Normal Values for Renal Function Tests (RFT)
Glomerular Filtration Rate (GFR)
Normal GFR in young adults is approximately 120-130 mL/min per 1.73 m², with values declining by approximately 1% per year after age 40 years. 1, 2
- GFR ≥90 mL/min per 1.73 m² is considered normal kidney function 1
- GFR 60-89 mL/min per 1.73 m² represents mildly decreased kidney function (Stage 2 CKD if kidney damage is present) 1
- GFR <60 mL/min per 1.73 m² represents loss of half or more of normal adult kidney function and is associated with increased risk for complications 1
Critical Clinical Pitfall: Never Use Serum Creatinine Alone
Serum creatinine concentration should never be used as the sole means to assess kidney function, as GFR must decline to approximately half the normal level before serum creatinine rises above the upper limit of normal. 1
- Among patients with normal serum creatinine measurements, one in five had asymptomatic renal insufficiency when assessed by creatinine clearance methods 3
- When serum creatinine significantly increases, GFR has already decreased by at least 40% 3
- A serum creatinine of 1.2 mg/dL can represent a creatinine clearance of 110 mL/min in a young adult but only 40 mL/min in an elderly patient 3
Serum Creatinine
The upper limit of normal for serum creatinine is approximately 1.4 mg/dL (124 μmol/L), but this value is profoundly affected by age, sex, muscle mass, and race. 1, 3
- Normal serum creatinine in young adult men and women is approximately 0.7-1.3 mg/dL, but this fails to increase with age despite declining GFR due to age-related muscle mass loss 2
- In elderly patients, serum creatinine does not reflect the age-related decline in GFR because of concomitant decline in muscle mass that reduces creatinine generation 1
- Individuals with higher muscle mass naturally produce more creatinine, elevating serum levels independent of kidney function 4
Albuminuria/Proteinuria
Normal albumin-to-creatinine ratio (ACR) in young adults is approximately 10 mg/g, with ACR ≥30 mg/g considered abnormal. 1
- ACR <30 mg/g (category A1): Normal to mildly increased 1
- ACR 30-300 mg/g (category A2): Moderately increased (formerly "microalbuminuria") 1
- ACR >300 mg/g (category A3): Severely increased 1
- Sex-specific cut points: ACR >17 mg/g in men and >25 mg/g in women may be considered abnormal 1
Recommended Assessment Method
Clinical laboratories should report estimated GFR (eGFR) using the CKD-EPI equation in addition to serum creatinine, as this provides the most accurate assessment of kidney function in adults. 1
- The CKD-EPI equation has less bias than the MDRD equation, especially at GFR ≥60 mL/min per 1.73 m², with improved precision and greater accuracy 1
- For medication dosing decisions, use the Cockcroft-Gault formula to estimate creatinine clearance, as most drug dosing studies have historically used this formula 3
- In adults with eGFR 45-59 mL/min per 1.73 m² without other markers of kidney damage, consider measuring cystatin C to confirm CKD 1
Special Population Considerations
In elderly patients (>70 years), the BIS1 equation may provide more accurate eGFR estimation than CKD-EPI or MDRD when only serum creatinine is available. 5
- Standard eGFR equations systematically underestimate renal function in the oldest patients 3
- In patients with exceptionally high muscle mass (athletes, bodybuilders), standard equations underestimate true GFR 4
- For obese patients, use the mean value between actual and ideal body weight in the Cockcroft-Gault formula 3