Fig. 4: Algorithm for renal biopsy evaluation in patients suspected to have MGRS.

Kidney biopsy analysis in patients suspected to have monoclonal gammopathy of renal significance (MGRS) should include light microscopy (including staining the paraffin sections with haematoxylin and eosin, periodic acid−Schiff, Masson trichrome, Jones methenamine silver and Congo red). Immunofluorescence studies conducted on frozen tissue should include staining for IgG, IgM, IgA, C1q, C3 and κ and λ light chains. Finally, transmission election microscopy should be conducted. This standard renal biopsy approach enables diagnosis of MGRS in the majority of affected patients. In some individuals, ancillary techniques are needed to establish the diagnosis, including mass spectrometry, immunogold electron microscopy, immunofluorescence staining for IgG subtypes and paraffin immunofluorescence. The indications for these ancillary techniques are detailed in Table 2. AKI, acute kidney injury; eGFR, estimated glomerular filtration rate; MGUS, monoclonal gammopathy of undetermined significance; MIDD, monoclonal immunoglobulin deposition disease; TMA, thrombotic microangiopathy.