Understanding Glomerular Filtration Rate (GFR) in Chronic Kidney Disease
The GFR lab test determines how well your kidneys are filtering waste products from your blood, which directly measures your kidney function. 1
What GFR Measures
- GFR is the best overall measure of kidney function in both health and disease, representing the specific filtering capacity of the kidneys 1, 2
- It quantifies how efficiently your kidneys filter creatinine and other waste products from your blood 1
- Normal GFR in young adults is approximately 120-130 mL/min/1.73 m², which naturally declines with age 1, 2
- GFR below 60 mL/min/1.73 m² represents loss of half or more of normal adult kidney function 1, 2
How GFR Is Measured
- The initial test for GFR evaluation is typically creatinine-based estimated GFR (eGFRcr), which uses serum creatinine levels and prediction equations 1
- These equations take into account serum creatinine concentration plus variables such as age, sex, race, and body size 1
- When more accurate assessment is needed, cystatin C measurements can be added (eGFRcr-cys) 1
- For highest accuracy, GFR can be directly measured using plasma or urinary clearance of exogenous filtration markers 1, 3
Clinical Significance of GFR
- GFR is used to diagnose and stage chronic kidney disease (CKD) 1, 2
- CKD Stage 1: GFR ≥90 mL/min/1.73 m² with evidence of kidney damage
- CKD Stage 2: GFR 60-89 mL/min/1.73 m² with evidence of kidney damage
- CKD Stage 3a: GFR 45-59 mL/min/1.73 m²
- CKD Stage 3b: GFR 30-44 mL/min/1.73 m²
- CKD Stage 4: GFR 15-29 mL/min/1.73 m²
- CKD Stage 5: GFR <15 mL/min/1.73 m² or on dialysis
- GFR helps guide treatment decisions and medication dosing 1
- Declining GFR correlates with increasing complications of CKD 1, 2
Important Considerations
- Serum creatinine alone is an inadequate measure of kidney function as it's affected by factors other than GFR, such as muscle mass, diet, and medications 1, 2
- Some patients maintain seemingly normal creatinine levels despite declining GFRs 1, 4
- In elderly patients, serum creatinine may not reflect age-related decline in GFR due to concomitant decline in muscle mass 2, 5
- Certain conditions may require more accurate GFR assessment methods:
Clinical Application
- GFR should be monitored regularly in patients with CKD to track disease progression 1, 2
- A GFR below 60 mL/min/1.73 m² for 3 or more months, with or without kidney damage, is diagnostic of CKD 1, 2
- Early detection of decreased GFR allows for timely interventions that may slow CKD progression and reduce complications 1, 2
Remember that while GFR is crucial for assessing kidney function, it should be interpreted alongside other markers of kidney damage, such as proteinuria, for a complete evaluation of kidney health 1, 6.