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).
Figure 1: Images of a cast (mixed hyaline / finely granular) containing numerous calcium oxalate monohydrate crystals. Urine specimen prepared with Sternheimer-Malbin stain. Panel A: bright field illumination; panel B: dark field illumination; panel C: polarized light; panel D: phase contrast microscopy; (400X).
Figure 2: Images of a cast (mixed finely granular / waxy) containing 2 calcium oxalate monohydrate crystals. Urine specimen with hyperbilirubinuria and prepared with Sternheimer-Malbin stain. Panel A: bright field illumination; panel B: dark field illumination; panel C: polarized light (partial); panel D: phase contrast microscopy; (400X).
Figure 3: Images of casts containing leucine crystals (arrows). Urine specimens from patients with acute kidney injury and cirrhosis and/or cholestasis (hyperbilirubinuria). Panel A: granular cast, bright field illumination; panel B: hyaline casts, phase contrast microscopy; panel C: granular casts, dark field illumination; (400X).
The use of polarized light is critical to identify crystal-containing casts.
Figure 4: Images of a cast containing 2 leucine crystals. Urine specimen from a patient with acute kidney injury and cirrhosis (hyperbilirubinuria). Panel A: bright field illumination; panel B: polarized light, note the “Maltese cross” sign; (400X).
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.
Figure 5: Images of hyaline casts likely containing ciprofloxacin crystals exhibiting clump-like (arrows) and needle-like shapes. Urine specimen from a patient with acute kidney injury visualized under bright field illumination (400X).
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).
