Comparison Between Colorimetric and Subjective Evaluation of Intradermally Injected Histamine-Induced Erythema in Healthy Dogs.

BACKGROUND: Currently used methods for the interpretation of intradermal allergen test results either ignore the erythema of the wheals (objective method) or evaluate it subjectively (subjective method, and global wheal score [GWS]). OBJECTIVES: To compare the efficacy of GWSs in differentiating between wheals induced by intradermal injection of a negative control and histamine at three different concentrations, using either the objective colorimetric measurement of erythema or the subjective evaluation of erythema as absent, weak, or strong. ANIMALS: Seven clinically healthy dogs. MATERIALS AND METHODS: Duplicate intradermal injections of a negative control, histamine 0.0001%, histamine 0.001%, and histamine 0.01% were administered on the lateral thorax of the dogs. A colorimeter was used to measure erythema before and 15 min after the injection and, at the same time, three blinded investigators scored erythema subjectively. RESULTS: The subtraction of the first colorimetric measurement of erythema obtained before intradermal injection from the first postinjection objective measurement (ΔEfirst) outperformed three other objective measurement methods and yielded GWSs capable of differentiation among the four solutions with efficiencies comparable to those of GWSs based on subjective erythema assessment. However, the stronger correlation between the later and wheal diameters may indicate bias in the subjective evaluation of erythema. CONCLUSIONS AND CLINICAL RELEVANCE: A single colorimetric measurement of erythema before and after intradermal injection of a negative control and histamine at three different concentrations, followed by calculation of their difference, can replace the subjective evaluation of erythema for the calculation of GWSs and avoid possible bias.