Renal Clearance Formula
The renal clearance formula calculates how efficiently the kidneys filter a substance from blood.
Includes GFR, creatinine clearance, and worked examples.
The Formula
Renal clearance is the volume of plasma from which the kidneys completely remove a given substance per unit time. It tells us how efficiently the kidneys are filtering that substance from the blood. Different substances have different clearances — some are freely filtered, some are reabsorbed, and some are actively secreted.
Variables
| Symbol | Meaning | Unit |
|---|---|---|
| C | Clearance of the substance | mL/min |
| U | Urine concentration of the substance | mg/mL |
| V̇ | Urine flow rate | mL/min |
| P | Plasma concentration of the substance | mg/mL |
Glomerular Filtration Rate (GFR)
The gold standard for measuring GFR is inulin clearance — inulin is neither reabsorbed nor secreted by the tubules, so its clearance exactly equals the filtration rate. Normal GFR is approximately 125 mL/min in young healthy adults, or about 180 liters of plasma filtered per day.
Creatinine Clearance (Clinical Approximation)
Clinically, creatinine clearance approximates GFR because creatinine (a muscle metabolism waste product) is freely filtered and only slightly secreted. It slightly overestimates true GFR (~10–20%) but requires only a 24-hour urine collection and a blood test.
Cockcroft-Gault Estimated GFR
This equation estimates creatinine clearance from age, weight, and serum creatinine — no urine collection needed. It is widely used in clinical practice to detect kidney disease and adjust drug dosages.
Example 1
Patient: urine creatinine 120 mg/dL, urine flow 1.2 mL/min, plasma creatinine 1.0 mg/dL
C = (1.20 mg/mL × 1.2 mL/min) / 0.010 mg/mL
C = 1.44 / 0.010
Creatinine clearance = 144 mL/min — within normal range
Example 2
Cockcroft-Gault: 65-year-old male, 70 kg, serum creatinine 1.4 mg/dL
eGFR = [(140 − 65) × 70] / (72 × 1.4)
= [75 × 70] / 100.8 = 5250 / 100.8
eGFR ≈ 52 mL/min — CKD Stage 3 (mild to moderate impairment)
Interpreting Clearance Values
- C = GFR: Substance is freely filtered, neither reabsorbed nor secreted (e.g., inulin)
- C < GFR: Net tubular reabsorption (e.g., glucose, amino acids — normally C = 0)
- C > GFR: Net tubular secretion (e.g., para-aminohippuric acid / PAH — used to measure renal plasma flow)
CKD Stages by GFR
- G1 (≥90 mL/min): Normal or high — kidney damage markers present
- G2 (60–89 mL/min): Mildly decreased
- G3a (45–59 mL/min): Mild to moderately decreased
- G3b (30–44 mL/min): Moderately to severely decreased
- G4 (15–29 mL/min): Severely decreased — prepare for renal replacement
- G5 (<15 mL/min): Kidney failure — dialysis or transplant