CONTENIDO DEL TFG

Ideally, the specimen for routine urinalysis should be exam- ined while fresh. If this is not possible, then it should be refrigerated until examined. Specimens left at room

temperature will soon begin to decompose, mainly due to the presence of bacteria in the sample. Urea-splitting bacteria produce ammonia, which then combines with hydrogen ions to produce ammonium, thereby causing an increase in thepHof the urine. This increase in pH will result in the decomposition of any casts which may be present, because casts tend to dissolve in alkaline urine. If glucose is present, the bacteria may use it as a source of energy which could then result in a false-negative test for glycosuria. Even if bacterial contamination is not present, some urinary com- ponents such as blood cells and casts still tend to deterio- rate on standing. However, if the pH of the sample is low and the specific gravityis high (1.015), deterioration will take longer to occur. Table 3-1 emphasizes these potential changes in unpreserved urine specimens.

There are times when a urine specimen must be saved for a longer period of time than is recommended. This is a com- mon occurrence when specimens are sent to commercial laboratories for analysis. There are several chemical preserv- atives that can be added to the specimen but most of them interfere in some way with the testing procedure. For

this reason, the routine use of preservatives is not recom- mended.

PRESERVATIVES

Preservatives that can be used to preserve random screening specimens include toluene, formalin, thymol, formalde- hyde-generating preservative tablets, and chloroform, boric acid, and chlorhexidine.

Formalin(1 drop/30 mL urine) is a good preservative for

urinary sediment but if used in too large a concentration it will precipitate protein and will give a false-positive test for reducing substances.2

Toluene(2 mL/100 mL urine) preserves ketones, pro-

teins, and reducing substances, but it is not effective against bacteria already present in the urine. Because toluene floats on the surface of the urine, it may be difficult trying to sep- arate the preservative from the specimen for testing. In addition, toluene is flammable.3

Thymol(one small crystal) is an adequate but rarely used

preservative for most urinary constituents. Thymol inter- feres with the acid precipitation test for protein, but it does not interfere with reagent strip tests for protein.3

Preservative tablets(1 tablet/30 mL urine), commercially

available, usually act by releasing formaldehyde. At this concentration the formaldehyde will not interfere with the test for reducing substances, but higher concentrations will result in false positives. Formaldehyde increases the specific gravity by 0.005/1 tablet/30 mL.4

Chloroformhas been used for inhibiting bacterial growth,

but it is not recommended for the routine specimen because it causes changes in the characteristics of the cellu- lar sediment.5

Boric acidpreserves formed elements but interferes with

the pH reading.6Boric acid is the preservative used in tubes used to preserve urine for culture and sensitivity. The Becton Dickinson brand is available with a gray-stoppered evacuated tube containing boric acid and sodium formate. This tube is not to be confused with the gray-stoppered blood collection tube that contains sodium fluoride and potassium oxalate.

Chlorhexidineprevents bacterial growth and is useful as

a glucose preservative.7The Becton Dickinson manufac- tures a red/yellow–stoppered conical evacuated tube that contains chlorhexidine, ethylparaben, and sodium propi- onate. Although specimens transferred to this tube for transport are stable for 72 hours, if not protected from light will yield erroneous bilirubin and urobilinogen results.6

TIMING OF COLLECTION

Arandomsample is usually sufficient for the performance

of most urinary screening tests; but, since the first specimen voided in the morning (first-morning)is more concentrated,

Table 3-1 Changes Occurring to Unpreserved Urine

POTENTIAL

CHANGE CHANGE OCCURRING OVER TIME

Color Oxidation of substances

Clarity Increased turbidity due to proliferation of bacteria or precipitation of chemical substances

Odor Increasing strength due to proliferation of bacteria

pH Increases as bacteria convert urea to ammonia and the loss of CO2from the

specimen

Bilirubin Decreasing due to photooxidation and hydrolysis

Glucose Decreases due to metabolism by microorganisms

Ketones Volatilization

Nitrite Increasing due to proliferation of bacte- ria, but also decreasing as bacteria con- tinue to convert nitrite to nitrogen Urobilinogen Decreasing due to oxidation Crystals Appearing due to cooling of the

specimen

Cells and Casts Decreasing due to cellular degeneration Microorganisms Increasing due to proliferation

28 Graff’s Textbook of Routine Urinalysis and Body Fluids

it is usually the specimen of choice. Samples collected ran- domly during the day are sometimes so dilute due to increased fluid consumption that they tend to give a false picture of the patient’s health.

There are some tests that are best if performed on speci- mens obtained at certain times of the day. For example, gly- cosuria is more readily detected on samples taken 2–3 hours after eating (postprandial), whereas urobilinogenis best eval- uated in a specimen collected in the early afternoon (2–4 PM

collection).

Because urinary substances are excreted in varying con- centrations throughout the day, it is necessary to collect timed specimens to accurately quantitate some substances such as creatinine, glucose, total protein, electrolytes, hor- mones, and urea. The most commonly used sample is the 24-hour specimen. In this procedure, the patient empties the bladder and discards the urine. This is usually done about 8 AM. All urine is collected for 24 hours thereafter,

including the sample at 8 AMthe next day. The container

that is used for the 24-hour specimen should be kept in the refrigerator during the entire collection period. Various chemical preservatives may need to be added to the collec- tion container depending on the substance to be tested. For some tests, such as creatinine and protein, refrigeration alone is sufficient. To get an accurate test result, it is impor- tant that all urine excreted during the timed period be col- lected. It is also important that the timing be exact. Because of the difficulty that is sometimes encountered when obtaining 24-hour collections, physicians sometimes order 12-hour or 2-hour timed specimens. However, if not prop- erly collected, these can give misleading results.

EXAMINATION OF PHYSICAL