regulate blood pressure. This is why kidney disease and hypertension are so closely linked.

Produce Hormones

Secrete erythropoietin (EPO) to stimulate red blood cell production and activate vitamin D for calcium absorption and bone health.

Maintain Electrolyte Balance

Precisely regulate sodium, potassium, calcium, phosphate, and bicarbonate levels — essential for nerve signalling, muscle contraction, and heart rhythm.

Key Kidney Markers

These are the blood markers your doctor uses to assess kidney function. Each tells a different part of the story.

Creatinine

A waste product from normal muscle metabolism. Your muscles produce creatinine at a relatively constant rate, and healthy kidneys filter it out efficiently. When kidney function declines, creatinine accumulates in the blood.

Reference Range	Men	Women
Typical	60–110 µmol/L	45–90 µmol/L

Higher in muscular individuals and those who eat a lot of red meat. Lower in elderly or frail patients. Creatine supplements will raise levels independently of kidney function.

eGFR (Estimated Glomerular Filtration Rate)

The gold standard measure of kidney function. It estimates how many millilitres of blood your kidneys filter per minute, calculated from serum creatinine, age, sex, and ethnicity using the CKD-EPI equation.

Reference Range	Men	Women
Typical	>90 mL/min/1.73m²	>90 mL/min/1.73m²

Less accurate at extremes of body size (very muscular or very frail patients). Declines naturally with age — an eGFR of 60–89 in an 80-year-old may be normal ageing, not disease.

BUN / Urea

Blood Urea Nitrogen (BUN) measures urea, a waste product from protein digestion. The BUN:Creatinine ratio helps distinguish pre-renal causes (dehydration, heart failure) from intrinsic kidney disease.

Reference Range	Men	Women
Typical	2.5–7.8 mmol/L	2.5–7.8 mmol/L

Rises after high-protein meals, with dehydration, GI bleeding, or catabolic states. A BUN:Creatinine ratio >20:1 suggests a pre-renal cause (e.g., dehydration) rather than intrinsic kidney damage.

Cystatin C

A newer and increasingly preferred kidney marker. Produced by all nucleated cells at a constant rate, cystatin C is not affected by muscle mass, diet, or sex — making it more accurate than creatinine alone.

Reference Range	Men	Women
Typical	0.56–0.98 mg/L	0.56–0.98 mg/L

Particularly useful in elderly patients, amputees, vegetarians, and bodybuilders where creatinine-based eGFR may be misleading. Can be combined with creatinine for a more accurate eGFR calculation (CKD-EPI cystatin C equation).

Electrolytes (Na, K, Cl, HCO₃)

The kidneys are the primary regulators of electrolyte balance. Sodium controls fluid volume and blood pressure. Potassium is critical for heart rhythm. Chloride maintains acid-base balance alongside bicarbonate.

Reference Range	Men	Women
Typical	Na 135–145, K 3.5–5.0 mmol/L	Na 135–145, K 3.5–5.0 mmol/L

Potassium rises in advanced CKD (dangerous above 6.0 mmol/L — cardiac risk). Low bicarbonate indicates metabolic acidosis, common in Stage 4–5 CKD. Sodium disorders often reflect fluid balance, not salt intake.

Uric Acid

A breakdown product of purines (from DNA and certain foods). Approximately 70% of uric acid is excreted by the kidneys. Elevated levels cause gout and are associated with kidney stone formation and CKD progression.

Reference Range	Men	Women
Typical	0.20–0.43 mmol/L	0.15–0.36 mmol/L

Higher in men and post-menopausal women. Rises with red meat, organ meats, shellfish, and alcohol (especially beer). Diuretics and low-dose aspirin also increase uric acid levels.

CKD Staging: How Kidney Disease Is Classified

Chronic Kidney Disease is classified into five stages based on eGFR. Early stages are often silent — most people with Stage 1–3a have no symptoms at all. This is why routine blood testing is so important.

Approximately 10% of the global population has some form of CKD, and millions are undiagnosed.

Stage 1

eGFR ≥90

Normal or High

Kidney damage with normal or increased filtration. Often no symptoms.

Affects ~3.3% of the adult population

Recommended Actions

Monitor annually, manage risk factors (diabetes, blood pressure), healthy lifestyle.

Stage 2

eGFR 60–89

Mildly Decreased

Mild reduction in kidney function. Usually discovered incidentally on blood tests.

Affects ~3.0% of the adult population

Recommended Actions

Monitor every 6–12 months, control blood pressure, review medications.

Stage 3a

eGFR 45–59

Mild to Moderate

Moderate reduction. Waste products may start building up. Fatigue possible.

Affects ~4.3% of the adult population

Recommended Actions

Nephrology referral, dietary changes, manage phosphate and calcium, monitor every 3–6 months.

Stage 3b

eGFR 30–44

Moderate to Severe

Significant function loss. Anaemia, bone disease, and fluid retention may develop.

Affects ~1.8% of the adult population

Recommended Actions

Active nephrology management, erythropoietin if anaemic, dietary protein restriction, avoid nephrotoxins.

Stage 4

eGFR 15–29

Severely Decreased

Severe reduction. Symptoms become more pronounced: nausea, swelling, fatigue, itching.

Affects ~0.4% of the adult population

Recommended Actions

Prepare for renal replacement therapy (dialysis or transplant), vascular access planning, dietitian review.

Stage 5

eGFR <15

Kidney Failure

Kidneys can no longer sustain life without intervention. Dialysis or transplant needed.

Affects ~0.1% of the adult population

Recommended Actions

Dialysis (haemodialysis or peritoneal) or kidney transplant. Palliative care if appropriate.

Important: A single eGFR reading does not diagnose CKD. The decline must be sustained for at least 3 months with evidence of kidney damage (protein in urine, structural abnormalities, or biopsy findings) to confirm a diagnosis.

Risk Factors for Kidney Disease

Knowing your risk profile is the first step in prevention. The more risk factors you have, the more important regular kidney function testing becomes.

Diabetes

Major cause

The leading cause of CKD, responsible for approximately 40% of kidney failure cases worldwide. High blood sugar damages the glomerular capillaries over time.

High Blood Pressure

Major cause

The second leading cause. Sustained hypertension damages the small blood vessels in the kidneys, reducing their filtering capacity. Target <130/80 mmHg for kidney protection.

Family History

High risk

First-degree relatives of CKD patients have a 2–3x increased risk. Polycystic kidney disease (PKD) is directly inherited and affects 1 in 1,000 people.

Age Over 60

High risk

Kidney function naturally declines with age. After 40, eGFR drops approximately 1 mL/min/year. By age 70, many people have an eGFR of 60–75 without overt disease.

Obesity

Moderate risk

BMI >30 independently increases CKD risk by 2–7x. Excess weight forces kidneys to work harder (hyperfiltration), accelerating damage. Weight loss can slow progression.

Smoking

Moderate risk

Accelerates kidney function decline by 2–3x. Damages blood vessels, raises blood pressure, and reduces blood flow to the kidneys. Quitting slows progression.

NSAID Overuse

Moderate risk

Regular use of ibuprofen, naproxen, or diclofenac reduces blood flow to the kidneys. Chronic use can cause interstitial nephritis or papillary necrosis.

What Affects Your Results

Kidney markers are influenced by many non-kidney factors. Understanding these helps you avoid unnecessary worry and ensures accurate interpretation.

Marker	Can Be Falsely High When	Can Be Falsely Low When	Best Practice
Creatinine	High muscle mass, recent intense exercise, creatine supplements, high red meat intake, certain medications (e.g., trimethoprim)	Low muscle mass, elderly/frail, liver disease, vegetarian diet, amputees	Avoid intense exercise 24h before test; note supplements
eGFR	Low creatinine (frail patients) — eGFR appears falsely reassuring	High creatinine (muscular patients) — eGFR appears falsely alarming	Consider cystatin C-based eGFR for patients at body composition extremes
Urea / BUN	High-protein meal beforehand, dehydration, GI bleeding, catabolic states, corticosteroids	Low-protein diet, liver disease, overhydration, pregnancy	Fast for 8–12 hours; stay normally hydrated
Uric Acid	Alcohol (especially beer), red meat, organ meats, seafood, diuretics, fasting	Low-purine diet, allopurinol, high-dose vitamin C	Avoid alcohol and purine-rich foods 48h before test

Time of day matters: Creatinine levels are most stable in the morning. For the most consistent trending over time, try to have your blood drawn at a similar time of day for each test.

Protecting Your Kidneys

Most kidney damage is preventable or can be slowed significantly with early intervention. These evidence-based strategies protect kidney function at every stage.

Stay Well Hydrated

Aim for 1.5–2 litres of water daily (more in hot weather or with exercise). Consistent hydration helps kidneys flush waste efficiently. Avoid chronic dehydration, but excessive water intake is unnecessary.

Control Blood Pressure

Target <130/80 mmHg if you have CKD or diabetes. ACE inhibitors and ARBs are preferred because they protect kidney function beyond just lowering blood pressure.

Manage Blood Sugar

Keep HbA1c below 7% (53 mmol/mol) if diabetic. SGLT2 inhibitors (e.g., dapagliflozin, empagliflozin) now have strong evidence for kidney protection even in non-diabetic CKD.

Limit NSAIDs

Avoid regular use of ibuprofen, naproxen, and diclofenac. Use paracetamol (acetaminophen) as a safer alternative for pain relief. If you need NSAIDs, use the lowest dose for the shortest time.

Reduce Sodium Intake

Aim for <2,000 mg sodium per day. High sodium raises blood pressure and increases proteinuria (protein in urine), both of which accelerate kidney damage. Read food labels carefully.

Do Not Smoke

Smoking accelerates kidney decline by 2–3 times. Quitting at any stage slows progression. Smoking also interferes with blood pressure medications, reducing their effectiveness.

Track Your Kidney Function Over Time

Upload your blood test results and SmarterBlood will track your creatinine, eGFR, and other kidney markers over time — helping you and your doctor spot trends early, when intervention matters most.

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Medical Disclaimer: This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories. Always discuss your results with a qualified healthcare professional. If you have concerns about your kidney function, consult your doctor or a nephrologist.