A creatinine urine test measures the amount of creatine in your urine. The test can help you doctor evaluate how well your kidneys are functioning. This is useful for diagnosing or ruling out kidney disease and other conditions affecting the kidneys. The creatine in the urine can vary a lot based on diet,exercise and hydration levels, so a spot check is not as helpful. For a 'spot' urine test you collect one sample preferably early in the morning.