Eosinophils in psoriasis: A systematic review and meta-analysis introducing a study quality assessment tool for diagnostic pathology studies

Background: The histopathologic features of psoriasis are well-documented, but recent studies have highlighted atypical features, such as eosinophils, in clinically confirmed cases. Methods: A systematic review exploring eosinophils in psoriasis was performed. A novel quality assessment tool (SQAT-Path) we designed for cross-sectional pathology studies was employed. Results: Five studies (N = 218) were identified. The pooled prevalence of dermal eosinophils in psoriasis was 46% (95% confidence interval, 0.27–0.66). The prevalences of 1 to 5 lesional eosinophils (24%) compared to >5 eosinophils (26%) were similar. There was no association between eosinophils and prior treatment. There was also no association between eosinophils and spongiosis. In SQAT-Path, studies scored between 9 and 18 (out of a maximum of 27: “fair” to “good”), consistent with the ratings using other assessment tools. Conclusion: Eosinophils were found in approximately half of systematically studied and published cases of psoriasis. When present, their quantity is variable, with the likelihood of having greater than 5 eosinophils in a biopsy section comparable to having between 1 and 5. Greater than 5 eosinophils, as an isolated finding, would not be typical of psoriasis, but should not preclude its diagnosis without considering the overall histologic context.