Evaluation and relevance of isolated test reactions to cobalt.

During a 5-year period, 286 of 4,034 (7.1%) standard tested eczema patients reacted to cobalt. Fifty (1.2%) showed "isolated" cobalt reactions (i.e. without simultaneous reaction to chromium and/or nickel). A follow-up study of 36 of these patients included a serial dilution test (SDT). Eleven of 15 patients with negative reactions in the SDT had demonstrated weak reactions in the previous standard test, whereas stronger initial reactions had been noted in 19 of 21 patients who reacted positively. Sixteen SDT-positive and six SDT-negative patients were definitely or probably exposed to cobalt in their working or home environments. The majority of patients with "isolated" cobalt sensitivity also had other contact sensitivities (especially to rubber chemicals) or atopic dermatitis. Therefore in some cases of "isolated" cobalt reactions another contact sensitivity or skin disease may be a prerequisite for the development of cobalt allergy. It is however possible that the simultaneous presence of reactivities to different test substances is only a reflection of general skin vulnerability. An uncertainty in testing methodology must also be considered when evaluating test reactions to cobalt. Finally, present results show that cobalt allergy unassociated with contact sensitivity to other substances or other skin diseases is rare.