Urine is made up of water and dissolved chemicals, such as sodium and potassium. It also contains urea, which is made when protein breaks down, and creatinine, which is formed from muscle breakdown. Normally, urine contains specific amounts of these waste products. If these amounts are not within a normal range, or if other substances are present, it may be a sign of a certain disease or condition. A 24-hour urine collection is a simple lab test that measures what’s in your urine. The test is used to check kidney function. A 24-hour urine collection is done by collecting your urine in a special container(s) over a full 24-hour period. The container(s) must be kept cool until the urine is returned to the lab.