Your doctor can measure the level of creatinine by doing a simple blood test. A normal adult creatinine is about 1.0, regardless of age. We use the level of creatinine to determine the “glomerular filtration rate” or “GFR” – in other words, how well the glomeruli (those tiny filters in the kidneys) are filtering out waste products. The GFR is calculated using a mathematical equation, or sometimes by having you collect urine for 24 hours and measuring how much creatinine is in the urine. The 24-hour urine test is called the “creatinine clearance” and is similar to the GFR.
Kidney function normally declines as we age, so even though the normal blood creatinine is still 1.0 when you are 70-years old, the kidneys filter waste products less efficiently than when you were age 20. The higher the GFR, the better the kidney function. A GFR of 60-100 is generally considered normal (depending on your age and whether you are male or female). Most people do not have symptoms of decreased kidney function until the GFR is 20-30 or less, and some people don’t feel sick until the GFR is as low as 10-15. It is important for anyone with kidney disease to “know your GFR number” and once you know your blood creatinine, you can even calculate your own GFR by using the GFR Calculator on this website. (Go to DaVita.com’s GFR Calculator to learn your GFR.)