Urine preservatives - Specimen collection:

Urine preservatives: preservatives have different roles but are usually added to reduce bacterial action or chemical decomposition or to solubilize constituents that might otherwise precipitate out of solution. Another application is to decrease atmospheric oxidation of unstable compounds. Some specimens should have no preservative added because of possibility of interference with analytical methods. One of the most satisfactory forms of preservation of urine specimens is refrigeration immediately after collection. Urine preservative tablets contain many compounds that act on decreasing the pH of the urine or releasing of formaldehyde. Formalin is also used, but in large amounts it precipitate urea and inhibit certain reactions.

Acidification below pH 3 is widely used to preserve 24 hr urine specimens and is particularly useful for specimens for calcium, steroid, vanillylmandelic acid (VMA) determination (10 ml of HCl, 6 mol/L, per 24 hr excretion).

Sulfamic acid (10 g/L urine) is also used to educe pH. Boric acid ( 5 mg/30 ml) has been also used During acidification, precipitation of urates will occur, thereby rendering a specimen unsuitable for measurement of uric acid. Although thymol and chloroform were widely used in the past to preserve specimens for chemical and microscopic examination. Toluene is the only organic solvent that is still in use as a preservative. A mild base, such as sodium bicarbonate or small amount of NaOH, is used to preserve porphyrins, urobilinogen, and uric acid. When timed collection is complete, the specimen should be delivered without delay to the laboratory.