preservation of urine specimens

There is no substitute for a fresh urine specimen, and in all cases the analysis should be performed as soon as possible. A delay in analysis leads to a degeneration of the formed elements and decomposition of chemical constituents. Occasionally, however, the analysis has to be delayed, or a specimen must be shipped. When such situations occur, deterioration of the specimen may be inhibited by the use of some form of preservation .
The methods most commonly employed for preservation are the following

a. Refrigeration. The best general method of preservation up to eight hours is refrigeration at 4 to 6 C. Refrigerated specimens are warmed to room temperature before performing an analysis

b. Toluene (Toluol). If only the chemical contents of the urine are of interest, as with most 24-hour specimens, toluene may be used. Toluene merely lies on the surface of the urine, forming a thin layer and acting as a physical barrier to air and bacteria. However, anaerobic bacteria, if present, are not inhibited. To measure portions of the specimen, it is necessary either to remove the toluene or to pipet from below the surface

c. Formalin (10 percent). Ten percent formalin is an excellent preservative for the formed (microscopic) elements in urine. About four drops of formalin may be used for each 100 ml of urine. However, it interferes with some qualitative chemical tests, and it should not be used when the glucose concentration is to be determined

d. Boric Acid (0.8 percent). Boric acid is a satisfactory preservative for general purposes. It will not interfere with examinations for protein, sugar, or ketone bodies

e. Thymol (10 percent in Isopropanol). Thymol is another general purpose preservative. Approximately 10 ml of the prepared solution is used for each (24-hour collection)

f. Sodium Fluoride. Sodium fluoride may be used as a preservative for urine samples when one is concerned with glucose as it prevents glycolysis. However NaF does inhibits glucose tests using reagent strips

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Refrigeration. The best general method of preservation up to eight hours is refrigeration at 4 to 6 C. Refrigerated specimens are warmed to room temperature before performing an analysis
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