Red blood cell casts are urinary casts whose presense in the urine is always pathologic, and is strongly indicative of glomerular damage, which can occur in glomerulonephritis from various causes or vasculitis, including Wegener's granulomatosis, systemic lupus erythematosus, post-streptococcal glomerulonephritis or Goodpasture’s syndrome. They can also be associated with renal infarction and subacute bacterial endocarditis.
Some times red blood cells stick together and form red blood cell casts. Such casts are indicative of glomerulonephritis, with leakage of RBC's from glomeruli, or severe tubular damage. They are a yellowish-brown color and are generally cylindrical with sometimes ragged edges; their fragility makes inspection of a fresh sample necessary. They are usually associated with nephritic syndromes.