Key Terms1

The study of urine is the oldest clinical laboratory test still performed. Historically, only the physical characteristics of urine were evaluated—color, clarity, odor, and taste. The latter characteristic—taste—has not been performed for centuries because of chemical methods that can be used to assess the “sweetness” of urine. The physical characteristics of urine continue to play an important part in a routine urinalysis. The presence of disease processes and abnormal urine components can be evident during the initial physical examination of urine.

The characteristic yellow color of normal urine is principally due to the pigment urochrome.1 A product of endogenous metabolism, urochrome is a lipid-soluble pigment that is present in plasma and excreted in urine. Patients in chronic renal failure, with decreased excretion of urochrome, may exhibit a characteristic yellow pigmentation of their skin caused by deposition of urochrome in their subcutaneous fat. Because urochrome production and excretion are constant, the intensity of the color of urine provides a crude indicator of urine concentration and the hydration state of the body. A concentrated urine is dark yellow, whereas a dilute urine is pale yellow or colorless. Urochrome, similar to other lipid-soluble pigments, darkens on exposure to light.² This characteristic darkening is often observed in urine specimens that are improperly stored. Small amounts of urobilin (an orange-brown pigment) and uroerythrin (a pink pigment) also contribute to urine color. Urobilin and uroerythrin are normal urine constituents; uroerythrin is most evident when it deposits on urate crystals, producing a precipitate often described as brick dust.

The terminology used to describe urine color often differs among laboratories. Regardless of the terminology used, an established list of terms should be available and used by all personnel in the laboratory. These terms should reduce ambiguity in color interpretation and improve consistency in the reporting of urine colors.³ Terms such as straw and beer brown should be replaced with light yellow and amber. The term bloody should be avoided. Although bloody is descriptive, it is not a color; red or pink would be more appropriate. Table 5.1 lists some appropriate terms to describe the color of urine and the substances that can cause these colors.

A fresh brown urine can indicate the presence of blood, hemoglobin, or myoglobin. Distinguishing among these substances is difficult, particularly between hemoglobin and myoglobin, because all three produce a positive chemical test for blood. Red blood cells are confirmed by microscopic examination, whereas the discrimination between hemoglobin and myoglobin requires additional urine chemical testing and possibly an evaluation of the blood plasma. Chapter 6 provides further discussion on the differentiation of hemoglobin and myoglobin in urine.

Bilirubin is another substance that can contribute to urine color. It is a byproduct of hemoglobin catabolism and has a characteristic yellow color. When present in sufficient amounts in urine or plasma, bilirubin imparts a distinctive amber coloration. However, upon standing or improper storage, bilirubin oxidizes to biliverdin, causing the urine to take on a greenish hue. Bilirubin is also susceptible to photo-oxidation by artificial light or sunlight; therefore, specimens must be stored properly to avoid degradation of this component. This photosensitivity is temperature dependent; optimal specimen stability is obtained by storing the specimens at low temperatures in the dark.

Some substances are colorless and normally do not contribute to urine color. However, upon standing or improper storage, they convert to colored compounds. Urobilinogen, a normal constituent in urine, is colorless, whereas its oxidation product urobilin is orange-brown. Porphobilinogen, a colorless and chemically similar (tetrapyrroles) substance, is a solute found in the urine of patients with abnormal porphyrin metabolism (heme synthesis). Porphobilin, the oxidation product of porphobilinogen, can impart a pink color to urine. As a result, urine that contains these substances can change color over time; this may alert the laboratorian to its presence and the need for additional testing. However, these color changes are often subtle and take hours to develop.

A multitude of urine colors results from ingested substances, and often the colors have no clinical significance. Highly pigmented foods such as fresh beets, breath fresheners containing chlorophyll, candy dyes, and vitamins A and B can impart distinctive colors to urine. Included in this group of ingested substances are numerous medications, some of which are used specifically to treat urinary tract infection. Other medications are present because they are eliminated from the body in the urine. Table 5.2 lists commonly encountered drugs and the colors they impart to the urine. It is worth noting that phenazopyridine, a urinary analgesic used in the treatment of urinary tract infections and often encountered in the clinical laboratory, imparts a distinctive yellow-orange coloration (similar to orange soda pop) with a thick consistency to the urine. This drug-produced color frequently interferes with the color interpretation of chemical reagent strip tests; alternative chemical testing methods (e.g., tablet tests, chemistry tests) must be used with these urine specimens.

Pathologic conditions can be indicated by the presence of certain analytes and components that color the urine. Substances such as melanin, homogentisic acid, indican, porphyrins, hemoglobin, and myoglobin or components such as red blood cells provide evidence of a pathologic process. In each case, urine suspected of containing these components requires additional chemical testing and investigation. Many of these substances are discussed individually along with the metabolic diseases that produce them in Chapters 6 and 8.

However, contaminants—substances not produced in the urinary tract—can also color the urine; these include fecal material, menstrual blood, and hemorrhoidal blood.

In summary, the color of urine is actually a combination of the colors imparted by each constituent present. To evaluate urine color consistently, the criteria outlined in Box 5.1 are necessary. Without attention to these details and to the use of established terminology, consistent reporting of urine color is not possible.

Moderate to large amounts of protein (albumin) in urine cause a stable white foam to be produced when the urine is poured or agitated (Fig. 5.1). Similar to egg albumin, the foam that develops is thick and long lasting. In addition, a larger volume of foam is easily produced by agitation of this urine compared with urine in which protein is not present.

When bilirubin is present in sufficient amounts, the foam if present will be characteristically yellow (see Fig. 5.1). This coloration may be noticed when the urine is being processed and the physical characteristics recorded. Although not definitive, this distinctive yellow coloration of the foam provides preliminary evidence for the presence of bilirubin.

Most substances that intensify or change the color of urine do not alter the color or characteristics of the urine foam. In other words, despite significant changes in urine color, the foam, if forced to form by agitation, remains white and readily dissipates.

Foam characteristics noted in the physical examination are not reported in a routine urinalysis; instead, they serve as preliminary and supportive evidence for the presence of bilirubin and abnormal amounts of protein in the urine. These suspected substances must be detected and confirmed during the chemical examination before either substance is reported.

Clarity, along with color, describes the overall visual appearance of a urine specimen. It is assessed at the same time as urine color and refers to the transparency of the specimen. Often called turbidity, clarity describes the cloudiness of the urine caused by suspended particulate matter that scatters light. The criteria outlined in Box 5.1 for assessing urine color also apply when evaluating urine clarity. An established list of descriptive terms for clarity used by all laboratory personnel ensures consistency in reporting and eliminates ambiguity. Table 5.3 defines common clarity terminology and provides a list of substances that produce these characteristics.

Table 5.3

Clarity Terms

Term	Definition	Possible Causes
Clear	No (or rare) visible particles; transparent	All solutes present are soluble. Note: The possibility of an abnormal solute such as glucose, proteins (albumin, hemoglobin, myoglobin), or bilirubin is not ruled out.
Hazy or slightly cloudy	Visible particles present; newsprint can be read when viewed through urine tube	Clarity varies with the substance and the amount present: • Blood cells—red blood cells (RBCs), white blood cells (WBCs) • Contaminants: powders, talc, creams, lotions, feces, etc.
Cloudy	Significant particulate matter; newsprint is blurred or difficult to read when viewed through urine tube	• Crystals of normal or abnormal solutes • Epithelial cells • Fat (lipids, chyle) • Microbes—bacteria, yeast, trichomonads • Contaminants: powders, talc, creams, lotions, feces, etc. Only gold members can continue reading. Log In or Register to continue Share this: Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Like this: Like Loading... Related Related posts: Urine Specimen Types, Collection, and Preservation Quality Assessment and Safety Renal Function and Assessment Renal and Metabolic Disease Stay updated, free articles. Join our Telegram channel Join Tags: Fundamentals of Urine and Body Fluid Analysis Oct 18, 2022 | Posted by admin in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Routine Urinalysis—the Physical Examination Full access? Get Clinical Tree Get Clinical Tree app for offline access Get Clinical Tree app for offline access %d