Cobalt allergy: suitable test concentration, and concomitant reactivity to nickel and chromium

Cobalt allergy is frequent, but knowledge about exposure is limited. The patch test concentration and relevance of positive reactions are sometimes questioned.

To assess the suitability of cobalt 1% versus 0.5% for patch testing, and to analyse the co-occurrence of allergy to cobalt, chromium, and nickel.

Consecutive dermatitis patients (n = 656) were patch tested with cobalt chloride 0.5% and 1%, potassium dichromate 0.5%, and nickel sulfate 5%, all in petrolatum. Reactions were assessed on day (D)3, and on D6 or D7, and the reactivity and development of reactions were analysed.

Allergy to any metal was shown in 31% of patients, allergy to cobalt in 14%, allergy to chromium in 7%, and allergy to nickel in 20%. A significant proportion (37%) of cobalt allergy cases were missed by cobalt 0.5% versus 1%, whereas the reactivity profiles were similar. Cobalt allergy was solitary, without concomitant allergy to chromium or nickel, in 50% of patients.

Cobalt chloride 1% pet. is more suitable for patch testing than 0.5%. Solitary cobalt allergy is as frequent as concomitant allergy to cobalt and nickel or chromium. Sources of skin exposure to metals need to be identified for prevention of contact allergy. This is, owing to large knowledge gaps, particularly demanding for cobalt.