Red blood cell cast

Urinary casts are microscopic cylindrical structures produced by the kidney and present in the urine in certain disease states. They form in the distal convoluted tubule and collecting ducts of nephrons, then dislodge and pass into the urine, where they can be detected by microscopy.

They form via precipitation of Tamm–Horsfall mucoprotein which is secreted by renal tubule cells, and sometimes also by albumin in conditions of proteinuria. Cast formation is pronounced in environments favoring protein denaturation and precipitation (low flow, concentrated salts, low pH). Tamm–Horsfall protein is particularly susceptible to precipitation in these conditions.

Casts were first described by Henry Bence Jones (1813–1873).

As reflected in their cylindrical form, casts are generated in the small distal convoluted tubules and collecting ducts of the kidney, and generally maintain their shape and composition as they pass through the urinary system. Although the most common forms are benign, others indicate disease. All rely on the inclusion or adhesion of various elements on a mucoprotein base—the hyaline cast. "Cast" itself merely describes the shape, so an adjective is added to describe the composition of the cast. Various casts found in urine sediment may be classified as follows.

The most common type of cast, hyaline cast are solidified Tamm–Horsfall mucoprotein secreted from the tubular epithelial cells of individual nephrons. Low urine flow, concentrated urine, or an acidic environment can contribute to the formation of hyaline casts, and, as such, they may be seen in normal individuals in dehydration or vigorous exercise. Hyaline casts are cylindrical and clear, with a low refractive index, so that they can easily be missed on cursory review under brightfield microscopy, or in an aged sample where dissolution has occurred whereas, on the other hand, phase contrast microscopy leads to easier identification. Given the ubiquitous presence of Tamm–Horsfall protein, other cast types are formed via the inclusion or adhesion of other elements to the hyaline base.

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