Lesson 1 Collecting, Preserving, and Examining Urine Samples
Collecting and Preserving Specimens
1-1. IMPORTANCE OF URINALYSIS
Purification of the circulating blood is crucial for the continuation of life. The urinary system is a group of organs that serves to remove from the blood nonvolatile waste products that cannot be removed through the respiratory system. The organ that has the major role in both filtration and reabsorption of necessary substances is the kidney. Consequently, urinalysis is an extremely valuable tool for demonstrating pathological conditions in the excretory system and as an index for the general metabolic condition of an individual. There are several different kinds of samples used in urinalysis.
1-2. RANDOM SPECIMEN
A random urine specimen is satisfactory for most qualitative tests and may be collected at any time. However, factors such as the type and amount of food consumed, and the performance of exercise must be considered when interpreting results. For example, an elevated urine sugar may be obtained after an exceedingly high carbohydrate meal. All random specimens should be freshly voided and delivered to the laboratory as quickly as possible. Urine is an excellent culture medium for many types of microorganisms. As bacterial growth and metabolism increase, decomposition of the urine proceeds rapidly. If a delay of several hours is unavoidable, the urine specimen should be kept in the refrigerator.
1-3. FIRST MORNING VOID
A first morning sample is collected when the patient rises in the morning. Only a clean container is necessary. Early morning specimens are used most frequently for analysis due to their day-to-day consistency. It is the most concentrated of the urine samples and is used for qualitative analysis. It is also essential for preventing false- negative pregnancy tests and for evaluating orthostatic proteinuria.
1-4. TWO-HOUR POSTPRANDIAL
This specimen is collected two hours after the patient has eaten a meal and requires only a clean container. The specimen is tested for glucose, and the results are used to monitor insulin therapy in patients with diabetes mellitus.
1-5. TWENTY-FOUR HOUR SPECIMEN
a. A 24-hour specimen is required in order to obtain significant results in the quantitative analyses. It is essential that a clean container and the proper preservative be used. The 24-hour specimen is made up of the total urinary output for a specific 24- hour period and to obtain an accurate timed specimen. It is necessary to begin the collection period with an empty bladder and end the collection period with an empty bladder. The procedure is as follows:
(1) Day one. First thing in the morning, the patient should void and discard that specimen, after which the remainder of urine is collected for the next 24-hours. The patient should be instructed to urinate into a separate urine collection cup and pour the contents into the 24-hour collection container (caused by the possibility of splashing preservative onto exposed skin).
(2) Day two. At the same time as the beginning of the first collection, the patient voids and adds this urine to previously collected urine.
(3) Storage. The patient should be advised to store partial collections at 4-6oC and deliver the completed 24-hour urine collection to the laboratory as soon as possible after completion.
b. Upon arrival in the laboratory, the 24-hour specimen must be thoroughly mixed and the volume accurately measured and recorded. Only an aliquot is needed for testing, but the amount saved must be adequate to permit repeat or additional testing.
1-6. SPECIAL METHODS OF URINE COLLECTION
When bacteriological studies are to be done, special collection techniques may be necessary to avoid contamination of the specimen.
Catheterization. Catheterization is used for some bacteriological tests performed on urine. However, even the most careful sterile technique cannot entirely prevent contamination of the bladder and the upper urinary tract during the passage of the catheter. This method is not used very often as it causes the patient much discomfort.
Midstream (Clean Catch) Specimen. A midstream specimen is used more often than a specimen from catheterization. Although this method does not eliminate contamination as much as catheterization, it is satisfactory if it is carefully collected.
With men, the glans penis should be adequately exposed and cleaned with soap or a mild antiseptic solution. The initial flow of urine should be allowed to escape, but the midstream urine should be collected in a sterile container.
With women, the urethral opening should be plainly exposed and well cleaned with soapy cotton balls. The area should be thoroughly rinsed with sterile, water-saturated cotton balls. The female patient should void the first portion of urine forcibly and then allow the midstream portion of about 20 to 100 ml to be caught in a sterile container.
Suprapubic Aspiration. Urine may be collected by external introduction of a needle into the bladder. The bladder is sterile under normal conditions. This collection method provides a sample for bacterial culture free of extraneous contamination and may be used for cytological studies.
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:
Refrigeration. The best general method of preservation up to 8 hours is refrigeration at 4-6șC. Refrigerated specimens are warmed to room temperature before performing an analysis.
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.
Formalin (10 percent). Ten percent formalin is an excellent preservative for the formed (microscopic) elements in urine. About 4 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.
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.
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.
Chloroform. Chloroform may be used as a preservative, but it interferes with some chemical tests and may cause cellular changes.
Sodium Fluoride. Sodium fluoride may be used as a preservative for urine samples when one is concerned with glucose. It inhibits tests for glucose on the reagent strip.
Sodium Carbonate. To preserve urobilinogen in urine requires special precautions. To assure alkalinity, a half-teaspoonful of sodium carbonate is placed in the specimen bottle before the urine is voided into the bottle.
Strong Mineral Acids. Analysis for amino acids, delta-aminolevulinic acid, and total nitrogen requires acidification with a strong mineral acid (for example, hydrochloric acid to pH 3.0).