Amyloid Casts

The differential diagnosis of acute renal failure in patients with multiple myeloma includes light chain cast nephropathy, amyloidosis, and monoclonal immunoglobulin deposition disease, among other possibilities. On occasion, patients with light chain cast nephropathy have large lamellated silver-positive casts with prominent peripheral spicules (Figure 1 and 2) instead of the more characteristic fractured and refractile PAS-negative casts. Such casts usually exhibit light chain restriction by immunofluorescence, and they have been designated in the literature as “intratubular amyloid” or “tubular amyloid casts” because they show congophilia and apple-green birefringence with polarized light (Figure 3). However, in the absence of glomerular, vascular, or interstitial amyloid deposits, such congophilic casts do not necessarily indicate systemic amyloidosis.