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

## TL;DR

This study explores using a colorimeter to objectively measure skin redness in dogs after histamine injections, aiming to improve allergy testing accuracy.

## Contribution

The study introduces colorimetry as a novel objective method to assess histamine-induced erythema in dogs, contrasting with subjective interpretations.

## Key findings

- Colorimetry detected increased redness at the center of strong histamine wheals compared to weak solutions and saline.
- No flare reaction was observed in dogs, unlike in humans.
- Reliable erythema measurements were obtained using specific colorimeter data processing methods.

## Abstract

An intradermal test is commonly performed in dogs with allergies to determine whether they are sensitized to various environmental allergens. Unfortunately, the interpretation of test results is subjective and not highly reproducible. We investigated whether this could be improved by using a colorimeter, a device that measures the redness of the skin. The colorimeter was used to measure the redness of wheals that were induced in seven healthy dogs by intradermal injections of strong, intermediate and weak solutions of histamine and saline. Our results showed that colorimetric measurements detected a greater increase in skin redness at the center of the wheals induced by the strong histamine solution, compared to those induced by the weak histamine solution and by the saline. Additionally, we observed no increase in skin redness outside the histamine-induced wheals in these healthy dogs. This finding contrasts with observations in humans, where a red area, known as “the flare reaction”, surrounds the histamine wheals.

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 (ΔE) at the center (ΔEc), but not at the border, of the H1 wheals from the ΔE of NC and H3 wheals. Reliable ways to calculate ΔEc 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.