CD03 Intraregional variation in response to allergen in patch testing

Abstract

Patch testing has been the gold-standard method for diagnosis of allergic contact dermatitis. Allergens are applied in chambers to the back with patches removed 48 h later and readings are undertaken at 48 h and 72–96 h. The upper back has historically been considered the optimal site of application given the extensive area and reduced chances of unintended removal of patches. There is limited literature on intraregional variations in response to allergens. One study utilizing Doppler flowmetry revealed a highly significant difference between inflammatory reactions on the upper and lower back (van Strien GA, Korstanje MJ. Site variations in patch test responses on the back. Contact Dermatitis 1994; 31: 95–6). We carried out analysis of 22 cases of suspected nickel and cobalt allergy in our contact dermatitis unit at a tertiary dermatology centre. Patients had been referred to the Contact Dermatitis Investigation Unit for exclusion of metal allergy either preoperatively or postoperatively, when they gave a history suggestive of metal allergy. Nickel and cobalt test patches were applied to the upper back as part of the standard series and were repeated on the lower back. Tests were applied and readings undertaken according to the International Contact Dermatitis Research Group guidelines, applied on Finn Chambers on Scanpor tape. Our first reading at 48 h found two patients to have tests that were negative to nickel on the upper back but positive on the lower back. Five patients had more positive reactions on the lower back, while three patients had greater reaction on the upper back. Day 4 readings showed greater positive reaction on the lower back in six cases. One case was positive on the upper back and negative on the lower back, and six cases were positive on the lower back and negative on the upper back. For cobalt on day 2, there was a more positive reaction in two patients on the upper back, and one patient had a positive reaction on the lower back but was negative on the upper back. On day 4, there were stronger positive reactions seen on the upper back compared with the lower back in four positive cases. Our early data suggest increased reaction on the lower back compared with the upper back for nickel. We postulate this might be secondary to the mild immunosuppressive action of sun exposure on the upper back. These results suggest that around 10% of cases of nickel allergy may be missed if the patient is only tested on the upper back, resulting in a risk of false negative result. In conclusion, in cases where it is important to exclude a particular allergen, the authors recommend that an additional test on the lower back should be considered.