The patch test is the most reliable method for diagnosing a metal allergy. However, interpretation of patch test results depends on the experience of the investigator and on the conditions of the individual patient. In the case of patch tests for metal salts, irritant reactions such as pustular or follicular reactions are quite common. Therefore, one must be careful not to interpret irritant reactions as allergic responses, and objective standards to evaluate erythema in patch tests need to be established.In the present study, we attempted to objectively evaluate patch test results for metal salts using an erythema index meter. We used the Erythema Index Difference (EID) (patch test site erythema index (EI) minus the mean EI of healthy skin outside the patch test area and at the unit tape site) rather than the EI itself in order to eliminate the effects of the patch test unit and individual differences. We measured the EID over time in patients with suspected metal allergies to various metals. Significant differences in EID were observed among ++ assessments, + assessments and irritant reactions based on International Contact Dermatitis Research Group (ICDRG) standards. In considering changes in EID over time, allergic and irritant reactions can be discriminated with high sensitivity (73.3%) and specificity (91.2%).These results suggest that the measurement of erythema over time with an erythema index meter is useful for the objective assessment of metal patch test reactions.