Diagnosis of renal amyloidosis using Congo red fluorescence.

Early diagnosis and classification of amyloid deposition and differentiation from other glomerular fibrillar deposits relies on routine Congo red (CR) histochemistry. Congo red fluorescence (CRF) is an alternative method based on examination of the CR-stained section by ultraviolet (UV) light. The aim of this study is to investigate the usefulness of CRF, especially when applied to frozen kidney sections. Congo red fluorescence was applied to sections of frozen kidney biopsies prospectively and to paraffin sections retrospectively. The findings of CRF were compared to CR staining in bright light. Prospectively, 15 cases of amyloidosis were diagnosed on frozen sections and identical CR staining was found in all of the paraffin-stained sections. There were no false positives or negatives. Retrospectively, 146 renal biopsies previously stained with CR were re-evaluated with CRF. Eighty-seven CR positive cases were confirmed by CRF, and one new case was identified. Congo red fluorescence is simple to perform and more pronounced, therefore easier to evaluate than CR in bright light. Congo red, when combined with immunohistochemistry, is still visible under UV whereas CR is masked in bright light. Although not widely used, the CRF method for detecting amyloid is simple to use with a high specificity and sensitivity, and may be applied successfully to frozen sections.