Discoid lupus erythematosus (DLE) can be clinically and histologically challenging to diagnose. Some laboratories provide immunofluorescence testing, but this requires fresh tissue. C3d staining for the diagnosis of DLE in formalin-fixed paraffin-embedded specimens was used to determine the sensitivity and specificity of detection of this complement protein. We tested 59 cases of DLE and a control set composed of 59 cases of lichen planus (LP), 9 cases of LP-like keratosis, 5 cases of cheilitis, and 3 cases of mucositis. Blinded assessment of the C3d staining was carried out by three pathologists. Fisher’s exact test, the independent t-test, and kappa analysis were used for analyzing the sensitivity and specificity of C3d, which were 44% and 97%, respectively. C3d staining in dermoepithelial junctions confirmed DLE and excluded LP. C3d staining in DEJ is specific for DLE but has low sensitivity.
