Ethylmercuric chloride: the responsible agent in thimerosal hypersensitivity.

The causative agent of thimerosal allergy (sodium ethylmercury thiosalicylate) has not previously been thoroughly investigated. To evaluate whether the organic mercury component or the thiosalicylic acid molecule induces thimerosal sensitization, 23 patients positive to thimerosal were patch tested with ethylmercuric chloride, thiosalicylic acid and 8 different derivatives of mercury. To date, ethylmercuric chloride has not been tested in thimerosal allergy. 19/23 patients (82%) showed positive patch test reactions to ethylmercuric chloride. 4/23 patients negative to ethylmercuric chloride reacted positively to thimerosal 0.1% but not to thimerosal 0.05%. 8/23 patients (35%) also reacted to other mercurials. 20 controls negative to thimerosal showed negative patch test reactions to ethylmercuric chloride. Neither patients nor controls reacted to thiosalicylic acid. These results indicate that testing with thimerosal 0.1% leads to false-positive reactions and that the ethyl mercury component is the responsible agent in thimerosal allergy.