Cross-reactivity patterns of cobalt and nickel studied with repeated open applications (ROATs) to the skin of guinea pigs

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Abstract
The relevance of patch-test reactivity to chemicals on cross-challenge is hard to state, but it is generally assumed that the patient might risk a relapse of contact dermatitis when exposed to the cross-reacting compound(s). To study relevance by using the repeated open application test (ROAT) and applying the inducing allergen cobalt chloride (COCl2) or nickel sulfate (NiSO4) as well as the possibly cross-reacting compound (NiSO4 or COCl2) topically to guinea pigs. Animals were induced according to the guinea pig maximization test (GPMT) method, patch tested and then treated for 10 days using ROATs. Sensitivity thresholds were determined with serial dilution tests. Guinea pigs induced with COCl2 reacted in patch testing (100%) and in ROATs to COCl2 (93%) but not to NiSO4. Animals induced with NiSO4 reacted in patch testing to NiSO4 (100%) but not to COCl2, and in the ROATs to NiSO4 (41%) and less to COCl2. Our results support the assumption that the concomitant patch test reactivity is due to multiple sensitizations rather than cross-reactivity. We previously found that animals induced with palladium chloride (PdCl2) also reacted to NiSO4 on patch testing but not in the ROATs, indicating that the results from patch testing might overestimate the risk of a relapse. ROATs in patients with solitary and/or concomitant sensitivity to COCl2, NiSO4 or PdCl2 are desirable.

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