We previously discussed the effect of overhydrating prior to specimen collection. Overhydrating 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 non-detectable.
Urine creatinine is often used by laboratories as a correction factor, or an internal standard to correct for sample over-dilution. Even creatinine has its limitations in this regard, as over-dilution could render the creatinine concentration of urine to very low levels, of which the excretion rates of measurand per creatinine becomes 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.
