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Creatinine: What is it and why is it important?
Serum creatinine level and "creatinine clearance" are different ways of determining kidney function. Here is a technical explanation for those that are interested:
Creatinine is a protein produced by muscle and released into the blood. The amount produced is relatively stable in a given person. The creatinine level in the serum is therefore determined by the rate it is being removed, which is roughly a measure of kidney function. If kidney function falls (say a kidney is removed to donate to a relative), the creatinine level will rise. Normal is about 1 for an average adult. Infants that have little muscle will have lower normal levels (0.2). Muscle bound weight lifters may have a higher normal creatinine. Serum creatinine only reflects renal function in a steady state. After removing a kidney, if the donor's blood is checked right away the serum creatinine will still be 1. In the next day the creatinine will rise to a new steady state (usually about 1.8). If both kidneys were removed (say for cancer) the creatinine would continue to rise daily until dialysis is begun. How fast it rises depends on creatinine production, which is again related to how much muscle one has. A baby may need dialysis when the creatinine reaches 2, whereas a normal adult may be able to hold off until 10, or higher.
Creatinine clearance is technically the amount of blood that is "cleared" of creatinine per time period. It is usually expressed in ml per minute. Normal is 120 ml/min for an adult. It is roughly, inversely related to serum creatinine: If the clearance drops to one half of the old level, the serum creatinine doubles (in the steady state). So for an adult, serum creatinine of 2 is roughly a creatinine clearance of 60 ml/min; creatinine 3 is roughly a clearance of 30; creatinine of 4 is roughly a clearance of 15, etc. So why didn't the creatinine rise to only 2 when a kidney was removed? (I said it would rise to 1.8) The answer is that the remaining kidney "hyperfilters" and seems to work harder, therefore kidney function is not quite halved.
Usually, an adult will need dialysis because symptoms of kidney failure appear at a clearance of less than 10 ml/min. Creatinine clearance has to be measured by urine collection (usually 12 or 24 hours). It is a more precise estimate of kidney function than serum creatinine since it does not depend on the amount of muscle one has.
Note that the units used in the United States are milligrams per deciliter (mg/dl). To convert to international units (micromoles per liter) multiply the creatinine (in mg/dl) by 88. Thus a serum creatinine of 2mg/dl is the same as 176 micromoles per liter.
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