Urine Specimen Types, Collection, and Preservation

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Urine Specimen Types, Collection, and Preservation

Learning Objectives

Key Terms1

The purposes of performing tests using urine are (1) to aid in the diagnosis of disease; (2) to screen for asymptomatic, congenital, or hereditary disease; (3) to monitor disease progression; and (4) to monitor therapy effectiveness or complications.1 Many urine tests are available in clinical and commercial laboratories. These tests may be quantitative assays that determine the level of a substance in the urine, such as assays that measure electrolytes, proteins, hormones, and other metabolic substances (e.g., porphyrins). Other urine tests are qualitative or screening tests. They are used to detect the presence or increased amount of a substance, such as rapid pregnancy tests, tests to detect microbial DNA and RNA (e.g., chlamydia, trichomonas), or a routine urinalysis—which provides a real-time “snapshot” of a person’s urinary tract and metabolic status.

The most commonly performed urine test is a routine urinalysis test. It is economical and provides valuable patient health information to healthcare providers. A routine urinalysis evaluates three aspects of the urine: 1) its physical characteristics, 2) its chemical composition, and 3) the microscopic sediment elements (e.g., epithelial cells, blood cells, casts, mucus) suspended in it. See Chapters 5, 6, and 7 for a detailed discussion of these three examinations that comprise a routine urinalysis.

To obtain accurate test results, urine specimen integrity must be maintained. If the urine specimen submitted for testing is inappropriate (e.g., if a random specimen is submitted instead of a timed collection) or if the specimen composition has changed because of improper storage conditions, testing will produce results that do not reflect the patient’s condition. In such situations, the highest quality reagents, equipment, expertise, and personnel cannot compensate for the unacceptable specimen. Therefore written criteria for urine specimen types, instructions for proper collection and preservation, appropriate specimen labeling, and a handling timeline must be available to all personnel involved in urine specimen procurement.

Why Study Urine?

Urine is actually a “fluid biopsy” of the kidneys and can provide a fountain of information about the health of an individual (Fig. 2.1). The kidneys are the only organs that can have their functional status evaluated by such a noninvasive means. In addition, because urine is an ultrafiltrate of the plasma, it can be used to evaluate and monitor body homeostasis and many metabolic disease processes.

Usually, urine specimens are readily obtainable, and their collection inconveniences a patient only briefly. Some individuals are uncomfortable discussing body fluids and body functions. Good verbal and written communication with each patient in a sensitive and professional manner can ensure the collection of a quality urine specimen. The ease with which urine specimens are obtained can lead to laxity or neglect in educating the patient and in stressing the importance of a proper collection. Note that if the quality of the urine specimen is compromised, so is the resultant urinalysis.

Specimen Types

The type of specimen selected, the time of collection, and the collection technique are usually determined by the tests to be performed. The three basic types of urine specimens are first morning, random, and timed collections (Table 2.1). Note that the ideal urine specimen needs to be adequately concentrated to ensure, upon screening, the detection of chemical components and formed elements of interest. These factors also depend on the patient’s state of hydration and the length of time the urine is held in the bladder.

First Morning Specimen

To collect a first morning specimen, the patient voids before going to bed and immediately on rising from sleep collects a urine specimen. Because this urine specimen has been retained in the bladder for approximately 8 hours, the specimen is ideal to test for substances that require concentration or incubation for detection (e.g., nitrites, protein) and to confirm postural or orthostatic proteinuria. Formed elements such as white blood cells, red blood cells, and casts are more stable in these concentrated acidic urine specimens. Because the number of epithelial cells present can be significant, these specimens may be used for cytology studies. The high osmolality of first morning specimens maintains the morphology of cellular components and reduces degeneration of renal casts.2 However, the high concentration of salts in these specimens can crystallize on cooling to room temperature (e.g., amorphous urates) and interfere with routine processing for cytologic studies. If the cellular morphology in this specimen type is determined to be suboptimal (i.e., signs of degeneration present), a random urine specimen can be collected.

Although the first morning urine is usually the most concentrated and is frequently the specimen of choice, it is not the most convenient to obtain. It requires that the patient pick up a container and instructions at least 1 day before his or her appointment; in addition, the specimen must be preserved if it is not going to be analyzed within 2 hours of collection.

Random Urine Specimen

For ease and convenience, routine screening is most often performed on random urine specimens. Random specimens can be collected at any time, usually during daytime hours and without prior patient preparation. Because excessive fluid intake and strenuous exercise can directly affect urine composition, random urine specimens may not accurately reflect a patient’s condition. Despite this, random specimens are usually satisfactory for routine screening and are capable of detecting abnormalities that indicate a disease process.

With prior hydration of the patient, a random clean catch urine specimen is ideal for cytology studies. Hydration consists of instructing the patient to drink 24 to 32 ounces of water each hour for 2 hours before urine collection. Most cytology protocols require collection of these specimens daily for 3 to 5 consecutive days. This increases the number of cells studied, thereby enhancing the detection of abnormality or disease. One method that can be used to increase the cellularity of a urine specimen is to have the patient exercise for 5 minutes by skipping or jumping up and down before specimen collection.

Timed Collection

Because of circadian or diurnal variation in excretion of many substances and functions (e.g., hormones, proteins, glomerular filtration rate) and the effects of exercise, hydration, and body metabolism on excretion rates, quantitative urine assays often require a timed collection. Timed collections, usually 12-hour or 24-hour, eliminate the need to determine when excretion is optimal and allow comparison of excretion patterns from day to day. Timed urine specimens can be divided into two types: those collected for a predetermined length of time (e.g., 2 hours, 12 hours, 24 hours) and those collected during a specific time of day (e.g., 2 PM to 4 PM). For example, a 4-hour or 12-hour specimen for determination of urine albumin, creatinine, and the albumin-to-creatinine ratio can be collected anytime and is an ideal specimen to screen for microalbuminuria. In contrast, a 2-hour collection for determination of urinary urobilinogen is preferably collected from 2 PM to 4 PM—the time when maximal excretion of urobilinogen is known to occur (in most individuals).

Accurate timing and strict adherence to specimen collection directions are essential to ensure valid results from timed collections. For example, if the two first morning specimens are included in a single 24-hour collection, the results will be erroneous because of the additional volume and analyte added. Box 2.1 summarizes a protocol for the timed collec-tion of a 24-hour specimen. This same protocol is applicable to any timed collection. A rule of thumb is to empty the bladder and discard the urine at the beginning of a timed collec-tion and to collect all urine subsequently passed during the collection period. At the end time of the collection, the patient must empty his or her bladder and include that urine in the timed collection.

Depending on the analyte being measured, a urine preservative may be necessary to ensure its stability throughout the collection. In addition, certain foods and drugs can affect the urinary excretion of some analytes. When this influence is known to be significant, the patient needs to be properly instructed to avoid these substances. Written instructions should include the test name, the preservative required, and any special instructions or precautions. The most common errors encountered in quantitative urine tests are related directly to specimen collection or handling, such as loss of specimen (i.e., not collecting all urine excreted in a timed collection), inclusion of two first morning samples, inaccurate total volume measurement, transcription error, and inadequate preservation.

Collection Techniques

Routine Void

A routine voided urine specimen requires no patient preparation and is collected by having the patient urinate into an appropriate container. Normally the patient requires no assistance other than clear instructions. These routine specimens, whether random or first morning, can be used for routine urinalysis. For other collection procedures, the patient may require assistance, depending on the patient’s age and physical condition or the technique to be used for collection (Table 2.2).

Table 2.2

Urine Collection Techniques
Collection Technique Description Use
Routine void

Midstream clean catch

Catheterized, urethral

Catheterized, ureteral

Suprapubic aspiration

Oct 18, 2022 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Urine Specimen Types, Collection, and Preservation
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