Kidney Function Tests: BUN, Creatinine, eGFR Explained

The kidneys filter approximately 180 liters of blood daily, removing waste products and maintaining fluid/electrolyte balance. Kidney function tests assess how well this filtration system is working:

• Creatinine: A waste product from normal muscle metabolism, filtered entirely by the kidneys. Normal: 0.7-1.3 mg/dL for men, 0.6-1.1 mg/dL for women. Higher levels indicate reduced kidney filtration. However, creatinine is also influenced by muscle mass — a muscular person may have higher creatinine without kidney disease.
• BUN (Blood Urea Nitrogen): A waste product from protein metabolism. Normal: 7-20 mg/dL. BUN is less specific than creatinine because it is also affected by protein intake, dehydration, liver function, and GI bleeding.
• eGFR (Estimated Glomerular Filtration Rate): Calculated from creatinine, age, sex, and (in some equations) race. This is the most clinically useful measure of kidney function.

eGFR staging of kidney function:

• Stage 1: eGFR 90+ (normal function, but other signs of kidney damage present)
• Stage 2: eGFR 60-89 (mildly decreased)
• Stage 3a: eGFR 45-59 (mild-to-moderate decrease)
• Stage 3b: eGFR 30-44 (moderate-to-severe decrease)
• Stage 4: eGFR 15-29 (severely decreased)
• Stage 5: eGFR below 15 (kidney failure)

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Elevated creatinine or reduced eGFR can result from various causes:

Acute causes (reversible):

• Dehydration (most common cause of mildly elevated creatinine)
• NSAID use (ibuprofen, naproxen reduce renal blood flow)
• Contrast dye from imaging procedures
• Urinary tract obstruction (kidney stones, enlarged prostate)
• Acute infection or illness

Chronic causes:

• Diabetes (the leading cause of chronic kidney disease, accounting for 44% of cases)
• Hypertension (the second leading cause)
• Glomerulonephritis (autoimmune kidney inflammation)
• Polycystic kidney disease (genetic)
• Long-term NSAID or PPI use

An elevated BUN:creatinine ratio (above 20:1) suggests pre-renal causes like dehydration, GI bleeding, or high protein intake. A ratio below 10:1 may indicate liver disease or malnutrition.

Important: a single abnormal result does not diagnose kidney disease. Chronic kidney disease requires demonstrated abnormality lasting 3+ months. Always retest before drawing conclusions.

Evidence-based kidney protection strategies are especially important for those with diabetes, hypertension, or existing CKD:

• Blood pressure control: The most important modifiable factor. Target below 130/80 mmHg for kidney protection. ACE inhibitors and ARBs have specific kidney-protective effects beyond blood pressure lowering.
• Blood sugar control: HbA1c below 7% significantly reduces diabetic nephropathy progression.
• Hydration: Adequate water intake (2-3 liters daily for most adults) supports kidney function. Chronic dehydration concentrates toxins and increases kidney workload.
• Limit NSAIDs: Regular NSAID use reduces kidney blood flow. Use acetaminophen as a first-line pain reliever when possible.
• Moderate protein intake: For established CKD (stage 3+), protein restriction to 0.6-0.8g/kg daily may slow progression. For healthy kidneys, higher protein intake is not harmful.
• Annual screening: If you have diabetes, hypertension, or family history of kidney disease, annual creatinine/eGFR and urine albumin-to-creatinine ratio (UACR) screening is recommended. UACR above 30 mg/g is an early sign of kidney damage, often appearing before eGFR changes.