Increase in concentration of patch test allergen reduces patch test occlusion time to 12 hours without affecting patch test reactivity in patients with parthenium dermatitis.

BACKGROUND: Patch testing is the standard method to diagnose contact allergy. Patches are applied for 48 hours, which is inconvenient to patients in tropical weather. Therefore, we evaluated different patch test occlusion times with increased concentrations of an allergen to determine if occlusion time can be reduced without compromising patch test reactivity. METHODS: Patch test positive patients with parthenium dermatitis were enrolled and patch tested using five different concentrations (10%, 4%, 2%, 1%, and 0.5%) of parthenium extract. The patches were applied in triplicate. The first set was removed after 12 hours, whereas the second and third sets were removed after 24 and 48 hours, respectively. Readings were performed at 24, 48, and 96 hours. RESULTS: Fifty patients with parthenium dermatitis were included. The positive patch test reaction rates were comparable in all three sets at 24- and 48-hour readings irrespective of the occlusion time. All were positive, with 10%, 4%, and 2% concentrations at 96-hour reading with an occlusion time of 12 hours. CONCLUSION: An occlusion time of 12 hours seems adequate to elicit positive patch test reaction at a 96-hour reading if the concentration of patch test allergen can be increased, that is, from 1% to 2% in these patients.