In addition to the timing aspects of a 24-hour sample, the importance of a complete collection must be stressed when providing instructions to patients. Certain measurands are known to fluctuate throughout the day. Catecholamines and cortisol are known to be elevated following periods of high stress. Failure to collect the urine sample after the high-stress period will likely lead to a falsely low determination of those excreted components. Also, first-morning urine samples tend to be more concentrated, and failing to collect this sample could lead to the underestimation of a number of excreted components.
Patients should be asked to confirm that all excreted urine had been added to the collection jug and that the volume is representative of the entire day of voided urine.