Allergic reactions to lower concentrations of nickel sulfate and formaldehyde often appear later than reactions to higher concentrations.

BACKGROUND

A late-appearing patch test reaction may be a sign of active sensitization or represent a delayed elicitation reaction.

OBJECTIVES

To retrospectively study the effect of concentration on the time course of allergic reactions to routine concentration dilution series of formaldehyde and nickel sulfate.

METHODS

We tested concentration dilution series of 2%, 1%, 0.32% and 0.1% formaldehyde and 5%, 1.6%, 0.5% and 0.16% nickel sulfate, respectively. The last readings were performed on day 4 to day 6. We included patients with allergic reactions to either of the two lowest concentrations in each dilution series and whose tests had been read three times.

RESULTS

Forty-two nickel-allergic and 23 formaldehyde-allergic patients fulfilled the inclusion criteria. In 26 (62%) of the nickel-sensitive patients, reactions to lower concentrations appeared later than reactions to the highest concentration. Of the formaldehyde-sensitive patients, 17 (74%) developed one or two allergic reactions to lower concentrations later than reactions to the highest concentration, and one (4%) patient developed allergic reactions to lower concentrations sooner than a reaction to the highest concentration. The remaining patients showed all allergic reactions at the same reading.

CONCLUSIONS

In these selected, relatively strongly sensitized patients, allergic reactions to lower concentrations quite regularly appeared later than reactions to higher concentrations.