The finding of fibrils on electron microscopy of a renal biopsy specimen may be an extremely important clue in establishing the diagnosis. In fact, for many of the conditions listed below, the diagnosis may be based PURELY on a detailed ultrastructural characterization of the fibril, as in fibrillary or immunotactoid glomerulopathy.
The first step in the differential diagnosis of fibrillary glomerulopathies is to divide the condition into an IMMUNOGLOBULIN-MEDIATED versus a NON-IMMUNOGLOBULIN MEDIATED process, which is typically done by immunofluorescent staining against different heavy and light chain classes.
IMMUNOGLOBULIN-MEDIATED fibrillary glomerulopathies include:
-MIDD (monoclonal immunoglobulin deposition disease).
-type 2 cryoglobulinemia.
-immunotactoid glomerulopathy (see picture)
NON-IMMUNOGLOBULIN-MEDIATED fibrillary glomerulopathies include:
-diabetic fibrillary glomerulopathy.
(note: the latter two on this list are pretty rare).
Another crucial distinction here is whether the biopsy specimen stains negative or positive for CONGO RED (indicating the beta-pleating sheet structure of precipitated amyloid protein).