Renal function tests
Renal clearance is the measurement of the kidney’s ability to remove substances from the plasma.
Clearance measurements are used to determine Renal Blood Flow (RBF), Renal Plasma Flow (RPF), Glomerular Filtration Rate (GFR), Filtration Fraction (FF) and how different substances are handled by the kidney tubules (reabsorbed or secreted) and to compare the kidney function values for diagnostic purposes.
Plasma clearance
Plasma clearance is used to express the ability of the kidneys to clean or clear the plasma of various substances. e.g., If the plasma passing through the kidneys contains 1 mg of a substance in each ml and 1 mg of this substance is also excreted into the urine each minute, then 1 ml/min of the plasma is cleared of the substance.
Plasma clearance is an excellent measure of kidney function and the clearance rate of different substances are determined by analyzing the concentration of substance simultaneously in plasma and urine and measuring the rate of urine formation.
Inulin clearance as an estimate of GFR
A substance to measure GFR must be freely filtered at the glomerulus and should not be reabsorbed or secreted by the tubular epithelium after it enters the nephron. Inulin, a fructose polysaccharide is the substance that is most commonly used.
Mannitol is another polysaccharide used to estimate GFR.
Creatinine clearance
In clinical conditions it can be used to measure GFR and kidney functions. Creatinine is freely filtered and not reabsorbed by the tubules. In some species (not in dogs) about 10% is secreted by the tubules. It can’t be used in birds because creatinine is either secreted or reabsorbed from the tubules to a greater extent.
Measurement of RPF
The substance used must be freely filtered at the glomerulus and must not be reabsorbed from the tubular lumen and must be secreted by the tubular epithelium so that all of the substances in the blood perfusing the tubules is removed before the blood leaves the kidney. Therefore, if all the substance in the plasma that perfuses the kidneys is excreted in the urine the rate of its excretion is the same as its plasma load. Para Amino Hippuric acid (PAH) is used to measure RPF.
Plasma load and tubular load
Plasma load of a substance is the total amount of substance in the plasma that passes through the kidneys each minute. Eg: If the concentration of glucose in the plasma is 100 mg/100 ml and 600 ml of plasma passes through both the kidneys each minute, then the plasma load of glucose is 600 mg/min.
A fraction of plasma load that is filtered as GF is referred to as tubular load.
For example, if 125 ml of GF is formed each minute with a glucose concentration of 100 mg% then the tubular load of glucose is 125 mg (100 x 1.25) glucose/min.