CPC05 34BE12 is the most reliable marker for keratin-derived primary cutaneous amyloidosis

Abstract Congo red (CR) is traditionally the main stain used by pathologists to detect amyloid deposition. However, CR has limitations in the detection of keratin-derived amyloid (amyloid-k) in skin biopsies. These include variable sensitivity and lack of specificity, with staining also observed in solar elastosis, dermal collagen and other cutaneous deposits, partly reflecting different staining protocols. Previous studies suggest that immunohistochemistry (IHC) for cytokeratin (CK) may be superior to CR in confirming primary cutaneous amyloidosis (PCA). We investigated the staining intensity of CR in comparison with a range of commercially available CK IHC in 15 retrieved cases of PCA. Tissue sections were cut from formalin-fixed paraffin-embedded blocks and stained with CR, CK5, AE1/AE3, MNF116 and 34BE12. The intensity of staining was scored as negative (−), weak (+1), moderate (+2) or strong (+3). Our cases included nine female and six male patients with a mean age of 46.8 years, including examples of macular amyloidosis or lichen amyloidosis. 34BE12 was the only marker that consistently highlighted the amyloid deposition with strong (3+) staining in all 15 cases, while the other markers gave varied results. 34BE12 is a mouse-derived monoclonal antibody specific to high-molecular-weight keratin, recognizing CK1, CK5, CK10 and CK14, thus differing from other commercially available CK IHC. Two previous studies also demonstrated 100% positivity for 34BE12, although they did not comment on the comparative strength of staining with other markers. Many centres continue to employ CR as the primary diagnostic tool in cases of possible PCA, which may lead to misdiagnosis. We propose 34BE12 as a sufficient and cost-effective marker for amyloid-k-derived PCA.