Use of Colorimetry for the Measurement of Intradermally Injected Histamine-Induced Erythema in Healthy Dogs: A Proof-of-Concept Study.

The interpretation of intradermal test results may improve if wheal erythema is measured objectively using colorimetry. Our aim was to find the necessary number of erythema colorimetric measurements, taken before and after intradermal injections of histamine 0.01% (H1), histamine 0.001% (H2), histamine 0.0001% (H3) and a negative control (NC). We also aimed to assess whether erythema should be measured at the wheal center or border, and to investigate whether erythema appears beyond histamine wheals (flare), as observed in humans. Duplicate injections of the four solutions were administered on the lateral thorax of seven healthy dogs and erythema was measured by removing and repositioning the colorimeter probe. Colorimetry can differentiate the change in erythema (E) at the center (Ec), but not at the border, of the H1 wheals from the E of NC and H3 wheals. Reliable ways to calculate Ec were to use the first positive a* axis value of the colorimeter before and after the injection or to obtain the first seven positive a* axis values, excluding the highest and lowest, and calculated the mean of the remaining five. Unlike in humans, intradermal injection of histamine, even at a concentration of 0.01%, did not induce a flare reaction in healthy dogs.