Why do you refrigerate a urine sample after collecting?

Answer 1

in short, decomposition and bacterial growth alter the urine sample unless some form of preservative (chemical and/or refrigeration) is used.

Many of the changes that occur in urine stored at room temperature relate to multiplication of bacteria. Preservatives for urinalysis, including refrigeration, generally work by inhibition of bacterial growth. The use of these chemical preservatives result in a specimen that is unsuitable for culture. Changes include an increase in urinary pH as urea is broken down to ammonia. In addition, casts tend to decompose, and red cells undergo hemolysis. Turbidity increases due to the growth of bacteria. The glucose level decreases due to metabolism by bacteria, while nitrates are converted to nitrite through bacteria action.

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A According to NCCLS Approved Guidelines on Urinalysis and Collection, Transportation, and Preservation of Urine Specimens, "In general, chemical preservatives should be avoided for urinalysis." (1) NCCLS recommends that urinalysis be performed within two hours of collection. If this is not possible, the urine specimen should be refrigerated as soon as possible after collection. The length of time a specimen can be held under refrigeration for adequate preservation has not been agreed upon -- different urine constituents remain unchanged under refrigeration for different lengths of time. Generally, the urinalysis should be completed within six to eight hours of collection.

Decomposition of urine begins within 30 minutes; therefore, ideally, urine should be examined within this time. Urine left at room temperature for more than two hours is usually considered unacceptable for examination. Refrigeration is generally adequate for constituents encountered in the routine urinalysis, with the exception of bilirubin and urobilinogen, which are labile to both heat and light. Refrigeration can result in the precipitation of urates or phosphates, which may obscure other pathologic constituents in the microscopic examination of the urine sediment. According to NCCLS, "If the urine is also to be cultured, it should be refrigerated during transit and held refrigerated until cultured."

There are urine preservation tubes commercially available for both urinalysis and culture that are said to preserve urine up to 72 hours without refrigeration. They are generally screw-on cap, leak-proof polypropylene tubes with conical bottoms and skirted freestanding bases, and each contains a preservative tablet such as mercuric oxide. The capacity of the tube is limited -- as little as 10mL. Culture tubes are also flat-bottomed and contain a different preservative such as boric acid-based preservative tablets. These tubes are not interchangeable; specimens that are to be cultured must be placed in the culture tube, and specimens for routine urinalysis, in the tube meant for urinalysis. Preservatives that interfere with any of the chemical tests included in the routine urinalysis are not acceptable. Since the preservation tubes are fairly small, a suitable collection container must be provided to the patient for collection of the sample, and the urine must be well mixed and poured into the preservation tub e -- all introducing a chance of processing error, plus the additional cost of the containers.

Many of the changes that occur in urine stored at room temperature relate to multiplication of bacteria. Preservatives for urinalysis, including refrigeration, generally work by inhibition of bacterial growth. The use of these chemical preservatives result in a specimen that is unsuitable for culture. Changes include an increase in urinary pH as urea is broken down to ammonia. In addition, casts tend to decompose, and red cells undergo hemolysis. Turbidity increases due to the growth of bacteria. The glucose level decreases due to metabolism by bacteria, while nitrates are converted to nitrite through bacteria action.

Other changes include a darkening of the urine color, due to oxidation of colorless chromogens such as the oxidation of urobilinogen to urobilin, or a change in color as bilirubin is oxidized to biliverdin. In alkaline urines of a low specific gravity, cells and casts begin to lyse. Leukocytes are especially subject to lysis as the urine stands at room temperature. If urine specimens contain glucose, the presence of leukocytes will result in decreased levels of glucose due to metabolism by the cells. If ketones are present, the level will decrease as acetone is converted to water and carbon dioxide or the ketone volatilizes.

In summary, refrigeration appears to be the best solution when urinalysis cannot be performed within two hours of collection. Your recommendation that all specimens be placed in coolers containing freezer packs until they are brought to the urinalysis section (assuming it is impossible to refrigerate the specimens at the collection site) seems to be an acceptable alternative to examination within two hours of collection.