2.3 Case-control study
To determine the potential biomarker of arsenic-related pruritus, we recruited 77 cases with severe itch, 95 cases with mild itch, and 57 controls with no itch from the participants in Shimen, and tested their serum levels of known biomarkers for itch, including β-endorphin, substance P, nerve growth factor, histamine, interleukin-31, IgE, endothelin-1, and 5-hydroxytryptamine. Participants of the three groups were roughly matched by age and sex. Participants with eczema, psoriasis or other conditions that may cause itch were excluded.