Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
The blood sample is sent to a lab, where the creatinine level is tested. The lab specialist uses combines your creatinine level with several other factors to estimate your glomerular filtration rate (GFR). Different formulas are used for adults and children. The formula will include some or all of the following:
- Creatinine measurement
Certain drugs can interfere with test results. Tell your doctor about all medicines you are taking, including over-the-counter ones.
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there might be some throbbing.
The GFR test measures how well your kidneys are filtering a waste called creatinine, which is produced by the muscles. When the kidneys aren't working as well as they should, creatinine builds up in the blood.
Your doctor may order this test if there are signs that your kidneys are not working properly. It may also be done to see how far kidney disease has progressed.
The test is recommended for people with chronic kidney disease and those at risk for it due to:
According to the National Kidney Foundation, normal results range from 90 - 120 mL/min. Older people will have lower normal GFR levels, because GFR decreases with age.
Normal value ranges can vary slightly among different laboratories. Talk to your doctor about what your specific test results mean.
Levels below 60 mL/min for 3 or more months are a sign of chronic kidney disease. Those with GFR results below 15 mL/min are a sign of kidney failure.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but can include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
The creatinine clearance test, which involves a 24-hour urine collection, can also provide an estimate of kidney function. However, this method is not always accurate.
GFR can increase during pregnancy.
K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Kidney Disease Outcome Quality Initiative. Am J Kidney Dis. 2002;39(2 Suppl 1):S1-246.
Johnson CA, Levey AS, Coresh J, Levin A, Lau J, Eknoyan G. Clinical practice guidelines for chronic kidney disease in adults: Part II. Glomerular filtration rate, proteinuria, and other markers. Am Fam Physician. 2004;70:1091-1097.
Snyder S, Pendergraph B. Detection and evaluation of chronic kidney disease. Am Fam Physician. 2005;72:1723-1732, 1733-1734.