The evaluation of Congo red staining combined with fluorescence microscopy in the diagnosis of primary cutaneous amyloidosis

Primary cutaneous amyloidosis (PCA) is a chronic pruritic skin disease. The apple-green birefringence of Congo red-stained amyloid under a polarized light microscope (CR-PLM) remains the gold standard in the diagnosis of PCA. However, there are some limitations to this approach. In this study, eighty-two paraffin-embedded biopsy skin samples were collected from patients with a clinical diagnosis of PCA. The sections were respectively stained with hematoxylin-eosin (HE), crystal violet (CV), and Congo red (CR) and observed under a light microscope. CR-stained sections were also observed under a polarized light microscope (CR-PLM) or an ultraviolet (UV)-emitted fluorescence microscope (CR-UFM). Further, 35 cases clinically diagnosed with psoriasis, lichen planus, and prurigo nodularis were selected as the negative control group. The positive rate of amyloid protein detected by CR-UFM (81.71%) was significantly higher than that detected by CR-PLM (70.73%, p = 0.004), CR staining (56.10%, p < 0.001), CV staining (30.49%, p < 0.001), or HE staining (28.05%, p < 0.001). In the control group, 34 (97.14%) cases were negative for amyloid deposits in CR staining, CR-PLM, and CR-UFM sections. The relative number of positive dermal papillae observed by CR-UFM (0.35 ± 0.27) was much more than that observed by CR-PLM (0.15 ± 0.17, p<0.001), CR staining (0.12 ± 0.16, p < 0.001), CV staining (0.07 ± 0.12, p < 0.001), or HE staining (0.05 ± 0.12, p < 0.001). The intensity of fluorescence by CR-UFM was significantly greater than that of the appl-green birefringence by CR-PLM (p < 0.001). Moreover, the amyloid was easily distinguished from the surrounding tissues using the CR-UFM method. In conclusion, the CR-UFM method was superior to CR-PLM, CR staining, CV staining, and HE staining in diagnosing PCA.