Most urinary casts have a fairly homogenous aspect throughout their entirety and are clearly classifiable as either hyaline, granular (with all their variants), waxy, or of some cellular nature. However, in many instances urinary casts are not homogenous and contain features of more than one type of cast. Those casts are called mixed. In this post, we will center the discussion on mixed acellular casts.
Mixed acellular casts can be grossly divided in those containing elements of hyaline and granular casts (Figures 1 and 3), those containing features of more than one type of granular cast (Figures 2 and 3) and those containing elements of granular and waxy casts (Figure 2).
Because hyaline casts are found in conditions of tubular stasis and in the absence of overt tubular injury, a mixed hyaline/granular cast is considered a snapshot of an evolving process of a acute kidney injury when a functional or pre-renal state is gradually transforming into one of established acute tubular injury. As the tubular injury increases in magnitude or duration, light and dark granularity can be captured in a single cast. Furthermore, a granular/waxy cast can be commonly found in cases of overt and sustained acute tubular injury. There, the presumption is that a granular cast (typically muddy brown) may evolve into a broad waxy cast over time as the tubular atrophy becomes more pronounced.
In summary, mixed casts may be seen as snapshots of the natural history of acute tubular injury. However, it must be acknowledged that conclusive demonstration of this chronological perspective is lacking.