Typically, water is permitted without restriction prior to sample collection and, unless there is a compromise in kidney function, will have minimal effect on components measured in the blood. One exception is that of the artificial lowering of hemoglobin and hematocrit related to recent consumption of large volumes of water or other beverages just ahead of sample collection, which could transiently increase the plasma volume. Likewise, dehydration may artificially increase the hematocrit and hemoglobin concentration due to decreased plasma volume. Dehydration is also known to increase blood urea nitrogen (BUN) levels due to reduced renal blood flow.
For urinary analytes, excessive hydration has a direct effect on the excreted concentrations of electrolytes and metabolites. In random urine samples, creatinine serves as a determinant of the relative concentration of the urine. This is often used to control the variable dilutional effects, since many reference intervals are established per concentration of creatinine.