About blood casts
Blood casts are of prime clinical significance. Even a rare finding is indicative of renal "glomerular" bleeding. This finding can obsolete the need for invasive urologic tests to determine the origin of the hematuria. Blood casts are mostly found in glomerulonephritis. Since the final diagnosis of GN is obtained through a biopsy, precaution is a rule before reporting blood casts. Finding blood casts in the routine batch is a rare event. Everyone should be concious that over identification of blood casts is as bad for credibility as failure to recognize them.
Blood casts are usually found in a context of a proteinuria (slight or heavy), and hematuria. The hematuria is characterized by a high percentage of red cells dysmorphocytosis. Specimens with these figures, especially dysmorphocytosis, should systematically be scanned for blood casts. In some cases of GN, the proteinuria is minimal.
Like any natural coloration, the orange red color (Burnt sienna), typical of the blood casts, is best seen at low power field (obj.10x). At higher magnification, colors have a faded tint.
Red blood cells casts (erythrocytic casts)
Red blood cells casts are
described as hyaline casts, containing ghost red blood cells, or
as hyaline casts filled with numerous orange red erythrocytes.
The unpigmented form of red blood cells casts is more frequent.
Care must be taken not to confuse red blood cells casts with RBC
coated mucus threads mimic casts. This situation will usually
also show similar structures with completely different shapes.
The true blood casts
The true blood casts have a matrix resembling to
the waxy casts; the square ends and the typical notchs are
frequently seen. The color is the typical orange red, and the
texture is homogenous and rather granular. Identifiable red blood
cells are rare. In typical cases, it is not rare to see cast
fragments that would easily be unnoticed in some other
situations.
It seems that the true blood casts are a bit more specific to glomerular hematuria than the red blood cells casts. The cast inclusions are made of hematine arising from red cells degeneration, and from fibrinolysis of microclots.
Distinction between the true blood casts and the red blood cells casts is not always obvious. We are using a simple rule: if the matrix is hyaline, the cast is reported as a red blood cells cast, and if the matrix is waxy, the cast is reported as a true blood cast. Since both casts have a nearly indentical clinical value, this simplification is acceptable.
Hemoglobin casts / Myoglobin casts
Hemoglobin casts and myoglobin casts are hyaline casts having a protein pigmented matrix. In these cases, the pigmentation is acquired through the incorporation of the pigmented protein not by a breakdown process. The myoglobin or hemoglobin reaches the urinary space through filtration. This situation does not necessarily imply a glomerular disease. Myoglobin casts can be seen in massive muscular breakdown (crush syndrome) while hemoglobin casts can be seen in intensive intravascular hemolysis as, in malaria and catastrophic transfusion error. The dirty brown casts (heme degradation products pigmented granular casts) are sometimes difficult to differentiate from the blood casts or the hemoglobin casts. These casts are treated elsewhere.