Assessment of the 'no eosinophils' rule: are eosinophils truly absent in pityriasis lichenoides, connective tissue disease, and graft-vs.-host disease?

Eosinophils are often present in the inflammatory infiltrate of an interface dermatitis, but the diagnostic specificity of eosinophils in interface dermatitis has not been formally evaluated. We retrospectively identified 97 examples of interface dermatitis with clinically confirmed diagnoses, including lupus erythematosus (LE), lichen planus, pityriasis lichenoides (PL), graft-vs.-host disease (GVHD), dermatomyositis (DM) and drug reaction. Diagnoses were clinically confirmed by at least two dermatologists. Slides were reviewed in a blinded fashion by at least two dermatopathologists. The average eosinophil count per 10 ×200 (×20 objective) fields was lowest for PL (0.2), DM (0.3), GVHD (0.4), and LE (0.5) [defined as Group 1] and was higher for lichen planus, drug reactions, erythema multiforme (major and minor) and viral exanthems [defined as Group 2]. Distinction between Group 1 and Group 2 was maximized using an eosinophil count cutoff of 1.1. In conclusion, eosinophils are usually rare to absent in PL, DM, most forms of LE and GVHD. While final interpretation requires a composite assessment of all features, our results suggest that the presence of even a single eosinophil within nine or ten ×20 fields argues against a diagnosis of PL, DM or LE.