The causes of a decrease and increase in the glomerular filtration rate

The glomerular filtration rate (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.

The GFR is influenced by extrarenal factors. 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 decline in GFR up to 40 ml / min in chronic renal pathology indicates a marked renal failure, a drop of up to 15-5 ml / min - on the development of terminal CRF.

Some drugs (eg, cimetidine, trimethoprim) reduce the tubular secretion of creatinine, contributing to an increase in its serum concentration. Antibiotics of the cephalosporin group, due to interference, lead to false-positive results in determining the concentration of creatinine.

Laboratory criteria of stages of chronic renal failure 

Stage	Phase	Blood creatinine, mmol / l	GFR,% of due
I - latent	A	Norm	Norm
	B	To 0.18	Up to 50
II - azotemic	A	0.19-0.44	20-50
	B	0.45-0.71	10-20
III - uremic	A	0.72-1.24	5-10
	B	1.25 and above	Below 5

The increase in 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 _Ks. endogenous creatinine can be up to 30% higher than the true volume of the glomerular filtrate.

The amount of clearance of endogenous creatinine is affected by the secretion of creatinine by the cells of the kidneys, 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, antagonists _of H2-histamine receptors (usually cimetidine at a dose of 1200 mg 2 hours before the start of the collection of 24-hour urine) are prescribed , which 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).

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 blood serum is marked on the scale 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. The tubular reabsorption (CR) 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] × 100. 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.