A doctor administers a GFR or glomerular filtration rate test, by analyzing a blood sample for creatine levels, according to the National Kidney Foundation. The doctor computes the GFR test result through a formula that uses the creatine levels, along with the patient's age, race, sex and body size.
GFR tests give information about the health of the kidneys, according to MedlinePlus. Most doctors order the test through a reputable lab. The patient stops certain medications, such as antacids and antibiotics, that might affect the test results for a short period before the test. The lab technician draws a blood sample and measures the percentage of creatine in the sample. Low creatine levels mean that the kidneys are not functioning properly. Alternatively, some doctors have patients give blood several times over a 24-hour period to get average creatine levels from a larger sample.
The technician or doctor plugs the creatine level from the blood sample and information about the patient into a standardized formula to estimate the actual glomerular filtration rate of the kidneys, according to the National Kidney Foundation. Results between 90 and 120 are normal for most adults. Numbers below 90 require more testing, including a urine test for excess albumin or other abnormalities. Monthly GFR test results under 60 for three months in a row are indicative of chronic kidney disease.