The Foundation for IgA Nephropathy
What is the significance of creatinine?
Creatinine is a natural by-product of muscles doing work in your body. It starts out as creatine phosphate, and it ends up as a waste product in your blood which is then eliminated in urine. This waste product can be easily measured in both blood and urine, and, because it is released at a steady rate by your skeletal muscles, it is an excellent indicator of kidney function. Unlike urea, which also measures kidney function to some extent, creatinine is only slightly affected by the meat proteins you eat. As a result, it is a more precise, more specific measure of your kidney function than urea is.
There are two kinds of tests involving creatinine
Creatinine clearance. This is obtained from a measured 24 hour urine collection. It measures the actual amount of creatinine which has been excreted, and as such, it is roughly the equivalent of GFR (glomerular filtration rate, which is the percentage kidney function that most kidney patients are familiar with). It is only roughly equivalent to GFR because some creatinine is also cleared into the urine by the renal tubules (this is called tubular secretion) and this is in addition to the creatinine which is cleared via the glomeruli (which is what glomerular filtration rate measures). Normally, about 10% of creatinine clearance is due to tubular secretion, but in cases of chronic renal insufficiency, the proportion of creatinine clearance which is due to tubular secretion as opposed to glomerular filtration can rise to 30% and go as high as 50%. Because creatinine clearance is measured in a real urine collection (usually but not necessarily 24 hours), errors in collection are relatively common. Labs try to detect under or over-collection, but it may not always be possible. Overcollection of urine (that is, over more than 24 hours) will overestimate kidney function (making it look better than it is), and undercollection (some urine not collected) will underestimate kidney function.
Serum creatinine. This is obtained from a simple blood test. Normally, during the course of follow-up, serum creatinine will be checked frequently - that is, whenever blood work is done. Serum creatinine is the major component in the various equations used to estimate GFR, and as a patient, it is what you will use most in tracking how your kidney function is doing. Patients should keep in mind that serum creatinine will vary a little, up and down, from one test to another, and therefore, they should not attach too much importance to these apparent "improvements" or "worsening" of kidney function. In addition to an error margin of about 10%, serum creatinine results may vary depending on which lab method was used (a 10-20% variation is possible), and being dehydrated will increase it (usually with a disproportionate rise in urea or BUN). It may also rise during "flare-ups" which cause a bit of acute renal insufficiency, and other illnesses such as colds, flus, etc. People on our Yahoo Groups-based email support group often ask whether exercise or eating more protein affects it. The answer is that serum creatinine will not be significantly affected by exercise or how much protein one eats, but it will vary to some extent. The important thing is what the general trend is over time. Please note that serum creatinine does not rise above normal until creatinine clearance has already declined to half of normal (50% kidney function). So, by the time a blood test picks up an elevated serum creatinine, you probably have already lost half of your normal kidney function.