(c) Pus-casts (see [Fig. 62]), composed almost wholly of pus-corpuscles, are uncommon, and point to a chronic suppurative process in the kidney.
(d) True bacterial casts are rare. They indicate a septic condition in the kidney. Bacteria may permeate a cast after the urine is voided.
| FIG. 46.—Hyaline and granular casts, mucous threads, and cylindroids. There are also a few epithelial cells from the bladder (Wood). |
The structures most likely to be mistaken for casts are:
(1) Mucous Threads.—Mucus frequently appears in the form of long strands which slightly resemble hyaline casts (Fig. 46). They are, however, more ribbon-like, have less well-defined edges, and usually show faint longitudinal striations. Their ends taper to a point or are split or curled upon themselves, and are never evenly rounded, as is commonly the case with hyaline casts.
(2) Cylindroids.—This name is sometimes given to the mucous threads just described, but is more properly applied to certain peculiar structures more nearly allied to casts. They resemble hyaline casts in structure, but differ in being broader at one end and tapering to a slender tail, which is often twisted or curled upon itself (Fig. 46). They frequently occur in the urine along with hyaline casts, and have no definite pathologic significance.
(3) Masses of amorphous urates or phosphates or very small crystals (Fig. 47), which accidentally take a cylindric form, or shreds of mucus covered with granules, closely resemble granular casts. Application of gentle heat or appropriate chemicals will serve to differentiate them. When urine contains both mucus and granules, large numbers of these "pseudo-casts," all lying in the same direction, can be produced by slightly moving the cover-glass from side to side. It is possible—as in urate infarcts of infants—for urates to be molded into cylindric bodies within the renal tubules.
| FIG. 47.—Calcium-oxalate crystals, showing a pseudo-cast of small crystals (Jakob). |
(4) Hairs and fibers of wool, cotton, etc. These could be mistaken for casts only by beginners. One can easily become familiar with their appearance by suspending them in water and examining with the microscope ([Fig. 57]).
| FIG. 48.—Renal epithelium from nephritic urine: a, Polyhedral epithelium in nephritis of scarlet fever; b and c, different grades of fatty degeneration in renal epithelium in chronic nephritis (X400) (after Bizzozero). |