Normal Values for GFR and BUN:Creatinine Ratio
Normal GFR in healthy young adults is approximately 100-130 mL/min/1.73 m² for men and 90-120 mL/min/1.73 m² for women, with values remaining stable until age 40 before physiologically declining, while a normal BUN:creatinine ratio is typically less than 20. 1
Normal GFR Values
Age and Sex-Specific Ranges
Young adults (under 40 years): GFR values of approximately 100-130 mL/min/1.73 m² in men and 90-120 mL/min/1.73 m² in women represent normal kidney function 1
Mean GFR in healthy individuals: Approximately 107 mL/min/1.73 m² up to age 40, after which physiological decline begins 2
No significant sex difference: Meta-analysis of healthy potential kidney donors found no evidence for differences between men and women in separate age groups 2
Clinical Thresholds for Interpretation
GFR ≥90 mL/min/1.73 m²: Considered optimal kidney function and is the recommended threshold for living kidney donors 1
GFR 60-89 mL/min/1.73 m²: Represents mildly decreased kidney function but may be normal for older adults, particularly those over 70 years 1, 3
GFR <60 mL/min/1.73 m²: Represents loss of half or more of normal adult kidney function and is the threshold for defining chronic kidney disease (CKD) 4, 1
Reference interval from healthy volunteers: Studies show eGFR values >63.5 mL/min/1.73 m² in healthy adults, though these overlap with CKD stages 1 and 2 5
Age-Related Decline
Physiological decline: GFR remains constant until approximately age 40 years, then begins to decline in both men and women 1, 2
Elderly considerations: Approximately 17% of persons older than 60 years have an estimated GFR <60 mL/min/1.73 m², and values below 60 mL/min/1.73 m² could be considered normal in adults older than 70 years 1, 3
Normal BUN:Creatinine Ratio
Standard Reference Range
Normal BUN:creatinine ratio: Less than 20 is considered normal 6
Elevated ratio (≥20): Indicates various physiological conditions including prerenal azotemia, dehydration, high protein intake, gastrointestinal bleeding, or catabolic states 6
Clinical Significance for GFR Estimation
Impact on GFR estimates: A BUN:creatinine ratio of 20 or greater causes misestimation of CKD stage when using common eGFR equations (MDRD and CKD-EPI) 6
Concordance issues: More disagreement in CKD stage diagnosis occurs in patients with high BUN:creatinine ratio (weighted κ = 0.600 and 0.541) compared to low ratio (weighted κ = 0.816 and 0.758) 6
Clinical interpretation: GFR estimates for patients with BUN:creatinine ratio ≥20 should be interpreted cautiously, as the ratio suggests non-renal factors affecting measurements 6
Critical Clinical Caveats
When GFR Values May Be Misleading
Presence of other kidney damage markers: GFR >60 mL/min/1.73 m² does not exclude kidney disease if albuminuria or structural abnormalities on imaging are present 1
Body composition extremes: Extremes of muscle mass or weight can affect eGFR accuracy 1
Non-steady state conditions: Acute illness or recent surgery may produce inaccurate estimates 1
Hydration status: Significantly impacts measurements and should be considered when interpreting values 1
Assessment Requirements
Never use creatinine alone: Serum creatinine should not be used to assess kidney function; always calculate GFR 1
Persistent abnormalities required: CKD diagnosis requires persistent abnormalities for ≥3 months and should not be based solely on a single GFR measurement 1
Albuminuria assessment: Normal urinary albumin-to-creatinine ratio (UACR) is defined as <30 mg/g creatinine 4
Combined evaluation: Both GFR and albuminuria should be assessed together, as an eGFR persistently <60 mL/min/1.73 m² in concert with urine albumin >30 mg/g creatinine is considered abnormal 4