C4d as a Practical Marker for Cutaneous Amyloidosis

Cutaneous amyloidosis (CA) is defined by the accumulation of amyloid in the dermis; it might be primary or secondary. The diagnosis is based on histopathological findings with the demonstration of amyloid deposits, confirmed by Congo red stain under the polarized light. Studies on other diagnostic markers are ongoing in the literature. The aim of this study was to demonstrate the utility of C4d staining in the recognition of amyloid in CA and using it as an alternative or substitute marker for the diagnosis. In this retrospective study, 199 skin biopsies with a clinical provisional diagnosis of CA were analyzed, the Congo red stain was performed, and, in a subgroup (n = 97) with histopathological findings probably for CA, C4d immunohistochemistry was assessed. Forty-eight cases of CA were detected. Congo red birefringence was positive in all cases, whereas in 14 cases, it was faded. In these 14 cases, the diagnosis of CA was made by means of Congo red fluorescence and Thioflavin T because the histopathological findings were highly suggestive for CA. All CA cases were positive with C4d, and in 12 of the 49 inflammatory dermatoses, C4d was positive. The interpretation of C4d immunohistochemistry can be performed more easily and rapidly than Congo red evaluation. The sensitivity and specificity of C4d were 100% and 75.5%, respectively. In our experience, C4d staining was a useful method for detecting amyloid deposits in CA. Although Congo red staining is the gold standard for amyloid detection, we propose C4d immunohistochemistry as a routine screening method or hybrid transition while further investigations are completed.