Urinary casts are tiny tube-shaped particles that can be found when urine is examined under the microscope during a test called urinalysis.
Hyaline casts can be present in low numbers (0-1/LPF) in concentrated urine of otherwise normal patients and are not always associated with renal disease.
Hyaline casts are formed in the absence of cells in the tubular lumen. They consist of Tamm-Horsfall mucoprotein and have a smooth texture and a refractive index very close to that of the surrounding fluid. 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 more variable in size with irregular margins. Reduced lighting is essential to see hyaline casts in urine sediment preparations. Lighting can be reduced by lowering the substage condenser (or close the iris diaphragm).
Hyaline casts are far easier to visualize using phase contrast microscopy, but this is not available on standard microscopes. They become more visible with regular light microscopy if fat sticks to the protein matrix (Tamm-Horsfall mucoprotein) that makes up the hyaline cast (hyaline with fat) or particulate material from degenerating cells is present within the cast matrix (hyaline to finely granular cast). Note that when fat droplets stick to hyaline casts, they are still called hyaline and not fatty casts.
A few hyaline casts may occasionally be found in normal urine, and hyaline casts may be seen after strenuous exercise, during fever, or when undergoing diuretic therapy. Greater numbers of hyaline casts may be seen in association with proteinuria of prerenal (e.g., overflow proteinuria as in myeloma) or renal (e.g., glomerular disease) origin. In such cases it has been proposed that the presence of excessive serum protein in the tubular lumen promotes precipitation of the Tamm-Horsefall mucoprotein. Not all cases of proteinuria will have hyaline casts.
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