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