Thimerosal and the relevance of patch-test reactions in children.

Thimerosal (C(9)H(9)HgNaO(2)S) is a mercuric derivative of thiosalicylic acid that goes by many names. It has been used as a disinfectant (eg, Merthiolate) and as a preservative in some vaccines, cosmetics, tattoo inks, eye drops, and contact lens solutions. It remains one of the least likely chemicals to cause contact allergy and for this reason has been removed from many standard screening series. Positive patch-test reactions (demonstrating sensitization) may still be seen (especially in older children); however, these reactions are not clinically relevant and are expected to be fewer, reflecting changes in current vaccine formulations.