Normal kidney functions in a healthy adult show creatinine levels of 0.6 to 1.2 milligrams per deciliter in males and 0.5 to 1.1 milligrams per deciliter in females, according to MedicineNet. Older adults may have less creatinine in their blood, and athletes and those with muscular builds may have more.
Doctors determine normal kidney functions by a blood test called the glomerular filtration rate, which determines the level of creatinine within the blood, explains the Mayo Clinic. Creatinine levels may be high due to dehydration, taking certain medications, eating large amounts of protein or experiencing kidney problems. Doctors can administer a second blood test and possibly a urine test to confirm if the elevated creatinine level is due to kidney disease.