Overhydration of a patient may have profound effects on urinary markers of interest and, depending on the volume of water or other fluids consumed prior to and during a timed urine collection, may cause the desired measurands to be decreased or non-detectable.
Urine creatinine is often used by laboratories as an internal standard to check for completeness of the 24-hour urine collection. Even creatinine has its limitations in this regard, as over-dilution could render the creatinine concentration of urine to very low levels. The excretion rates of analyte per creatinine become inaccurate.
Many laboratories suggest instructing patients to limit the volume of fluids consumed within the collection period, often to 2,000 mL (2 liters) per day.
