Kidney Function Tests Explained: eGFR, Creatinine, and Urea

Kidney disease affects more than 3 million people in the UK — and the majority don't know they have it. The kidneys are remarkable organs, capable of maintaining normal function even when significantly compromised. By the time symptoms appear, disease is often already advanced. This is why the U&E (urea and electrolytes) blood test is one of the most important routine checks your GP can do.

What Do the Kidneys Do?

The kidneys are responsible for filtering waste products from your blood, regulating fluid and electrolyte balance, maintaining blood pressure, producing hormones that stimulate red blood cell production (erythropoietin), and activating vitamin D. When they're not functioning well, all of these processes are disrupted — which is why advanced kidney disease has such wide-ranging effects on the body.

The U&E Blood Test — What It Measures

eGFR — Estimated Glomerular Filtration Rate

eGFR is the headline kidney function marker. It estimates how many millilitres of blood your kidneys filter per minute, calculated from your creatinine level along with your age, sex, and ethnicity. A healthy young adult has an eGFR of around 90–120 mL/min/1.73m². eGFR naturally declines slightly with age — an eGFR of 60–89 in an older adult may be normal for their age rather than indicating disease.

Creatinine

Creatinine is a waste product of muscle metabolism that's normally filtered out by the kidneys. When kidney function declines, creatinine accumulates in the blood and the measured level rises. However, creatinine alone is an imperfect marker — it's heavily influenced by muscle mass. A muscular person will have a higher creatinine for the same level of kidney function as someone with less muscle. eGFR corrects for this.

Urea

Urea is a breakdown product of protein metabolism. Like creatinine, it rises when kidney function is impaired. However, urea is also influenced by protein intake (high-protein diets raise urea), hydration status, and the breakdown of blood in the gut (gastrointestinal bleeding raises urea significantly). It's most useful alongside creatinine and eGFR rather than in isolation.

Electrolytes — Sodium, Potassium, Bicarbonate

The kidneys tightly regulate the body's electrolyte balance. Sodium (normal 133–146 mmol/L) helps control fluid balance. Potassium (normal 3.5–5.1 mmol/L) is critical for heart muscle function — both high potassium (hyperkalaemia) and low potassium (hypokalaemia) can cause dangerous cardiac arrhythmias. Bicarbonate reflects the kidneys' ability to manage the body's acid-base balance.

eGFR Ranges and CKD Staging

eGFR (mL/min/1.73m²)	CKD Stage	Clinical Meaning
90 or above	Normal / Stage 1	Normal kidney function. Stage 1 only applies if there's also evidence of kidney damage (e.g. protein in urine).
60–89	Stage 2	Mildly reduced. Often within normal range for age. Warrants monitoring if persistent with other risk factors.
45–59	Stage 3a	Moderately reduced. Annual monitoring; review medications; cardiovascular risk assessment.
30–44	Stage 3b	Moderately-severely reduced. Closer monitoring; nephrology input may be needed.
15–29	Stage 4	Severely reduced. Active nephrology involvement; preparation for renal replacement therapy.
Below 15	Stage 5	Kidney failure. Dialysis or transplantation.

Causes of Reduced Kidney Function

The most common causes of chronic kidney disease in the UK are diabetes (diabetic nephropathy is the leading cause of kidney failure in the Western world) and hypertension (high blood pressure damages the small vessels in the kidneys over years). Other causes include recurrent urinary tract infections, kidney stones, certain autoimmune conditions (such as lupus or vasculitis), and inherited conditions like polycystic kidney disease.

Acute (sudden) rises in creatinine or falls in eGFR can be caused by dehydration, serious infections, heart failure, and — critically — certain medications. NSAIDs (ibuprofen, naproxen) and some contrast dyes used in imaging can cause acute kidney injury (AKI), particularly in people with existing reduced kidney function. If you have CKD and your GP has told you to avoid ibuprofen, this is why.

Proteinuria — The Other Key Marker

Your GP may also order a urine albumin-to-creatinine ratio (ACR) alongside the blood test. Protein in the urine (proteinuria) is an important independent marker of kidney damage — the kidneys normally retain protein, and when their filtering barrier is damaged, protein leaks through. An ACR above 3 mg/mmol is significant. Both eGFR and ACR together determine CKD stage and risk of progression.

What Happens if Your eGFR Is Reduced?

Finding a moderately reduced eGFR is not a crisis, but it does need investigation and monitoring. Your GP will likely: confirm the result with a repeat test, check urine for protein and blood, assess blood pressure, review your medication list for nephrotoxic drugs, check blood sugar if not recently done, and refer to nephrology if eGFR is below 30 or declining rapidly.

Slowing the progression of CKD focuses on the same measures that prevent cardiovascular disease: tight blood pressure control (target below 130/80 with ACE inhibitors or ARBs as preferred agents in people with diabetes or proteinuria), good blood sugar management in diabetes, avoiding NSAIDs, staying well hydrated, not smoking, and maintaining a healthy weight.

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