What is your diagnosis and what do you need to prove it?
The photomicrograph shows PAS negative intratubular casts that are focally broken and have a mild cellular reaction. While these casts are not a ‘textbook’ example, they are compatible with light chain cast nephropathy and immunofluorescence for kappa and lambda are necessary to determine restriction (lambda restricted in this case). Light chain cast nephropathy is a straightforward diagnosis in most cases, however, there is a spectrum to its morphology and when light chain restriction within tubules is present on immunofluorescence due diligence is needed to exclude this entity. In our practice we utilize a ‘PAS sandwich’ which includes serial sections for 1) Kappa IF, 2) PAS stain, 3) Lambda IF, and 4) Uromodulin IHC stain when we have a challenging case of suspected light chain cast nephropathy. This technique lets you directly compare individual casts for their light morphology (PAS), light chain status (kappa and lambda), and for the presence of uromodulin instead of light chains.
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