Kidney Function: What Creatinine and eGFR Reveal

What the Kidneys Do

The kidneys — two bean-shaped organs, one on each side of the spine — continuously filter the blood. Around 200 litres of blood pass through them every day. The kidneys remove waste products, regulate water and electrolyte balance, control blood pressure and produce hormones needed for red blood cell production. Their health directly determines the well-being of the whole body.

What Creatinine Is

Creatinine is a substance produced in muscles from creatine phosphate — a rapid energy source. Muscles generate it continuously, and the kidneys filter it out and excrete it in urine. When the kidneys are working well, little creatinine accumulates in the blood. When filtration is impaired, it builds up.

An important nuance: creatinine levels depend on muscle mass. Large, muscular people naturally have higher creatinine than slight or elderly individuals. The same number can therefore mean different things for different people — and this is where eGFR comes in.

What eGFR Is

eGFR (estimated glomerular filtration rate) shows how many millilitres of blood the kidneys filter per minute (mL/min/1.73 m²). It is calculated from creatinine level, age, sex and sometimes race — giving a more accurate picture of kidney function than creatinine alone.

Doctors generally use the following reference scale:

• eGFR ≥ 90 — normal or increased kidney function
• eGFR 60–89 — mildly reduced, but often a normal variant
• eGFR < 60 persisting for ≥ 3 months — chronic kidney disease
• eGFR < 15 — kidney failure; dialysis may be needed

Why Early Detection Matters

Chronic kidney disease is insidious: early stages typically produce no symptoms at all. The kidneys have enormous compensatory capacity — a person can lose up to 50% of function and still feel perfectly well (Chronic Kidney Disease — StatPearls, 2023). Symptoms such as swelling, fatigue and changes in urination usually appear only in later stages.

This is why regular blood testing is especially important for those with risk factors: type 2 diabetes, high blood pressure, a family history of kidney disease, older age, or frequent use of pain-relieving medications.

Factors That Can Affect Creatinine

Before interpreting a result, a few important circumstances are worth knowing:

Diet: A large serving of meat (especially cooked) shortly before the test can temporarily raise creatinine. Creatine monohydrate supplements, popular among athletes, also elevate creatinine.

Hydration: Dehydration concentrates the blood and can artificially inflate creatinine readings.

Medications: Certain drugs (e.g., ibuprofen, naproxen, some antibiotics) can affect kidney filtration. Always inform your doctor about all medications and supplements you take.

Muscle mass: As noted — athletes and very muscular people naturally have higher creatinine.

Lifestyle and Kidney Health

Hydration is one of the simplest ways to protect the kidneys. Adequate fluid intake helps them filter waste more effectively. There is no universal amount, but urine should be a pale yellow colour.

Diet: Very high protein intake (particularly from animal sources) can increase the filtration burden in people who already have kidney problems. A healthy person with normal kidney function does not need to restrict protein.

Blood pressure: Uncontrolled high blood pressure is one of the leading causes of chronic kidney disease — regular monitoring matters here too.

Over-the-counter medications: NSAIDs (ibuprofen, diclofenac and similar drugs) are harmless when used occasionally by healthy people, but regular use or high doses can damage the kidneys — especially if function is already reduced.

When to See a Doctor

If creatinine is elevated or eGFR is reduced, a doctor will typically repeat the test after a few weeks — a single measurement can be transient. A urine test is usually added as well (protein in the urine is an additional sign of kidney damage). The trend — the direction of measurements over time — matters more than any single data point.