The purpuric patch test in patients with allergic contact dermatitis from azo dyes.

The histopathological features of the purpuric patch test have been described in individual cases only. We report a series of patients with allergic contact dermatitis, who developed purpuric patch tests at the sites of allergens from the azo dye group. 105 patients were clinically evaluated and tested with the TRUE Test and the textile color & finish series (Chemotechnique Diagnostics) because of suspected clothing dermatitis. Positive results to the latter were found in 31 patients (29.5%). In 9 of these, purpuric patch tests were observed at the sites of the allergens Disperse Blue 124, 106 and 85. 10 biopsies were performed and studied. The histopathological changes of the purpuric patch test included: spongiosis (in 90% of cases), exocytosis (70%), and dilated blood vessels (100%) without signs of vasculitis, surrounded by an inflammatory infiltrate composed mainly of T lymphocytes. Extravasated erythrocytes were seen perivascularly, but also in the interstitium, surrounding the acrosyringium, at the dermoepidermal junction, and in the epidermis. Increased number of mast cells were found in 22.2% of cases. Disperse Blue 124, 106, and 85 are potent allergens that can elicit purpuric patch test reactions. The purpuric patch test in our cases was a manifestation of an allergic reaction, based not only on histopathological changes, but also on evolution and relevance of the patch tests.