Abstract
Background: Cold urticaria (ColdU) is a common type of chronic inducible urticaria. Patients with typical ColdU develop wheals in response to standard cold stimulation tests (CSTs), an ice cube (ICT) or TempTest ® . As of now, the evaluation of CST response is visual and subjective. Validated, robust, and objective test readouts lacking today are needed. Methods: We subjected 63 patients (39 with typical ColdU and 24 with atypical ColdU) and 15 healthy controls (HCs) to TempTest ® CSTs and critical temperature threshold assessments. Blood microcirculation photoplethysmography (PPG) measurements were performed 5 min before and 10 min after the ICT on the volar forearm. Results: PPG amplitudes reflected normal baseline skin blood perfusion in patients with typical or atypical ColdU. Ice cube CSTs induced a marked increase in blood perfusion and PPG amplitudes in typical but not atypical ColdU, with distinct pre-post CST changes in PPG amplitudes in the former. The ratio of post-provocation and baseline PPG amplitudes ( R PPG ) in typical ColdU patients exceeded that in atypical ColdU patients and HCs more than 3-fold. Almost all typical ColdU patients (98%), but only 13% of atypical ColdU patients and 7% of HCs had R PPG >3. PPG results matched those of CSTs in 94% of all tested individuals. Conclusion: Photoplethysmographic assessments of CST responses appear accurate and provide objective readouts. PPG may be of use in diagnosing ColdU, distinguishing typical and atypical ColdU, and more precise threshold testing.