Red cell casts contain intact and disintegrated or crenated red blood cells. A red cell cast appears yellow or reddish-brown when a large number of cells are enclosed within the matrix of the cast. The presence of red cell casts in the urine indicates bleeding into the nephron.
In order for a cast to be considered a red cell cast, the outlines of the red cells must be clearly visible in at least one area of the cast.
The image on the right is a red cell cast (400X magnification) viewed using brightfield microscopy.