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Section 2
Microscopic Examination of Organized Sediment


Urine sediment is divided into two groups, organized and unorganized.

Organized structures, which are primarily body cells and their derivatives, may be found in small numbers in all urine specimens. However, if they are present in any appreciable amount, they are usually associated with a pathological condition.

Catheterization of females may be required in rare cases to distinguish a pathological increase in the number of erythrocytes, leukocytes, and epithelial cells from an increase in these elements due to menstrual contamination. The principal organized structures in urine sediment are red blood cells, white blood cells, epithelial cells, and casts.


a. Appearance. The presence of large numbers of erythrocytes is pathological when contamination from menstrual discharge can be excluded. A few erythrocytes may be found in urine after exercise and are not considered pathologic. Red blood cellsí appearance varies considerably depending on the reaction, specific gravity, age, and so forth, of the specimen. Erythrocytes may be confused with yeast cells, fat droplets, or oxalate crystals, and therefore, should be positively identified by examination under high-dry objective. Yeast cells have a doubly refractile border which simulates the doughnut appearance of a red blood cell. Urate crystals may be red or reddish-brown, but they are usually much darker in appearance than red blood cells.

(1) Intact red blood cells (figure 3-1). In fresh urine, erythrocytes appear as lightly pigmented biconcave disks of uniform size. They are about 7 to 8 microns in diameter. They may be intact and have the characteristic shiny surface with a blue- green tint. When blood is present in a large amount, it may impart a color to the urine.

(2) Crenated red blood cells (figure 3-2). Crenated red blood cells, or crushed cells, frequently have star-like shapes with margins displaying numerous sharp edges. This is due to the effect of osmotic pressure removing the internal red blood cell fluid and thus collapsing the cell. This type of cell is often encountered in concentrated urine due to its hypertonicity.

Figure 3-1. Red blood cells.

Figure 3-2. Crenated red blood cells.

(3) "Ghost" red blood cells (figure 3-3). In dilute urine specimens, the swollen ghost or shadow cell is frequently found. These cells have a larger than normal diameter. The swelling of these cells is caused by fluid flowing into the cell as a result of altered osmotic pressure. Ghost red blood cells are not always uniform in size and they may be circular or oval.

Figure 3-3. "Ghost" red blood cells.

b. Blood in Urine. Blood in the urine is a serious condition; however a few erythrocytes may be found in urine after strenuous exercise.

(1) Menstrual discharge. Blood in a urine specimen from a female may be due to contamination from menstrual discharge. However, it is not possible to determine whether all of the blood or only part of the blood is due to contamination with menstrual discharge. All blood in a urine specimen must be reported when it is detected.

(2) Kidneys. Blood from the kidneys or upper urinary tract is usually hazy, reddish, or smoky-brown in color.

(3) Lower urinary tract. If blood comes from the lower urinary tract, it is often a brighter red and is not so thoroughly mixed with urine. Fresh blood settles to the bottom more quickly, and small clots may be present.

c. Three-Bottle Specimen. A clue as to the site of the bleeding may sometimes be obtained by having the patient void three separate portions.

(1) First portion. If the blood is contained mainly in the first portion of the urine specimen, the bleeding point is probably in the urethra.

(2) Second portion. If blood is mixed uniformly in the second portion of the urine specimen, as well as in the first and third portions, the bleeding site is probably in the kidney or ureter.

(3) Third portion. If most of the blood is mainly in the last portion, the bleeding site is probably in the bladder.

d. Alkaline Urine. In alkaline urine, red blood cells are small in size or may be entirely disintegrated. To differentiate between erythrocytes and leukocytes, yeast cells, or contaminants, a drop of 10 percent acetic acid is added to the sediment. Red cells, if present, will dissolve while other structures remain unaffected.

e. Associated Protein. Urine that contains blood is always proteinaceous. A very small amount of blood may not be observed macroscopically. If large numbers of red blood cells are present, a positive protein will be obtained from the supernatant fluid of a centrifuged specimen.


A few leukocytes are present in normal urine, particularly when much mucus is found. They are numerous only as a result of a pathological process. Catheterization or a "two-bottle test" may be required to distinguish urethral infection from infection of other parts of the genitourinary system. The two-bottle test is conducted in the manner of the three-bottle test described previously. However, only two portions of urine are obtained instead of three. If the greater portion of leukocytes is found in the first portion of the urine specimen, a urethral infection is indicated. If the greater portion of the leukocytes is found in the second portion, an infection involving some other part of the genitourinary system may be suspected. The presence of increased numbers of white blood cells or pus constitutes a condition called pyuria.

a. Macroscopic Appearance. When abundant, white blood cells form a white sediment resembling amorphous phosphates.

b. Microscopic Appearance (figure 3-4). Leukocytes are true cells with well developed nuclei. Most white blood cells are neutrophils and are stainable with neutral dyes. Under the microscope they appear as colorless granular spheres, about 10 to 15 microns in diameter, and larger than red blood cells most of the time. The granules are composed of normal neutrophilic granules and granular products of degeneration. Diluted acetic acid can dissolve the granules and thus allow the nuclear characteristics to be seen. In freshly voided urine, many white cells exhibit ameboid motion and assume irregular outlines.

c. Alkaline Urine. In alkaline urine, white blood cells are often swollen, very granular, and tend to adhere in clumps. The addition of a drop of 10 percent acetic acid not only allows differentiation from erythrocytes but brings the nuclei more clearly into view.

d. Acid Urine. In moderately acid urine, white blood cells are well preserved. In strongly acid urine, they may be shrunken and irregularly shaped, suggesting ameboid forms.

Figure 3-4. White blood cells (leukocytes).

e. Decomposing Urine. When the urine is decomposing, white blood cells are destroyed and converted into a gelatinous substance.

f. Emphasizing the Nuclear Structure. At times, the nuclei may be obscured or hidden by the granules. Nuclei may be brought clearly into view by running a little dilute acetic acid under the coverglass placed over the drop of urine before examining microscopically.

g. Albumin. When abundant, white blood cells add an appreciable amount of protein to the urine in the form of albumin. At times, it may be necessary to determine whether the albumin in a specimen is due solely to pus. It has been estimated that 80,000 to 100,000 white blood cells per cubic millimeter increase the albumin by about 1.1 percent. If a greater amount of albumin is present than can be accounted for by pus, the excess is probably derived from the kidney.


a. General Appearance. A few cells from the epithelium of various parts of the urinary tract occur in every specimen of urine. A marked increase in the number of these cells indicates some pathological condition at the site of their origin. They may occur in "blocks," "clumps," or "sheets" of cells. One should be extremely cautious about making statements concerning the origin of any individual cell; only a pathologist can finally confirm the sites of origin of the cells. In addition, most cells are greatly altered from their original shape, and, due to degenerative changes, may be so granular that the nucleus cannot be seen. Many contain fat globules or glycogen vacuoles.

Figure 3-5. Renal tubular cells.

b. Renal Tubular Cells (figure 3-5). Renal epithelial cells are small, spherical, or polyhedral cells, about 20 microns in diameter. They are about the size of a white blood cell or slightly larger, colorless, and contain a large round nucleus. These cells may be binucleate or tetranucleate. Granules are usually present in the cytoplasm. These cells are believed to have their origin in the kidneys and come from the convoluted tubules and the loop of Henle. When they are polygonal in shape, dark in color, granular, and contain a rather large nucleus, they probably come from the renal tubules.

Figure 3-6. Transitional epithelial cells.

c. Transitional Epithelial Cells (figure 3-6). Transitional epithelial cells are much larger than the renal tubular cells. They are two to four times the diameter of white blood cells and may have various forms. Some can have a distinct round or oval nucleus; others may be pear-or spindle-shaped with tail-like projections. These are referred to as "caudate." Transitional cells have their origin in the posterior urethra, bladder, and ureters; the caudate variety originates in the neck of the bladder and the pelvis of the kidney.

Figure 3-7. Squamous epithelial cells.

d. Squamous Epithelial Cells (figure 3-7). The most common type of epithelial cell found in urine is the squamous variety. These are large, flat cells that usually have a small distinct nucleus. There may be occasional granules in the cytoplasm. Squamous cells are derived from the ureters, the superficial layers of the urethra and, rarely, from Bowman's capsule. In female patients, many large, squamous cells are frequently seen in the urine. These cells are from the vagina and labia and have no significance in renal disease except for the nuisance they cause by obscuring other elements of urinary sediment. When the number of squamous epithelial cells renders a valid examination impossible, catheterized urine should be obtained.


Casts are proteinaceous products of the renal tubules, which act as molds for the casts. Casts, therefore, are tubular in shape and are a gelatinous impression of the kidney tubules. Their presence in the urine usually indicates some pathological change in the kidney, although the change may be slight or transitory. They are rarely found in the urinary sediment of normal individuals. Since casts are formed in and forced out of the renal tubules, they vary in shape and size according to the site of their origin. They may also differ in length, thickness, and consistency. A positive protein is often found when many casts are present.

Normal and occasionally abnormal plasma proteins constitute the source of the protein involved in cast formation. These proteins are not reabsorbed in the proximal convoluted tubules. In the distal convoluted tubules and the collecting ducts, acidification of the urine, and the relative concentration of solutes due to water reabsorption favor coagulation of protein. In addition, a marked decrease in urine flow and the presence of abnormal ionic or protein constituents encourage cast development. After formation, most casts are washed out of the tubules into the urine by increased hydrostatic pressure from behind, which causes the tubules to dilate around them. A simple way to visualize the formation and variety of these structures is to regard the process as a gel formation. This gelling process is similar to events that occur in the preparation of a gelatin dessert. When the proper temperature and concentration of gelatin are obtained in the solution, there is a sudden increase in viscosity. If sliced fruit has been added to the fluid mixture, the fruit fragments are included within the gelatinized mass. In much the same manner red blood cells, white blood cells, and epithelial cells become trapped in the gelatinized casts and thereby preserve a record of the tubular contents for examination in the urinary sediment.

If the urine is very dilute or alkaline, these casts dissolve. Therefore, it is imperative that the specimens be analyzed as soon as possible. Under the microscope, casts generally appear as clear, slightly refractive cylinders and are best recognized by using low power with dim light. However, all casts should be verified by using high power. Higher magnification is important in classifying casts as to type.


a. Hyaline Casts (figure 3-8). The simple hyaline cast is composed primarily of protein and has no inclusions. It is actually the basic material for all types of casts and is often referred to as a "hyaline matrix." Hyaline casts are colorless, homogenous, and semitransparent structures with cylindrical bodies that have parallel sides and rounded ends. The length of a hyaline cast varies. Generally it is straight, but occasionally may be slightly rounded or convoluted.

Figure 3-8. Simple hyaline casts.

(1) Diagnostic significance. Hyaline casts are the least significant of all casts. Small numbers appear after anesthesia, fever, or excessive exercise and in cases of renal congestion and irritation. However, as hyaline casts are associated with proteinuria, they can occur in virtually any kidney pathology.

(2) Microscopic identification. Since the refractive index of the surrounding medium is nearly identical with the refractive index of hyaline casts, such casts are almost invisible. They can only be seen in subdued light with the microscope condenser at its lowest adjustment.

Figure 3-9. Granular casts.

b. Granular Casts (figure 3-9). Granular casts are about the same size as hyaline casts and are composed of common hyaline material in which numerous granules are embedded. This granular material consists of protein, disintegrated leukocytes or erythrocytes, fats, and degenerated epithelial cells. These casts appear in practically every type of kidney disorder. They are generally divided into two basic categories:

(1) Coarsely granular casts. If the epithelial cells or other materials do not become immediately incorporated within the hyaline material, they tend to degenerate into coarse granules. These granules then adhere to the casts, thereby forming coarsely granular casts. Since coarsely granular casts contain large granules, they are darker in color than finely granular casts. They can even be dark brown as a result of altered blood pigments.

(2) Finely granular casts. As the coarsely granular casts slowly pass on down the tubules, the cell degeneration continues until the granules are very fine. Thus, finely granular casts show a further degeneration of granules that have become much smaller in size than the coarse type. Since finely granular casts contain many minute granules, they are usually more opaque than simple hyaline casts. They are grey to pale yellow in color.

Figure 3-10. Waxy casts.

c. Waxy Casts (figure 3-10). Waxy casts, like hyaline casts, are homogenous. However, they are more opaque than hyaline casts and are a waxy yellow in color, resembling a structure made from paraffin. They tend to be short and broad with irregular broken ends. They can be distinguished from hyaline casts by a higher refractive index. Their size varies, and, at times, they may be extremely large and irregular. Waxy casts are considered to have remained in the tubules for a long time and represent the final stage in the deterioration of granular casts. They are indicative of localized oliguria or anuria and occur in cases of severe chronic renal disease.

Figure 3-11. Fatty casts.

d. Fatty Casts (figure 3-11). The breakdown of the epithelial lining of the tubules may produce fat droplets instead of granules. These fat droplets are incorporated into the cast matrix to produce a fatty cast. Fatty casts are quite similar to waxy casts in appearance. However, the inclusion of the relatively large fat droplets makes them more refractile than either granular or waxy casts; they are lighter in color than waxy casts. Fatty casts are insoluble in acetic acid, but they are soluble in ether. They stain orange with Sudan III or black with osmic acid. Fatty casts are usually seen in degenerative tubular disease, associated with tubular deposition of fat and lipoid material.

e. Pigmented Casts.

Figure 3-12. Hemoglobin casts.

(1) Hemoglobin-pigmented casts (figure 3-12). Hemoglobin-pigmented casts are sometimes called true blood casts or fibrin clots. They contain hemoglobin from degenerated red blood cells. These casts are homogenous in texture with no perceptible cell margins; they are yellow to orange in color. The true blood cast must be distinguished from the hyaline red blood cell cast since they have different diagnostic implications. Some renal disorders increase the permeability of the glomerular membrane and, consequently, permit the passage of fibrinogen and numerous red blood cells into the glomerular filtrate. Such conditions can result in the formation of blood casts. The passage of fibrinogen through the glomerular membrane is significant because of the difference in the molecular size of serum globulin, serum albumin, and fibrinogen. As the fibrinogen molecule is larger than the albumin molecule, the passage of fibrinogen indicates a greater degree of glomerular damage than the passage of albumin.

(2) Myoglobin-pigmented casts. Myoglobin-pigmented casts are darker than hemoglobin-pigmented casts. The presence of myoglobin indicates muscular degeneration and glomerular damage.

(3) Bilirubin-pigmented casts. Casts pigmented with bilirubin are usually homogeneous and greenish-yellow in color. The presence of bilirubin provides microscopic evidence of liver disease.


Cells can often adhere to a cast or become trapped within the cast matrix. When these entrapped cells are numerous, their names are used to designate the cast.

Figure 3-13. White blood cell casts.

a. White Blood Cell Casts (figure 3-13). These casts are generally the same size and shape as hyaline casts, and are basically hyaline casts filled with leukocytes. An occasional white blood cell occurring within a cast has no serious implications; it is only when the casts are nearly or completely packed with leukocytes that they are designated as white blood cell casts. At times, it may be difficult to distinguish a white blood cell cast from a degenerated epithelial cell cast since the leukocytes have often degenerated and the details of the cell structure are not clear. White blood cell casts can be differentiated from epithelial casts by treating the cast with dilute acetic acid. This causes the nuclei of the leukocyte to become plainly visible. Identification is not difficult if the leukocytes are well preserved with visible nuclei and cell borders. White blood cell casts are a sign of intrinsic renal disease and are seen in suppurative diseases such as pyelonephritis and inflammatory conditions such as glomerulonephritis. If white blood cell casts are present, a bacteriological investigation of the urine is necessary.

Figure 3-14. Red blood cell casts.

b. Red Blood Cell Casts (figure 3-14). Red blood cell casts are hyaline casts containing erythrocytes and are usually orange to red in color. These casts are filled with intact erythrocytes, and one can readily distinguish the typical spherical shape of the cells as well as the distinct cell margins. Many red blood cells must be present in the matrix to call the structure a red blood cell cast. If only a few red blood cells are present, the cast is reported as hyaline with inclusions. As mentioned previously, if the erythrocytes have degenerated so that only the characteristic orange-red color of hemoglobin is present, the cast is termed a hemoglobin or true blood cast. Red blood cell casts are pathological and are usually indicative of bleeding into the tubules or of glomerular damage. Red blood cell casts are found in lupus, acute glomerulonephritis, bacterial endocarditis, and septicemias.

Figure 3-15. Epithelial cell casts.

c. Epithelial Cell Casts (figure 3-15). When epithelial cells are sloughed off from the tubules, they tend to coalesce (grow together) and subsequently adhere to or become incorporated within a protein matrix. Such a structure is called an epithelial cell cast. These casts are usually swollen and tinged with a yellow or brown color. Generally, these casts are about the same size and shape as hyaline casts. They may also resemble white blood cell casts, although the epithelial cells within the cast may be larger than the leukocytes and usually show more fatty and hyaline cytoplasmic degeneration. Nevertheless, since they are frequently confused with white blood cell casts, 10 percent acetic acid is used to bring out the nuclei and aid in recognition of the cells. As explained previously, [para 3-10b(1), (2)], if the epithelial cells have deteriorated, granular casts, and ultimately, waxy casts are formed. Epithelial cell casts can signify aseptic degeneration of the renal tubules. If fat is present within the degenerating epithelial cells, the nephrotic syndrome may be indicated. The ingestion of phosphorus, carbon tetrachloride, or bichloride of mercury results in tubular necrosis that is manifested by the presence of large numbers of tubular epithelial casts containing deteriorated cells.

d. Mixed Cell Casts. Mixed cell casts sometimes appear in urine. They are about the same size and shape as hyaline casts and may contain white blood cells, red blood cells, and epithelial cells, or any combination of these structures. They are classified according to the predominant element present.


Occasionally, due to inexperience, someone may identify a structure as a cast only to discover upon reexamination that the object looked like a cast, but was actually something else.

Figure 3-16. Cylindroids.

a. Cylindroids (figure 3-16). Cylindroids are an unusual type of hyaline cast and are often called pseudocasts. They are composed of clear hyaline material and have ends which taper to slender, twisted, or curled tails. They are often irregular and striated and may contain fat globules. Cylindroids are usually found in conjunction with hyaline casts and proteinuria, although their origin and process of formation are unclear. They have generally the same diagnostic significance as hyaline casts and could possibly result from inflammation in the renal pelvis or ureter.

Figure 3-17. Mucus threads.

b. Mucus Threads (figure 3-17). Mucus threads are long, slender, transparent strands, which can occur normally in small numbers. Increased numbers tend to be present in various urinary tract infections or irritations. They are often twisted into various formations, and this characteristic aids in distinguishing them from casts.

David L. Heiserman, Editor

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Revised: June 06, 2015