Kidney Function Tests: Understanding Creatinine, eGFR, and BUN

The Silent Organ Failure

Chronic kidney disease (CKD) is one of medicine's most dramatic examples of the gap between symptom onset and organ damage. The kidneys have extraordinary functional reserve — symptoms of kidney failure (fatigue, fluid retention, nausea, cognitive changes) typically do not appear until kidney function has fallen to approximately 15–30% of normal capacity.

This means standard clinical thresholds frequently catch CKD at a stage where significant scarring has already occurred. Early detection through routine testing — before symptoms appear — is where intervention is most effective and where progression can most reliably be slowed.

Creatinine

Creatinine is a waste product of muscle metabolism, produced at a relatively constant rate proportional to muscle mass and filtered out by the kidneys. As kidney filtration declines, creatinine accumulates in blood.

Limitations: Creatinine is heavily influenced by muscle mass. A muscular athlete and a frail elderly person can have identical creatinine levels with dramatically different kidney function. Creatinine also only rises meaningfully after 50% of filtration capacity is already lost.

Standard range: 0.6–1.2 mg/dL (men); 0.5–1.1 mg/dL (women)

eGFR (Estimated Glomerular Filtration Rate)

eGFR is calculated from serum creatinine along with age, sex, and race (the CKD-EPI equation). It estimates how many milliliters of blood your kidneys filter per minute — a direct measure of functional kidney capacity.

CKD Staging by eGFR

| Stage | eGFR (mL/min/1.73m²) | Description | |---|---|---| | G1 | 90 or above | Normal or high (with other evidence of kidney damage) | | G2 | 60–89 | Mildly decreased | | G3a | 45–59 | Mildly to moderately decreased | | G3b | 30–44 | Moderately to severely decreased | | G4 | 15–29 | Severely decreased | | G5 | Below 15 | Kidney failure |

An eGFR above 90 with no other abnormalities is normal. An eGFR declining progressively over serial measurements — even within the normal range — is clinically significant and warrants investigation.

UACR (Urine Albumin-to-Creatinine Ratio)

The UACR is arguably more sensitive than eGFR for early kidney damage detection. It measures the amount of albumin (a protein that healthy kidneys retain) leaking into the urine.

Interpretation:

• Below 30 mg/g: Normal
• 30–300 mg/g: Moderately increased (microalbuminuria) — early kidney stress
• Above 300 mg/g: Severely increased (macroalbuminuria) — established kidney disease

Microalbuminuria is not just a kidney marker — it reflects vascular endothelial damage throughout the body and is a powerful predictor of cardiovascular disease even in people without diabetes.

Blood Urea Nitrogen (BUN)

BUN measures urea, another nitrogenous waste product. BUN:Creatinine ratio provides diagnostic context:

• Ratio above 20: Suggests dehydration, high protein intake, or reduced kidney perfusion
• Ratio below 10: Suggests liver disease or low protein intake
• Normal ratio (10–20): Standard kidney function pattern

What Damages Kidneys Most

1. Chronic hypertension — the leading cause of CKD after diabetes
2. Type 2 diabetes and insulin resistance — glucose damages glomerular capillaries
3. Chronic NSAID use — ibuprofen and naproxen reduce prostaglandin-mediated renal blood flow
4. Dehydration — particularly during exercise or illness with inadequate fluid replacement
5. Contrast agents — used in CT scans can cause contrast-induced nephropathy in susceptible individuals