Anti-filaggrin antibodies, also named anti-keratin antibodies (AKA), are serum IgG labelling the stratum corneum of the rat oesophagus epithelium, detectable by indirect immunofluorescence (IIF). They constitute a specific marker for the diagnosis of rheumatoid arthritis (RA).^1–3

A laminar staining pattern is seen with most AKA-positive sera (fig 1A), but occasionally the cornified layer is diffusely labelled, and the image also shows foamy staining (fig 1B). Although most authors consider sera as AKA positive only when a linear laminated pattern is observed,^4,5 some consider the sera as AKA positive even when the staining is diffuse or foamy.⁶