Amorphous phosphates are a common finding in the urinary sediment (Fig. 1 and 2).
Distinguishing phosphates from urates
Morphologically, amorphous phosphates look pretty much identical to amorphous urates by bright-field and phase-contrast microscopy.
Contrary to amorphous urates, which usually show a clear glow in polarized light, amorphous phosphates exhibit no birefringence (1). Whereas urates form in acidic urine, phosphates are observed in alkaline urine (2). Importantly, macroscopic inspection of the sediment oftentimes allows separation of the two (2): Amorphous urates tend to give the sediment a brick-red hue. In contrast amorphous phosphates are characterized by a chalk-like white sediment (Fig. 3).
Elements in the urinary sediment that resemble casts but are not formed in the kidney tubules are referred to as pseudocasts (1).
It is not unusual for amorphous phosphates to aggregate in castlike structures, which can resemble granular casts (Fig. 4-7). The absence of a cast matrix typically enables the identification of these aggregates as pseudocasts.
(all images taken with iPhone 5, microscopic pictures through Leica DMLB microscope)
- Fogazzi GB. The urinary sediment: An integrated view. 3rd ed. Italy: Elsevier Srl; 2010.
- Mundt L, Shanahan K. Graff’s textbook of Urinalysis and body fluids. United States: Lippincott Williams and Wilkins; September 28, 2015.