Cross-reactivity patterns of palladium and nickel studied by repeated open applications (ROATs) to the skin of guinea pigs.

Cross-reactivity is usually studied with patch test techniques, but the relevance of a single 1-2 day exposure under occlusion can be questioned. To study relevance, animals were induced with PdCl2 or NiSO4 according to the guinea pig maximization test method and then treated for 10 days according to the repeated open application test (ROAT) method. Animals induced with PdCl2 reacted in the ROATs to PdCl2 (100%) but rarely to NiSO4. Animals induced with NiSO4 reacted in ROATs to the same degree with NiSO4 and PdCl2 (23-30%). The concordance between pre-ROAT patch test results and ROAT outcome was high for PdCl2 (100%) and low (10-40%) for NiSO4. Patch testing seems to overestimate the risk of skin reactions when guinea pigs sensitive to PdCl2 are treated topically with NiSO4. The finding from patch test studies that animals induced with NiSO4 react only to NiSO4 but not to PdCl2 was not confirmed. Repeated open applications more adequately mimic exposure conditions than does patch testing.