Variation and covariation in patch test reactivity to palladium and nickel salts

Concomitant reactions to palladium chloride (PdCl₂), sodium tetrachloropalladate (Na₂PdCl₄), and nickel hexahydrate sulphate (NiSO₄·6H₂O) are very common during patch testing and have mainly been explained by cross-sensitisation. Whether there is variation in reactivity to palladium or covariation to nickel and palladium is not known. The aim of this study was to investigate the variation in patch test reactivity to PdCl₂ and Na₂PdCl₄ over time and compare this to variation in patch test reactivity to NiSO₄·6H₂O. Fifteen females known to be sensitised to nickel and palladium were patch tested four times with 12-week intervals using a dilution series of NiSO₄·6H₂O, PdCl₂ or Na₂PdCl₄. Patch test reactivity to Na₂PdCl₄ was less variable compared to that for NiSO₄·6H₂O or PdCl₂. All test salts showed higher patch test reactivity during wintertime. No significant correlation was observed between the variation in patch test reactivity to Na₂PdCl₄ and PdCl₂ and the variation in patch test reactivity to NiSO₄·6H₂O during the entire test period. Patch test reactivity to Na₂PdCl₄ is less variable over time compared to that for PdCl₂ or NiSO₄·6H₂O. No clear covariation was identified between tests for palladium salts and NiSO₄·6H₂O. The variation in patch test reactivity found in this study could be due to seasonal changes.