Increased Diagnostic Specificity of Congo Red Stain for Amyloid: The Potential Role of Texas Red–Filtered Fluorescence Microscopy

Context.—The tissue diagnosis of amyloidosis is traditionally suggested by hematoxylin-eosin stain and confirmed by Congo red stain, both examined by routine light microscopy. Both false-positive and false-negative congophilia are well documented, limiting the sensitivity and specificity of the Congo red stain for the diagnosis of amyloidosis. Examination of Congo red–stained tissue by Texas Red–filtered fluorescence microscopy (TRFM) is known to enhance the amyloid-specific congophilia, thus increasing the diagnostic sensitivity. Objective.—To determine whether TRFM can mitigate the false positivity and thus improve the diagnostic specificity of the Congo red stain. Design.—Ninety-two tissue samples were categorized into 3 groups. Group I included 15 samples with tissue deposition of amyloid. Group II consisted of 63 samples in which amorphous eosinophilic structures reminiscent of amyloid were seen on hematoxylin-eosin–stained tissue sections. Group III included 14 samples in which amyloid and amyloid-like tissue were seen side by side. The final diagnosis of presence or absence of amyloidosis in each case was established by clinicopathologic correlation. The congophilic areas in each case were identified by light microscopy. The same areas were then examined by TRFM. Results.—TRFM enhanced congophilia, confirming the diagnosis of amyloidosis in all group I cases. Enhancement was not seen in 52 of the 63 group II cases. For group III cases, TRFM enhanced the amyloid-specific congophilia, but not the nonspecific congophilia, in all cases. Conclusions.—TRFM increases the diagnostic yield and specificity of Congo red–stained tissue sections for detection of amyloid.