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## Table of Contents## Creatinine ClearanceKidney function, or renal function, is measured in a number of ways. One method is to inject a substance that can only be excreted by the kidney, and take blood and urine samples over time. This is scientific and exact; and often done in research. In medical practice we take advantage of a substance called creatinine, which is produced naturally from muscle tissue and a nearly constant rate, and is almost totally eliminated by the kidney. ## Figure 1, Creatinine Molecule
Renal filtration is the process of clearing a substance from the body through the kidneys. I like to use the analagy of making spaghetti. After you boil a pot of spaghetti, one has to strain the water through a collander. Imagine the spaghetti as protein or large molecules and the water as fluid or small molecules. The collander holds the large molecules and the small molecules escape through the holes. The kidney works in much the same way. In figure 2, you can imagine the whole pot of spaghetti is poured into the kidney unit called a glomerulus. This entrance is called the afferent arteriole. Next, the blood travels through a complex of small leaky blood vessels. The glomerular blood vessel holds onto the large molecules and lets pass the small molecules into the glomerular capsule, where they pass downstream into the proximal convoluted tubule, where ultimately they end up in the urinary bladder as urine. ## Figure 2, Glomerulus
## Measuring Kidney FunctionCreatinine is not filtered; rather, it is secreted early from a capillary in the glomerulus called the proximal tubule. Since very little gets reabsorbed, it is a good natural compound in the body with which to measure kidney function. The rate of creatinine secretion is kept constant under normal circumstances. This assumption may deviate during times of illness or with the ingestion of certaiin medications. For example, in severe renal dysfunction, more creatinine is secreted giving an overestimation of creatinine clearance. Cimetidine and trimethoprim will reduce creatinine secretion and underestimate creatinine clearance. Also, active exercise or increased meat consumption will raise levels of creatinine in the blood stream. This will give an underestimate of creatinine clearance (if it does not get cleared, it bulids up in the blood stream). Other factors that affect kidney function is Age and Weight. As we age, our kidney function diminishes, and a random serum creatinine value will over estimate kidney function. As we gain weight, creatinine will fall and give an under estimate of kidney function. In summary, taking all these assumptions and factors into consideration, creatinine is the most commonly used measure of kidney function. Drawing blood and measuring creatinine is like taking a snap shot. Measuring urine secretion over 24 hours is like taking a motion picture: you get a more accurate measure of creatinine filtration. The result is then calculated as glomerular filtration rate (GFR). This is the gold standard of kidney function. The bottom line is the random measurement of creatinine can be translated into GFR using various formulae which have been developed by researchers over time. The Cockcroft-Gault equation has been used for decades, and is the most widely used equation to translate serum creatinine into GFR. A recent paper by Schaeffner has challanged the use of this equation for a better one. The Berlin Initiative Study (BIS) introduces two new equations and test all three against the gold standard and tests their validity. These three equations are listed below. In short, the best equation is the BIS2 equation, and the least accurate is the Cockroft-Gault equation ## Figure 3, Equations for Kidney Function## Cockcroft-Gault## BISI1## BISI2
## Accuracy DeterminationShaeffner and his researchers have determined the accuracy of these measures using GFR as a gold standard. Figiure 4 shows the results of his reasearch. The equation value is measured against the y-axis and the GFR is measured against the x-axis. The most accurate equation will have the least amount of variation, called variance in statistics. A measure of variance is called Standard Deviation (SD). As shown in figure 3, the BIS1 equation has the lowest SD. There are many other equations which add to the complexity; however, their accuracy or SD is not much better, and their complexity make their use less practical. Therefore, the BIS1, BIS2, and Cockroft-Gault are the three that I use. ## Figure 4## TablesI have listed a series of tables that are used to translate random serum creatinine levels into estimated glomerular filtration rate (eGFR). ## Cockcroft-Gault## Male Wt 125 lbs
Creatinine value on the left, Age across the top, and GFR inside the table.
Normal is 100. ## Male Wt 155 lbs
Creatinine value on the left, Age across the top, and GFR inside the table.
Normal is 100. ## Male Wt 210 lbs
Creatinine value on the left, Age across the top, and GFR inside the table.
Normal is 100. ## Female Wt 110 lbs
## Female Wt 145 lbs
## Female Wt 250 lbs
## BIS1## Male
## Female
## Reference1. Schaeffner,ES, et al, Two Novel Equations to Estimate Kidney Function in Persons Aged 70 Years or Older, Annals of Internal Medicine, 157-471-481, 2012. link |

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