Creatinine is a normal waste product produced by muscle metabolism. When your kidneys are functioning normally, they filter creatinine and other waste products out of your blood and send them out of the body through urine.A creatinine urine test measures the amount of creatinine in your urine and can help your doctor evaluate how well your kidneys filter waste.This is useful in diagnosing or ruling out kidney diseases and some other conditions.hen your kidneys are functioning normally, they filter creatinine and other waste products out of your blood and send them out of the body through urine.