Evaluation of a simple image‐based tool to quantify facial erythema in rosacea during treatment

Background Facial erythema is a common symptom in rosacea. To overcome subjectivity in scoring erythema severity, objective redness quantification is desirable. This study evaluated an image‐based erythema quantification tool to monitor facial erythema in rosacea patients during treatment and compared these values to clinical scores. Materials and Methods Twenty‐one rosacea patients were treated with topical ivermectin for 16 weeks. Clinical erythema scores and clinical photographs were taken at week 0, 6, 16 and 28. Using ImageJ, RGB images were split into red, green and blue channels to measure the green/red ratio of lesional skin compared with a green sticker. With CIELAB colour space, a* (indicating colour from green to red) of a lesional and non‐lesional facial site was measured, calculating ∆a*. Interobserver concordance and correlation between quantitative and clinical erythema values were determined. Results Treatment resulted in reduction of clinical erythema scores. No significant changes in red/green ratios were measured. Lesional a* and ∆a* significantly decreased from baseline to week 16 and 28 (P