Erythema‐index of clinical patch test reactions

Background: Methods for the precise non‐invasive and continuous grading of disease intensity are potentially of great use in a clinical setting. The erythema index offers a good method for assessing UVR induced erythema. Due to the development of a convenient hand‐held spectrometer, it is possible that the method could be used for the grading of eczematous reactions in a clinical setting as well. Objective: To assess the usefulness of the erythema index for the quantification of eczematous reactions using the Derma‐Spectrometer (Cortex technology, Hadsund, Denmark) in a clinical setting. Method: The erythema index of 56 patch test reactions ranging from +? to +++, was compared to regional controls and negative patch tests (189). The effects of intrumental application pressure was studied in 5 volunteers. Statistical analysis was carried out using Mann‐Whitney and Jonckheere‐Terpstra tests. Results: The erythema‐index was significantly higher in all degrees of patch test reactions than in uninvolved regional skin or negative patch tests. It also showed a significant positive trend for higher values in +, ++ and +++ reactions (P<0.0001). In direct comparison, the erythema index was unable to distinguish between?+ and +, or between ++ and +++ reactions. Only the distinction between + and ++ appeared significant (P<0.04). It is suggested that application pressure of the instrument is only of importance in high grades of erythema. Conclusion: Measurement of erythema index in eczematous reactions appear to be related to the intensity of the reaction, although the method does not reflect the many other changes which occur with increasingly strong reactions (oedema, etc.). The method therefore offers an objective quantitative supplement to clinical grading, especially in weak reactions. It is suggested that the method is better suited for experimental studies with a strict control of confounding factors.