Kidney Function Tests PDF

RENAL FUNCTION TESTS By: Dr: Mahmoud Gomaa Eldeib Biochemistry Dept., Faculty of Pharmacy, Sinai University Biochemistry and molecular biology Dept., Faculty of Pharmacy , Al-Azhar University  Functions of the kid...

RENAL FUNCTION TESTS By: Dr: Mahmoud Gomaa Eldeib Biochemistry Dept., Faculty of Pharmacy, Sinai University Biochemistry and molecular biology Dept., Faculty of Pharmacy , Al-Azhar University  Functions of the kidneys: - 1- Excretory function: the excretion of waste products. 2- Homeostatic function: regulation of the ECF volume and composition (regulation of water, electrolyte and acid–base balance) 3- Endocrine function: kidney perform the following ▪ Produce Rennin hormone that stimulate angiotensogen. ▪ Activates vitamin D by hydroxylation at C1 to produce 1,25 dihydroxy cholecalcifirol (Vit D3 active from). ▪ Produce Erythropiotin (stimulate erythropoiesis on the bone marrow). In order to achieve these functions, they receive a rich blood supply, amounting to about 25% of the cardiac output.  Functional unit of the kidney (nephron) Each kidney comprising about 1 million nephrons, which act as independent functional units. The glomeruli act as filters which are permeable to water and low molecular weight substances, but impermeable to macromolecules. This impermeability is determined by both size and charge, with proteins smaller than albumin (68 kDa) being filtered, and positively charged molecules being filtered more readily than those with a negative charge. The tubules return nutrients, fluids, and other substances that have been filtered from the blood, but the body needs, back to the blood. The remaining fluid and waste in the renal tubules become urine  Impaired renal function causes of impaired renal function 1- Pre-renal causes - reduced renal perfusion due to: A. Physiological response to hypovolaemia or a drop in blood pressure. this causes renal vasoconstriction and a redistribution of blood such that there is a decrease in GFR, but preservation of tubular function. Stimulation of vasopressin secretion and of the renin–angiotensin–aldosterone system causes the excretion of small volumes of concentrated urine with a low Na content. B. Congestive cardiac failure (CHF) If pre-renal causes are not treated adequately and promptly by restoring renal perfusion, there can be a progression to intrinsic renal failure. 2- Renal causes may be due to acute kidney injury or chronic kidney disease, with reduction in glomerular filtration. 3- Post-renal causes occur due to outflow obstruction, which may occur at different levels (i.e. in the ureter, bladder or urethra), due to various causes (e.g. renal stones, prostatism, genitourinary cancer). As with pre-renal causes, this may in turn cause damage to the kidney.  Renal function tests Tests of glomerular function In acute and chronic kidney injury, there is effectively a loss of function of whole nephrons, and because the process of filtration is essential to the formation of urine, tests of glomerular function are almost invariably required in the investigation and management of any patient with kidney disease. Measurement of glomerular filtration rate Clearance tests An estimate of the GFR can be made by measuring the urinary excretion of a substance that is completely filtered from the blood by the glomeruli and is not secreted, reabsorbed or metabolized by the renal tubules. Experimentally, inulin (a plant polysaccharide) has been found to meet these requirements. The volume of blood from which inulin is cleared or completely removed in 1 min is known as the inulin clearance, and it is equal to the GFR. Measurement of inulin clearance requires the infusion of inulin into the blood and is not suitable for routine clinical use. Creatinine clearance: The most widely used biochemical clearance test is based on measurements of creatinine in plasma and urine. Creatinine is an endogenous substance derived mainly from the turnover of creatine in muscle, and daily production is relatively constant, being a function of total muscle mass. A small amount of creatinine is derived from meat in the diet. Creatinine clearance is calculated using the following formula: Clearance =U× V˙/P mL/min U = urine creatinine concentration (μmol/L) V = urine flow rate (mL/min or (L/24 h) P = plasma creatinine concentration (μmol/L) Creatinine clearance in healthy adults is ∼120 mL/min, but a normal GFR for a small person will be lower than for a large person. Results can be corrected to a standard body surface area of 1.73 m2 using formulae that incorporate weight and height; this allows easier comparison between individuals. It should be noted that the clearance formula is valid only for a steady state, that is, when kidney function is not changing rapidly.  A larger amount, up to 10% of urinary creatinine, is actively secreted into the urine by the renal tubules and as a result, the creatinine clearance is higher than the true GFR. The difference is of little significance when the GFR is normal, but when the GFR is low (