Reference intervals for components excreted in a timed urine sample are based on the determined concentration of a measurand, as well as the time of collection. This means that a standard mode of timing is required to minimize the effect of underestimating or overestimating the period of time, and thus the volume collected.
For 24-hour collections, most laboratories employ the practice of instructing patients to arise on their day of collection and discard the first morning voided sample. This initiates the 24-hour period, during which ALL subsequent urine, including the first morning sample of the next day within the same 24-hours, is collected and transferred from a collection cup into the 24-hour jug. Improper timing may have undesired consequences on interpretation.