Renal clearance test

Renal clearance test

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  RENAL CLEARANCE TESTS By Dr. M . Anuswaru 3rd year Post graduate
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  • The kidneysare the vital organs of the body, performing the following major functions. 1. Maintenance of homeostasis : The kidneys are largely responsible for the regulation of water, electrolyte and acid-base balance in the body. 2. Excretion of metabolic waste products :The end products of protein and nucleic acid metabolism are eliminated from the body. These include urea, creatinine, creatine, uric acid, sulfate and phosphate. 3. Retention of substances vital to body : The kidneys reabsorb and retain several substances of biochemical importance in the body e.g. glucose, amino acids etc.
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  • 4. Hormonalfunctions : The kidneys also function as endocrine organs by producing hormone.  Erythropoietin, a peptide hormone, stimulates haemoglobin synthesis and formation of erythrocytes. 1,25- Dihydroxycholecalciferol (calcitriol)-the biochemically active form of vitamin D is finally produced in the kidney. lt regulates calcium absorption from the gut.  Renin, a proteolytic enzyme liberated by kidney, stimulates the formation of angiotensin ll which, in turn, leads to aldosterone production.  Angiotensin ll and aldosterone are the hormones involved in the regulation of electrolyte balance.
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  • The bloodsupply to kidneys is relatively large. • About 1200 ml of blood (650 ml plasma)passes through the kidneys, every minute. From this, about 120-125 ml is filtered per minute by the kidneys and this is referred to as glomerular filtration rafe (GFR). • With a normal GFR(12O-125 ml/min), the glomerular filtrate formed in an adult is about 175-180 litres per day, out of which onlv 1 .5 litres is excreted as urine. • Thus, more than 99% of the glomerular filtrate is reabsorbed by the kidneys.
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  Tests to assessrenal functions About two thirds of the renal tissue must be functionally damaged to show any abnormality by these tests. The kidney function tests may be divided into four groups : 1. Glomerular function tests : All the clearance tests (inulin, creatinine, urea) are included in this group. 2. Tubular function tests : Urine concentration or dilution test, urine acidfication test. 3. Analysis of blood/serum : Estimation of blood urea, serum creatinine, protein and electrolyte are often useful to assess renal function. 4. Urine examination : Simple routine examination of urine for volume, pH, specific gravity, osmolality and presence of certain abnormal constituents (proteins, blood, ketone bodies, glucose etc.) also helps to a limited degree to assess kidney functioning.
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  CLEARANGE TESTS • Theclearance tests, measuring the glomerular filtration rate (GFR) are the most useful in assessing the renal function. • The excretion of a substance can be expressed quantitatively by using the concept of clearance. • Clearance, in general, is defined as the volume of plasma that would be completely cleared of a substance per minute. • In other words, clearance of a substance refers to the milliliters of plasma which contains the amount of that substance excreted by kidney per minute. Clearance (C), expressed as ml/minute, can be calculated by using the formula C = U X V P U = Concentration of the substance in urine. V = Volume of urine in ml excreted per minute. P = Concentration of the substance in plasma.
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  • The clearanceof a given substance is determined by its mode of excretion. • The maximum rate at which the plasma can be cleared of any substance is equal to the CFR. • This can be easily calculated by measuring the clearance of a plasma compound which is freely filtered by the glomerulus and is neither absorbed nor secreted in the tubule. • Inulin (a plant carbohydrate, composed of fructose units) and Cr-EDTA satisfy this criteria. • Inulin is intravenously administered to measure GFR.
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  • In practice,however, measurement of clearance for the substances already present in the blood is preferred. • The two compounds , namely creatinine and urea, are commonly employed for this purpose. • Creatinine clearance ( ̴145 ml/min) is marginally higher than the CFR as it is secreted by the tubules. • Urea clearance ( ̴75 ml/min) is less than the CFR, since it is partially reabsorbed by the tubules.
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  Creatinine clearance test •Creatinine is an excretory product derived from creatine phosphate (largely present in muscle). • The excretion of creatinine is rather constant and is not influenced by body metabolism or dietary factors. • Creatinine is filtered by the glomeruli and only marginally secreted by the tubules. • The value of creatinine clearance is close to CFR, hence its • measurement is a sensitive and good approach • to assess the renal glomerular function.
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  • Creatinine clearancemay be defined as the volume (ml) of plasma that would be completely cleared of creatinine per minute. Procedure : ln the traditional method, creatinine content of a 24 hr urine collection and the plasma concentration in this period are estimated. The creatinine clearance (C) can be calculated as follows : C = U X V P U = Concentration of the creatinine in urine. V = Urine output in ml/min (24 hr urine volume divided by 24 x 6O) P = Concentration of creatinine in plasma. Creatinine concentration in urine and plasma should be expressed in the same units (mg /dl or mmol/l).
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  modified procedure :Instead of a 24 hr urine collection, the procedure is modified to collect urine for 1 hr, after giving water. The volume of urine is recorded. Creatinine contents in plasma and urine are estimated. The creatinine clearance can be calculated by using the formula referred before. Reference values : The normal range of creatinine clearance is around 120-145 ml/min. These values are slightly lower in women. In recent years, creatinine clearance is expressed in terms of body surface area. Diagnostic importance : A decrease in creatinine clearance value (< 75% normal) serves as sensitive indicator of a decreased GFR, due to renal damage. • This test is useful for an early detection of impairment in kidney function, often before the clinical manifestations are seen.
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  Urea clearance test •Urea is the end product of protein metabolism. After being filtered by the glomeruli, it is partially reabsorbed by the renal tubules. • Hence, urea clearance is less than the GFR and further it is influenced by the protein content of the diet. • For these reasons, urea clearance is not as sensitive as creatinine clearance for assessing renal function. • Urea clearance is defined as the volume (ml)of plasma that would be completely cleared of urea per minute. It is calculated by the formula Cm = U X V P Cm = maximum urea clearance U = Urea concentration in ( mg/ml) V = Urine excreted per minute in ml P = Urea concentration in plasma( mg/ml).
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  • The abovecalculation is applicable if the output of urine is more than 2 ml per minute . This is referred to as maximum urea clearance and the normal value is around 75 ml/min. Standard urea clearance : lt is observed that the urea clearance drastically changes when the volume of urine is less than 2 ml/min. This is known as standard urea clearance (Cs). normal value is around 54 ml/min. lt is calculated by a modified formula
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  Diagnostic importance : Aurea clearance value below 75% of the normal is viewed seriously, since it is an indicator of renal damage. • Bl lood urea level as such is found to increase only when the clearance falls below 50% normal. • As already stated, creatinine clearance is a better indicator of renal function.
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