Creatinine and eGFR: What Your Kidney Function Test Results Mean

What Is Creatinine?

Creatinine is a waste product produced by your muscles during normal metabolism. Your kidneys filter creatinine out of your blood and excrete it in urine. Because creatinine is produced at a roughly constant rate by muscle tissue, its level in the blood is a reliable indicator of how well the kidneys are functioning. When kidneys are impaired, creatinine builds up in the blood.

US reference ranges: 0.74–1.35 mg/dL for adult men and 0.59–1.04 mg/dL for adult women (LabCorp / Quest). Women have slightly lower values because they typically have less muscle mass. Athletes and people with significant muscle mass can have naturally higher creatinine without any kidney impairment.

What Is eGFR?

eGFR stands for estimated Glomerular Filtration Rate — it estimates how many millilitres of blood your kidneys filter per minute, adjusted for body surface area. It's calculated automatically from your creatinine value, age, and sex. eGFR is considered a more useful clinical measure than creatinine alone because it accounts for these individual differences.

eGFR (mL/min/1.73m²)	Kidney Function Stage	Interpretation
90 or above	Normal / Stage 1	Optimal kidney function
60–89	Mild decrease / Stage 2	Usually normal for older adults; monitor
45–59	Mild–moderate decrease / Stage 3a	Warrants monitoring and lifestyle attention
30–44	Moderate–severe decrease / Stage 3b	Nephrology referral often recommended
15–29	Severe decrease / Stage 4	Close medical management required
Below 15	Kidney failure / Stage 5	Dialysis or transplant consideration

What Affects Creatinine and eGFR?

Muscle mass: Because creatinine is a muscle by-product, people with more muscle (athletes, bodybuilders) naturally have higher creatinine and lower eGFR — which may flag as "low eGFR" without any kidney disease. Always interpret these values in context.

Diet: Eating a large amount of cooked red meat shortly before a blood test can temporarily raise creatinine. This is one reason fasting is recommended for comprehensive metabolic panels.

Hydration: Being dehydrated concentrates the blood, which can artificially elevate creatinine. Ensure you are well-hydrated before your blood draw.

Age: eGFR naturally declines with age — by about 1 mL/min/1.73m² per year after 40. An eGFR of 68 in a 75-year-old is very different from the same value in a 35-year-old.

Medications: Some medications including NSAIDs (ibuprofen, naproxen), certain antibiotics, and some blood pressure drugs can affect kidney function and creatinine levels.

BUN: The Other Kidney Marker

Your report also includes BUN (Blood Urea Nitrogen) — another waste product filtered by the kidneys. BUN alone is less specific than creatinine, but the BUN-to-creatinine ratio can help identify the source of kidney stress. A high BUN with normal creatinine often points to dehydration or high protein intake rather than true kidney impairment. The US normal range for BUN is 7–20 mg/dL.

When Should You Be Concerned?

A single slightly elevated creatinine or marginally low eGFR on one test is rarely cause for alarm, especially if you were dehydrated or had a high-protein meal. What matters far more is trend: if your eGFR is declining test-over-test, that's meaningful even if each individual value is within range.

Seek medical attention promptly if your eGFR is below 60 on a confirmed repeat test, if creatinine has doubled from your previous baseline, or if you have symptoms like persistent swelling in ankles/feet, reduced urine output, or foamy urine.

Protecting Kidney Function Long-Term

The most impactful things you can do for kidney health are controlling blood pressure (hypertension is the second leading cause of kidney disease in the US after diabetes), managing blood sugar, staying well-hydrated, avoiding regular NSAID use, and not smoking. Kidney damage is largely silent until significant function is lost — which is exactly why annual blood tests matter.