![]() ![]() The most commonly used endogenous marker for the assessment of glomerular function is creatinine. The inconvenience associated with the use of exogenous markers, specifically that the testing has to be performed in specialized centers, and the difficulty to assay these substances, has encouraged the use of endogenous markers. The most promising exogenous marker is the non-radioactive contrast agent, iohexol, especially in children. Other exogenous markers used are radioisotopes such as chromium-51 ethylene-diamine-tetra-acetic acid (51 Cr-EDTA), and technetium-99-labeled diethylene-triamine-pentaacetate (99 Tc-DTPA). It involves the infusion of inulin and then the measurement of blood levels after a specified period to determine the rate of clearance of inulin. Assessment of GFR using inulin, a polysaccharide, is considered the reference method for the estimation of GFR. It should not undergo extrarenal elimination.Īs no such endogenous marker currently exists, exogenous markers of GFR are used. This article provides an update on the relevant biochemical tests for the assessment of renal function. Worldwide, the most common causes of CKD are hypertension and diabetes. According to the National Institutes of Health, the overall prevalence of chronic kidney disease (CKD) is approximately 14%. Tests of renal function have utility in identifying the presence of renal disease, monitoring the response of kidneys to treatment, and determining the progression of renal disease. Assessment of renal function is important in the management of patients with kidney disease or pathologies affecting renal function. The functional unit of the kidney is the nephron, which consists of the glomerulus, proximal and distal tubules, and collecting duct. In particular, prerenal disease should not be excluded by a normal ratio since diminished urea production (due to decreased protein intake or underlying liver disease) can prevent the expected rise in BUN due to increased tubular reabsorption.The kidneys play a vital role in the excretion of waste products and toxins such as urea, creatinine and uric acid, regulation of extracellular fluid volume, serum osmolality and electrolyte concentrations, as well as the production of hormones like erythropoietin and 1,25 dihydroxy vitamin D and renin. In contrast to the potential utility of a high BUN/serum creatinine ratio, a normal ratio is of limited diagnostic utility. However, this problem is chronic and cannot explain an acute rise in the BUN out of proportion to any change in the serum creatinine concentration. ![]() The BUN/serum creatinine ratio can exceed 20:1 when loss of muscle mass in a chronically ill or older patient lowers creatinine production and, therefore, the serum creatinine concentration, independent of the GFR. ![]() The BUN will rise out of proportion to the serum creatinine when urea production is increased due to GI bleed (upper somewhat more than lower), tissue breakdown, or glucocorticoid therapy. Thus, a high ratio is suggestive of prerenal disease as long as some other cause of a high ratio is not present. The BUN/Cr ratio is normal at 10 to 15:1 in ATN (intrarenal), but is often greater than 20:1 in prerenal disease due to the increase in the passive reabsorption of urea that follows the enhanced proximal reabsorption of sodium and water. *** A BUN/Cr ratio that is high when BUN and Creatinine are either low or at the lower range of normal is not going to be clinically significant. In ATN, the reabsorption of BUN or secretion of Cr is decreased making the ratio to be normal. As they pass through the renal tubule, BUN is reabsorbed from the PCT (proximal convoluted tubule) while Creatinine is not reabsorbed but instead, more is secreted into the tube in the DCT. In normal circumstances, Both BUN and Cr are filtered. Renal damage causes reduced reabsorption of BUN, therefore lowering the BUN:Cr ratio. Some sources say the normal range is 10-15:1. Dehydration or hypoperfusion is suspected. BUN is disproportionately elevated relative to creatinine in serum. Urea is made in the liver as a by-product of protein metabolism. *Note that BUN=Blood urea nitrogen which is essentially a measurement of urea. The ratio may be decreased with liver disease (due to a decrease in the formation of urea) and malnutrition. It may also be seen with increased protein, from BI bleed, or increased protein in the diet. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as CHF or dehydration. The ratio of BUN to creatinine is usually between 10:1 and 20:1. We may look at the BUN / Creatinine ratio to help determine the cause of renal failure. ![]()
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