Identification of crystals and casts by microscopic examination of the urinary sediment is of clinical importance. Specifics about crystals and casts have been previously covered in preceding posts of this blog. Interestingly, crystalline structures can occasionally be embedded into urinary casts.
Examples include calcium oxalate (typically monohydrate) crystals (Figures 1-2) and leucine crystals (Figures 3-4).
The use of polarized light is critical to identify crystal-containing casts.
The clinical significance of finding crystal-containing casts compared to finding “free-floating” crystals is unknown. In some cases, crystals can be “trapped” into casts by virtue of a favorable pH and urinary chemical composition and may only represent innocent bystanders. In other cases, crystals immersed into hyaline or granular casts could relate to the specific pathogenesis causing an acute or chronic kidney insult. Therefore, it is important to interpret their presence taking into account the clinical context. Although uric acid crystals are commonly found in acidic urine, casts containing uric acid crystals are extremely rare.
In addition, drugs excreted into the urine can form crystals and those crystals may be found within casts. For instance, casts containing ciprofloxacin crystals can be found in individuals exposed to the antibiotic who present with acute kidney injury (Figure 5).