Final answer:
The most important thing to tell a patient when collecting a 24 hour urine test is to collect the first urine in the morning because it will be more concentrated and provide accurate results.
Step-by-step explanation:
The most important thing to tell a patient when collecting a 24 hour urine test is to collect the first urine in the morning. This is because the first urine will be more concentrated and will provide a better representation of the patient's urinary output and the substances being measured. The concentration of certain substances in urine can vary throughout the day, so it is important to collect the first urine to obtain accurate results.