In healthy persons, the urine contains small numbers of cells and other formed elements from the entire genitourinary tract: casts and epithelial cells from the nephron; epithelial cells from the kidney, pelvis, ureters, bladder, and urethra; mucous threads and spermatozoa from the prostate; and possibly RBCs or WBCs and an occasional cast. In renal parenchymal disease, the urine usually contains increased numbers of cells and casts discharged from an organ that is otherwise accessible only by biopsy or surgery (Tables 3.6 and 3.7). Urinary sediment provides information useful for both diagnosis and prognosis. It provides a direct sampling of urinary tract morphology.

The urinary sediment is obtained by pouring 1 mL of fresh, well-mixed urine into a conical tube and centrifuging the sample at a specific speed for 10 minutes. The supernatant is poured off, and 1 mL of the sediment is resuspended. A small drop is placed on a slide, cover-slipped, and examined microscopically.

The urine sediment can be broken down into cellular elements (RBCs, WBCs, and epithelial cells), casts, crystals, and bacteria. These may originate anywhere in the urinary tract. When casts do occur in the urine, they may indicate tubular or glomerular disorders.

Casts are the only elements found in urinary sediment that are unique to the kidneys. They are formed primarily within the lumen of the distal convoluted tubule and collecting duct, providing a microscopic view of conditions within the nephron. Their shapes are representative of the tubular lumen.

Cast width is significant in determining the site of origin and may indicate the extent of kidney damage. The width of the cast indicates the diameter of the tubule responsible for its formation. Cast width is described as narrow (as wide as 1–2 RBCs), medium-broad (3–4 RBCs), or broad (5 RBCs). The broad casts form in the collecting tubule and may be of any composition. Their presence usually indicates a marked reduction in the functional capacity of the nephron and suggests severe renal damage or end-stage renal disease.

The major constituent of casts is Tamm–Horsfall protein, a glycoprotein excreted by the renal tubular cells. It is found in normal and abnormal urine and is not detected by the urine dipstick method.