Those crystals which can be found in alkaline urine include triple phosphates (ammonium magnesium phosphates), amorphous phosphates, calcium carbonate, calcium phos- phate, and ammonium biurates, also called ammonium urates (Fig. 5-34 (page 75)).
Triple Phosphates
Triple phosphate(ammonium magnesium phosphate) crys-
tals can be present in neutral and alkaline urines. Triple phosphate crystals are colorless prisms with from three to six sides that frequently have oblique ends (Fig. 5-35 (page 75)). Triple phosphates may sometimes precipitate as feath- ery or fernlike crystals. Triple phosphate crystals are soluble in acetic acid.
Triple phosphate crystals are frequently found in normal urine but can also form urinary calculi. Pathologic condi- tions in which they may be found include chronic pyelitis, chronic cystitis, enlarged prostate, and when the urine is retained in the bladder.21
Amorphous Phosphates
Phosphate salts are frequently present in the urine in a non- crystalline, amorphous form (Fig. 5-36 (page 76)). These granular particles have no definite shape and they are usually visibly indistinguishable from amorphous urates. The pH of the urine helps distinguish between these two amorphous deposits as well as does their solubility proper- ties. Amorphous phosphates are soluble in acetic acid, whereas amorphous urates are insoluble. Amorphous phos- phates have no clinical significance.
Calcium Carbonate
Calcium carbonatecrystals are small, colorless crystals ap-
pearing in dumbbell or spherical forms, or in large granu- lar masses (Fig. 5-37 (page 76)). Calcium carbonate crystals are larger than amorphous and, when in clumps, they may appear to have a dark color. The mass of calcium carbonate crystals, as opposed to a clump of amorphous phosphates, will also be connected together around the edges.
Calcium carbonate crystals have no clinical significance, and they will dissolve in acetic acid with the resulting evo- lution of carbon dioxide gas.
Calcium Phosphate
Calcium phosphatecrystals are long, thin, colorless prisms
and can have one pointed end, be arranged as rosettes or stars (stellar phosphates), or appear as needles (Fig. 5-38 (page 76)). Calcium phosphate crystals may also form large, thin, irregular plates that may float on the surface of the urine (Fig. 5-39 (page 77)). Calcium phosphate crystals are soluble in dilute acetic acid. These crystals may be present in normal urine, but they may also form calculi.
Ammonium Biurates
Ammonium biuratecrystals, also referred to as ammonium
urates, are found in alkaline and neutral urine. However, they may occasionally be found in acidic urine.10,21Ammo- nium biurates are yellow–brown spherical bodies with long, irregular spicules (Fig. 5-40 (page 77)). Their appearance is often described with the term “thorn apple.” Ammonium biurates may also occur as yellow–brown spheroids without spicules (Fig. 5-41 (page 77)), although this form is not that common.
Figure 5-33. Bilirubin crystals (500).
Triple phosphate
Calcium carbonate Ammonium biurate
Amorphous phosphates Calcium phosphate
Figure 5-34. Crystals found in alkaline urine.
Figure 5-35. Triple phosphate crystals. Note the oblique ends of the prisms (200).
76 Graff’s Textbook of Routine Urinalysis and Body Fluids
Figure 5-36. Amorphous phosphates (400).
Figure 5-37. Calcium carbonate crystals. The small
arrowpoints out the typical “dumbbell” form which is next to a large mass of calcium carbonate crystals (400).
Figure 5-39. Calcium phosphate plate or phosphate sheath (200).
Figure 5-40. Ammonium biurate crystals (500).
Figure 5-41. Ammonium biurate crystals without spicules (500).
78 Graff’s Textbook of Routine Urinalysis and Body Fluids
Ammonium biurates dissolve by warming and are solu- ble in acetic acid, with the formation of colorless uric acid crystals after standing. The addition of sodium hydroxide will liberate ammonia.26Ammonium biurates are abnormal only if found in freshly voided urine.27
CASTS
Urinary casts are formed in the lumen of the tubules of the kidney. They are so named because they are molded in the tubules. Casts can form as the result of the precipita- tion or gelation of Tamm–Horsfall mucoprotein,28,29 the clumping of cells or other material within a protein matrix,15,30 the adherence of cells or material to the matrix,31 or by conglutination of material within the lumen.12 The renal tubules secrete a mucoprotein called Tamm–Horsfall protein which is believed to form the basic matrix of all casts.28 Some casts may also contain serum proteins but they are usually confined to the cast granules.29In waxy casts, serum proteins are present in a homogeneous distribution.32
Factors that are involved in cast formation include urinary stasis (marked decrease in urine flow), increased acidity, high solute concentration, and the presence of abnormal ionic or protein constituents. Cast formation usually takes place in the distal and collecting tubules because there the urine reaches its maximum concentra- tion and acidification.20,33,34Casts will dissolve in alkaline urine35 and in neutral urine having a specific gravity of 1.003 or less.36 The presence of casts in the urine is fre- quently accompanied by proteinuria. However, casts can be seen in the absence of protein,26 making microscopic examination of urine an important tool in the diction of casts.
Casts have nearly parallel sides and rounded or blunted ends, and they vary in size and shape according to the tubules in which they were formed. They may be convoluted, straight, or curved, and they may vary in length. The width of the cast indicates the diameter of the tubule responsible for its formation. Broad casts, which can be from two to six times wider than ordinary casts,11,22are formed either in pathologically dilated or atrophied tubules or in collecting tubules. Broad casts are frequently referred to as renal fail- ure casts.
Casts are always renal in origin, and they are important indicators of intrinsic renal disease. Disorders in which cast may be present include glomerular damage, tubular damage, renal inflammation, and renal infection. Classifi- cation of casts is made on the basis of their appearance and the cellular components that they contain. The dif- ferent types of casts are hyaline, red cell, white cell, epithelial cell, granular (coarse and fine), waxy, and fatty.
At times, it may be difficult to distinguish the various casts because of degeneration, or because the cast may contain a variety of structures (mixed casts). It has been proposed that as cellular casts degenerate they form gran- ular casts that in turn degenerate, forming waxy casts (Fig. 5-42).
Casts are cylindrical in shape and do not have dark edges. Occasionally, waxy casts may appear to have a thin dark edge but only because the shiny surface of the cast comes to an abrupt ending. Usually, this thin dark edge will disappear when the fine adjustment is turned slightly. Any structure, therefore, that has dark edges is most likely a piece of fiber. In addition, any structure with parallel sides that is flat in the middle with thick edges is probably also a fiber. Remember, renal tubules are round, so casts will be more or less circular and will be thicker in the middle.
Casts are reported according to type and the number that is present per low-power field (100). Ranges reported are usually none seen, 0–2, 2–5, 5–10, 10–20/LPF.