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

Cross-reactivity is usually studied with patch test techniques, but the rel evance of a single 1-2 day exposure under occlusion can be questioned, To s tudy relevance, animals were induced with PdCl2 or NiSO4 according to the g uinea pig maximization test method and then treated for 10 days according t o the repeated open application test (ROAT) method. Animals induced with Pd Cl2 reacted in the ROATs to PdCl2 (100%) but rarely to NiSO4. Animals induc ed 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 t o overestimate the risk of skin reactions when guinea pigs sensitive to PdC l2 are treated topically with NiSO4. The finding from patch test studies th at animals induced with NiSO4 react only to NiSO4 but not to PdCl2 was not confirmed. Repeated open applications more adequately mimic exposure condit ions than does patch testing.