Thursday, December 3, 2009

Hyaline Casts

Hyaline casts are the most common type of urinary casts. Formed in the absence of cells in the tubular lumen, hyaline casts are solidified Tamm-Horsfall mucoprotein secreted from the tubular epithelial cells of individual nephrons. They have a smooth texture and a refractive index very close to that of the surrounding fluid. 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.

They are very difficult to see in wet preparations of urine and must be distinguished from mucus strands. Generally, hyaline casts have parallel sides with clear margins and blunted ends, whereas mucus strands are irregular in size with irregular margins. Reduced lighting is essential to see hyaline casts in urine sediment preparations.

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. 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.

A hyaline cast