The researchers reviewed medical records of adults who were seen at the Mayo Clinic in Rochester, Minnesota, from 1996 to 2003 either for chronic kidney disease or as potential kidney donors. They found people who had had blood tests for creatinine and GFR measurements (iothalamate clearance tests). They calculated the MDRD equation estimate of GFR based on each person's age, sex, and creatinine value. They then compared the MDRD equation estimate with the actual GFR measurement in both groups of patients. Finally, they formulated a higher-order equation (a quadratic equation) that combined the age, sex, and creatinine values in a different manner and tested its ability to estimate GFR.