Reference intervals for components excreted in a 24-hour timed urine sample are based on the determined concentration of a measurand as well as the length of 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 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.
Urine specimens may also be random (any time), first-morning void, fasting, 2-hour post-prandial (after a meal), or timed for periods shorter than 24 hours.
