Renal Clearance Formula and Its Importance
Renal clearance provides essential information about excretory functions: filtration, tubular reabsorption, tubular secretion, and renal blood flow.
Table of Contents
Definition
The renal clearance value of a given substance is the volume of plasma, in milliliters, wholly cleared of that substance in one minute by the kidneys; it is expressed as milliliters per minute (ml/min).
Clearance compares the amount of substance removed from the plasma per unit time for a patient with the amount of substance excreted in the urine per minute. Plasma concentration rather than whole blood concentration is used to calculate renal clearance.
Normal plasma flow is 700 ml per minute. We know that plasma constitutes 55% of the blood, while the formed elements make up 45%. Therefore, when plasma flow is 55 mL/min, the total renal blood flow is 100 mL/min. Then, the total renal blood flow is approximately 100 × 700 / 55, or 1273 ml per minute.
Calculation of renal clearance of a given substance
Urine flow per minute X urinary concentration of the substance/: arterial plasma concentration.
Renal excretion rate =Urine flow per minute X urinary concentration of the substance.
Renal clearance of a given substance, the Renal excretion rate of the substance its arterial plasma concentration.
Calculation:
Suppose the substance is X, then the calculation is
The renal clearance rate of X=Urine flow per minute of X/plasma concentration of X.
If a substance is not metabolized in the tissues, the arterial level equals the plasma level of that substance.
The urea clearance (rate in ml per minute):
Urinary concentration of urea into urine flow per minute/plasma concentration of urea.
Average value
The average value of Urea is 20 mg per ml of urine.
The urine flow rate is 1 ml per minute.
Renal excretion rate of urea=20×1=20.
The plasma concentration of urea is 30 mg/dL.
Urea clearance = 20 X 100 /30 = 67 ml per minute
Utility of the Clearance Test
Renal clearance provides essential information about excretory functions: filtration, tubular reabsorption, tubular secretion, and renal blood flow.
1. Determination of glomerular filtration rate.
2. Estimation of renal plasma flow and renal blood flow.
3. Measurement of tubular filtration rate.
4. In the calculation of tubular secretory capacity.
5. In quantifying osmotic and free water clearance
6. Assessment of excretion of waste products.
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