The stability of some of the components excreted in the urine are affected by the change in pH that occurs in the collected sample. Urine pH often becomes more acidic upon collection due to the presence and ongoing metabolism of bacteria and viable cells that are excreted. Certain analytes are better suited for a pH that is closer to what is found within the urinary system, while others require a more acidic pH to minimize the deterioration of the desired measurands. The importance of such preservatives is stressed when collecting a 24-hour urine sample to minimize the degradation associated with variable handling and storage conditions.
The commonly used urine preservatives include boric acid, hydrochloric acid, acetic acid, and oxalic acid. Certain buffers and bicarbonate salts may also be used for specific measurands. Laboratories need to evaluate the effectiveness and potential interference of preservatives. Coordinators of urine sample collection need to educate patients as to the safe and effective handling of the preservatives when providing urine collection instructions.