The clearance of endogenous creatinine( a sample of reberg-tareyev)
The test of Reberg-Tareev allows to judge the glomerular filtration and tubular reabsorption in the kidneys. The test is based on the fact that creatinine is filtered only by glomerulus, is not practically absorbed and secreted by tubules in a small amount. The order of the sample is as follows: the patient wets in the morning, drinks 200 ml of water and then, on an empty stomach, in a state of complete rest, collects urine for a certain short time( 2 hours).In the middle of this period of time, blood is taken from the vein. Determine the concentration of creatinine in the blood and urine collected in 2 hours. Calculate the cleansing factor( Koç) or clearance of the endogenous creatinine : Koh =( M / Pl.) XD( ml / min), where M is the creatinine concentration in urine;Pl.- concentration of creatinine in plasma;D - minute diuresis in ml / min [equal to the amount of urine released in 2 hours( ml), divided by 120 min).Koch expresses the GFR.To determine GFR, urine collected per day can be examined.
Normally, GFR is 120 + 25 ml / min in men and 95 + 20 ml / min in women. GFR values are lowest in the morning, rise to maximum values during the day, and then decrease again in the evening. In healthy people, the decline in GFR occurs under the influence of heavy physical exertion and negative emotions;increases after drinking fluids and taking high-calorie food.
GFR is a sensitive indicator of the functional state of the kidneys, its decrease is considered one of the earliest symptoms of impaired renal function. Reduction of GFR, as a rule, occurs much earlier than a decrease in the concentration function of the kidneys and the accumulation of nitrogenous slags in the blood. In primary glomerular lesions, the failure of renal concentrating function is detected with a sharp decrease in GFR( approximately 40-50%).In chronic pyelonephritis, the distal tubules are mostly affected, and the filtration decreases later than the concentration function of the tubules. Violation of the concentration function of the kidneys and sometimes even a slight increase in the content of nitrogenous slags in the blood in patients with chronic pyelonephritis is possible in the absence of a decrease in GFR.
Extranenal factors affect GFR.Thus, GFR decreases with cardiac and vascular insufficiency, abundant diarrhea and vomiting, hypothyroidism, mechanical obstruction of urinary outflow( prostate cancer), liver damage. In the initial stage of acute glomerulonephritis, the decrease in GFR occurs not only because of impaired patency of the glomerular membrane, but also as a result of disorders of hemodynamics. In chronic glomerulonephritis, a decrease in GFR may be due to azotemia vomiting and diarrhea.
A persistent GFR decline of up to 40 ml / min in chronic renal disease indicates a marked renal failure, a drop of up to 15-5 ml / min - on the development of terminal CRF( Table).
Some drugs( eg, cimetidine, trimethoprim) reduce the tubular secretion of creatinine, contributing to an increase in serum concentration in the blood serum. Antibiotics of the cephalosporin group, due to interference, lead to false-positive results in determining the concentration of creatinine.
Table Laboratory criteria for stages of CRF
Elevated GFR is observed in chronic glomerulonephritis with nephrotic syndrome, in the early stage of hypertension. It should be remembered that with nephrotic syndrome, the clearance value of endogenous creatinine does not always correspond to the true state of GFR.This is due to the fact that in the nephrotic syndrome, creatinine is secreted not only by the glomeruli, but also secreted by the altered tubular epithelium, and therefore the endogenous creatinine Koch can be up to 30% higher than the true volume of the glomerular filtrate.
The clearance value of endogenous creatinine is affected by the secretion of creatinine by the cells of the renal tubules, so its clearance can significantly exceed the true value of GFR, especially in patients with kidney disease. To obtain accurate results, it is extremely important to completely collect the urine for a specified amount of time, incorrect collection of urine will lead to false results.
In some cases, to increase the accuracy of determining the clearance of endogenous creatinine, H2-histamine receptor antagonists( usually cimetidine at a dose of 1200 mg 2 hours before the start of the 24-hour urine collection) are prescribed that block the tubular secretion of creatinine. The clearance of endogenous creatinine, measured after taking cimetidine, is almost equal to true GFR( even in patients with moderate and severe renal failure).
The clearance of endogenous creatinine can quickly be calculated from the nomogram shown in Fig.[Appel G. B., Neu H. C., 1977].To do this, you need to know the body weight of the patient( kg), age( years) and the concentration of creatinine in the blood serum( mg%).Initially, the patient's age and body weight are joined by a straight line and the point on line A is marked. Then, the concentration of creatinine in the serum on the scale is marked and connected by a straight line to the point on line A, continuing it until it intersects with the endogenous creatinine clearance scale. The point of intersection of a straight line with the scale of clearance of endogenous creatinine corresponds to GFR.
Tubular Reabsorption. Tuberculosis reabsorption( CD) is calculated from the difference between glomerular filtration and minute diuresis( D) and is calculated as a percentage of glomerular filtration according to the formula: KR = [(GFR-D) / GFR] x100.Normally tubular reabsorption ranges from 95 to 99% of the glomerular filtrate.
Canalic reabsorption can vary significantly under physiological conditions, decreasing to 90% under water loading. The marked decrease in reabsorption occurs with forced diuresis, caused by diuretics. The greatest decrease in tubular reabsorption is observed in patients with diabetes insipidus. A persistent decrease in the reabsorption of water below 97-95% is observed in the primary and secondary wrinkled kidney and chronic pyelonephritis. Reabsorption of water can also decrease with acute pyelonephritis. In pyelonephritis, reabsorption decreases before GFR decreases. With glomerulonephritis, reabsorption decreases later than GFR.Usually, concomitant with a decrease in water reabsorption, failure of the concentration function of the kidneys is revealed. In this regard, a decrease in the reabsorption of water in the functional diagnosis of the kidneys of great clinical importance does not.
An increase in tubular reabsorption is possible with nephritis, nephrotic syndrome.