A Manual of Clinical Diagnosis

by James Campbell Todd

The significance of the different varieties is more readily understood if one considers their mode of formation. Albuminous material, the source and nature of which are not definitely known, probably enters the lumen of a uriniferous tubule in a fluid or plastic state. It there hardens into a mold, which, when washed out by the urine, retains the shape of the tubule, and contains within its substance whatever structures and débris were lying free within the tubule or were loosely attached to its wall. If the tubule be small and have its usual lining of epithelium, the cast will be narrow; if it be large or entirely denuded of epithelium, the cast will be broad. A cast, therefore, indicates the condition of the tubule in which it is formed.

The search for casts must be carefully made. The urine must be fresh, since hyaline casts soon dissolve when it becomes alkaline. It should be thoroughly centrifugalized. When the sediment is abundant, casts, being light structures, will be found near the top. In cystitis, where casts may be entirely hidden by the pus, the bladder should be irrigated to remove as much of the pus as possible and the next urine examined. In order to prevent solution of the casts the urine, if alkaline, must be rendered acid by previous administration of boric acid or other drugs.

(1) Hyaline Casts.—Typically, these are colorless, homogeneous, semitransparent, cylindric structures, with parallel sides and usually rounded ends. Not infrequently they are more opaque or show a few granules or an occasional cell, either adhering to them or contained within their substance. Generally they are straight or curved; less commonly, convoluted. Their length and breadth vary greatly: they are sometimes so long as to extend across several fields of a medium-power objective, but are usually much shorter; in breadth, they vary from one to seven or eight times the diameter of a red blood-corpuscle. (See Figs. [2], 41, 42, and [46].)

FIG. 41.—Hyaline casts showing fat-droplets and leukocytes (obj. one-sixth) (Boston).

FIG. 42.—Various kinds of casts: a, Hyaline and finely granular cast; b, finely granular cast; c, coarsely granular cast; d, brown granular cast; e, granular cast with normal and abnormal blood adherent; f, granular cast with renal cells adherent; g, granular cast with fat and a fatty renal cell adherent (Ogden).

Hyaline casts are the least significant of all the casts, and occur in many slight and transitory conditions. Small numbers are common following ether anesthesia, in fevers, after excessive exercise, and in congestions and irritations of the kidney. They are always present, and are usually stained yellow when the urine contains much bile. While they are found in all organic diseases of the kidney, they are most important in chronic interstitial nephritis. Here they are seldom abundant, but their constant presence is the most reliable urinary sign of the disease. Small areas of chronic interstitial change are probably responsible for the few hyaline casts so frequently found in the urine of elderly persons.

Very broad hyaline casts commonly indicate complete desquamation of the tubular epithelium, such as occurs in the late stages of nephritis.

(2) Waxy Casts.—Like hyaline casts, these are homogeneous when typical, but frequently contain a few granules or an occasional cell. They are much more opaque than the hyaline variety, and are usually shorter and broader, with irregular, broken ends, and sometimes appear to be segmented. They are grayish or colorless, and have a dull waxy look, as if cut from paraffin (Figs. 43 and [61]). They are sometimes composed of material which gives the amyloid reactions. Waxy casts are found in most advanced cases of nephritis, where they are an unfavorable sign.

FIG. 43.—Waxy casts (upper part of figure). Fatty and fat-bearing casts (lower part of figure) (from Greene's "Medical Diagnosis").

Casts which resemble waxy casts but have a distinctly yellow color, as if cut from beeswax (so-called "fibrinous casts"), are often seen in acute nephritis. They have less serious significance than the true waxy variety.