Glomerular Filtration Rate (GFR) and Renal Plasma Flow (RPF)
GFR is the rate at which the glomerulus filters plasma to produce an ultrafiltrate, measured in ml/min or ml/min per 1.73 m², while RPF is the volume of plasma delivered to the kidneys per unit time, typically 4-5 times higher than GFR in healthy individuals.
Glomerular Filtration Rate (GFR)
GFR represents the fundamental measure of overall kidney function and is used to diagnose, stage, and manage chronic kidney disease 1, 2. It quantifies how much blood the kidneys filter each minute 2.
Key Points About GFR:
- Units must always be specified as either ml/min per 1.73 m² (body surface area adjusted) or ml/min (unadjusted) 1
- GFR serves as the primary assessment tool for kidney function in clinical practice, public health, and research 2
- Used to determine drug dosages and ascertain prognosis for CKD-related events and mortality 2
GFR Categories (KDIGO Classification):
- G1: ≥90 ml/min per 1.73 m² (Normal to increased)
- G2: 60-89 ml/min per 1.73 m² (Mildly reduced)
- G3a: 45-59 ml/min per 1.73 m² (Moderately reduced)
- G3b: 30-44 ml/min per 1.73 m² (Moderately reduced)
- G4: 15-29 ml/min per 1.73 m² (Severely reduced)
- G5: <15 ml/min per 1.73 m² or treated by dialysis (Severely reduced) 1
Measurement Methods:
- Measured GFR: Uses clearance methods with exogenous filtration markers (e.g., inulin, iothalamate) 1
- Estimated GFR (eGFR): Calculated using equations (CKD-EPI, MDRD) based on endogenous markers like creatinine or cystatin C 1, 2
Important caveat: GFR and creatinine clearance are not synonymous terms 1.
Renal Plasma Flow (RPF)
RPF measures the volume of plasma flowing through the kidneys per unit time and is typically assessed using para-aminohippuric acid (PAH) clearance 3, 4.
Relationship Between GFR and RPF:
- Normal filtration fraction (FF) = GFR/RPF ≈ 0.20 5
- RPF is typically 4-5 times higher than GFR in healthy individuals
- There is a clear correlation between changes in RPF and changes in GFR (r=0.65; P < 0.003) 4
- When RPF increases substantially, GFR tends to rise as well, likely due to changes in intravascular oncotic pressure along the glomerular capillary 4
Clinical Significance:
During pregnancy, both GFR and RPF increase dramatically—GFR by 40-65% and RPF by 50-85%—due to profound reductions in both afferent and efferent arteriolar resistances 6. In cirrhosis with ascites, reduced RPF and altered filtration fraction can predict risk for hepatorenal syndrome 5.
Measurement Considerations:
Both GFR and RPF can be measured simultaneously using dual marker techniques (e.g., radiolabeled inulin for GFR and PAH for RPF) 3, 7. The renal extraction ratio of PAH (approximately 0.70 in mice) must be determined for accurate RPF calculations 3.