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An essay into the various methods of assessing glomerular filtration.

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0198622 An essay into the various methods of assessing glomerular filtration. The glomerulus is the main filter of the nephron, within the Bowman's capsule. It is semi-permeable, allowing water and soluble wastes to pass through and be excreted out of the Bowman's capsule as urine. The filtered blood passes out of the glomerulus into the efferent arteriole to be returned through the medullary plexus to the intralobular vein. In many renal diseases, it is the glomerulus's ability to filter blood that becomes impaired, and this is why assessing the glomerular function is so important in renal medicine. Glomerular function is assessed by measuring the glomerular filtration rate (GFR). It is frequently measured for initial patient assessment and also patient follow-up for many renal diseases. GFR is measured by performing a clearance study: the renal clearance of any substance represents the theoretical volume of plasma which is completely cleared of that substance in a given time period 3. To be representative of the GFR, the substance must be freely filtered at the glomerulus, and must not be metabolised or produced by the kidney, nor reabsorbed or secreted at the nephron. There have been many techniques developed to determine glomerular filtration rate, and therefore ultimately glomerular function, by measuring the glomerulus's ability to filter various substances in the plasma - both artificial and endogenous. ...read more.


The most commonly measured endogenous substance used to assess glomerular function is creatinine, because of cheapness and ease of replication 3. Creatinine is created by non-enzymatic dehydration of muscle 1 and is filtered at the glomerulus (although some is excreted by the tubules 7): its plasma concentration is therefore dependent on muscle mass and glomerular filtration rate. When measuring the clearance of creatinine, blood and urine samples are collected over 24-hours. The urine is then analysed for volume and creatinine concentration, and the blood is analysed for serum. Because creatinine is secreted by the tubules, glomerular filtration rate is overestimated when the kidney is normal, but in advanced renal failure less creatinine is filtered, so the amount secreted is proportionally greater, and thus the GFR is greatly over-estimated 4. There are also problems obtaining accurately timed urine samples, accurate creatinine measurements, and variation in generation rate of creatinine. A change in muscle mass will lead to a change in creatinine production, which could be mistaken for a change in glomerular filtration rate, as it takes up to 3 weeks for urinary excretion of creatinine to equilibrate. Stewed meat, such as 'goulash', has also been implicated in a doubling of plasma creatinine 1, which will result in a misleading measurement of glomerular filtration. A simplification of creatinine measurement is to use only a serum creatinine measurement to estimated glomerular function. ...read more.


In ordinary clinical practice, at present the measurement of plasma creatinine is an adequate method of estimating glomerular function provided that urine flow is normal. If not, then glomerular filtration should be measured using a constant intra-venous infusion of [51Cr] EDTA or inulin with blood samples. These methods are also essential for clinical trials. The new methods to estimate glomerular function, especially cystatin A and [99Tcm] DTPA, seem to have less margin for error and appear to be as accurate as inulin clearance. 1 Davidson A M, Oxford textbook of clinical 2nd ed. Oxford : Oxford University, 1998. Vol.1 Section 1 pp. 44-54 2 Finney H, Newman DJ, Price CP. Adult reference ranges for serum cystatin C, creatinine and predicted creatinine clearance. Ann.Clin.Biochem. 2000;37 ( Pt 1):49-59. 3 Gabriel R, Renal medicine, 3rd ed. London : Bailliere Tindall, 1988. pp 46-49 4 Gower PE, Handbook of nephrology, 2nd ed. Oxford : Blackwell Scientific, 1991. pp. 9-12 5 Prvulovich EM, Bomanji J B, The role of nuclear medicine in clinical investigation. BMJ 1998;316:1140-1146 6 Rehling M, Reliability of a 99mTc-DTPA gamma camera technique for determination of single kidney glomerular filtration rate. A comparison to plasma clearance of 51Cr-EDTA in one-kidney patients, using the renal clearance of inulin as a reference. Scand J Urol Nephrol 1986;20(1):57-62 7 Valtin H, Schafer JA, Renal function : mechanisms preserving fluid and solute balance in health, 3rd ed. Boston : Little, Brown , 1994. pp 50-54 ?? ?? ?? ?? 0198622 Word Count: 1193 Page 1 of 3 ...read more.

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