Is the Glomerular Filtration Rate (GFR) normal?

Understanding Normal Glomerular Filtration Rate (GFR)

Normal GFR values are generally above 60 mL/min/1.73 m² in adults under 70 years of age, with values below this threshold potentially indicating chronic kidney disease (CKD). 1

Definition of Normal GFR

• GFR is the best overall index of kidney function and is routinely used to diagnose CKD, monitor disease progression, and calculate appropriate drug dosages 1
• Normal GFR in young adults is approximately 120-130 mL/min/1.73 m² and physiologically declines with age 1
• GFR below 60 mL/min/1.73 m² represents loss of half or more of the adult level of normal kidney function 1

Classification of GFR Values

According to the Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines, GFR values are classified as follows:

• CKD Stage 1: GFR ≥90 mL/min/1.73 m² with evidence of kidney damage 1
• CKD Stage 2: GFR 60-89 mL/min/1.73 m² with evidence of kidney damage 1
• CKD Stage 3a: GFR 45-59 mL/min/1.73 m² 1
• CKD Stage 3b: GFR 30-44 mL/min/1.73 m² 1
• CKD Stage 4: GFR 15-29 mL/min/1.73 m² 1
• CKD Stage 5: GFR <15 mL/min/1.73 m² or on dialysis 1

Age Considerations

• In adults older than 70 years, GFR values below 60 mL/min/1.73 m² may be considered normal due to age-related decline in kidney function 2
• The age-related decline in GFR has traditionally been considered part of normal aging, but decreased GFR in the elderly is still an independent predictor of adverse outcomes such as death and cardiovascular disease 1

Methods of GFR Assessment

Estimated GFR (eGFR)

• Clinical laboratories should report an estimate of GFR using prediction equations in addition to reporting serum creatinine measurement 1
• The Modification of Diet in Renal Disease (MDRD) study and Cockcroft-Gault equations provide useful estimates of GFR in adults 1
• eGFR equations take into account serum creatinine concentration and variables such as age, sex, race, and body size 1
• Clinicians should not use serum creatinine concentration as the sole means to assess kidney function 1

Measured GFR (mGFR)

• In certain circumstances, direct measurement of GFR using clearance methods may be necessary 1
• Indications for measured GFR include: extremes of age and body size, severe malnutrition or obesity, diseases of skeletal muscle, paraplegia or quadriplegia, vegetarian diet, rapidly changing kidney function, and calculation of potentially toxic drug doses 1
• Iohexol plasma clearance is one accurate method for measuring GFR 3

Clinical Implications

• A GFR below 60 mL/min/1.73 m² for 3 or more months, with or without kidney damage, is diagnostic of CKD 1
• Decreased GFR is associated with complications including hypertension, anemia, malnutrition, bone disease, neuropathy, and decreased quality of life 1
• Early detection and treatment of CKD can prevent or delay progression to kidney failure 1

Common Pitfalls in GFR Interpretation

• Serum creatinine alone is an inadequate measure of kidney function as it is affected by factors other than GFR, such as muscle mass, diet, and medications 1, 4
• Minor elevations in serum creatinine can be consistent with substantial reduction in GFR 1
• In elderly patients, serum creatinine may not reflect age-related decline in GFR due to concomitant decline in muscle mass 1
• eGFR equations have limitations and may not be accurate in all populations, particularly at higher GFR levels 5

In summary, normal GFR is generally above 60 mL/min/1.73 m², but interpretation should consider age and other factors that might affect kidney function assessment.