here - Australian College of Veterinary Scientists

Each injection site is visually examined and gently palpated for changes in size,
erythema and turgidity. Reactions are scaled subjectively as with small animals with a
score of 0 (no increase in wheal size, using the negative control as a reference) to 4
(large increase in wheal size, using the positive control as a reference), with 2 and 3
reactions being somewhere in between. Reactions greater than 2/4 are considered to
be positive (16). Some clinicians elect to also measure the wheal diameter to provide a
more objective assessment of reactivity.
Interpretation of Results
What are we expecting to occur in an allergic individual? At a cellular level, relevant
allergens will theoretically bind and bridge reaginic IgE antibodies on the surface of
mast cells and result in mast cell degranulation resulting in wheal and flare reactions
(16).
It has been suggested that positive intradermal reactions be interpreted with caution
in horses (1). Various studies have proposed that irritant reactions may be responsible
for false positive results and allergen concentration in the test kit may need to be
adjusted in horses as compared with small animals. In particular, allergens reported to
evoke strong reactions in horses include Lucerne, grain mill dust, grain smuts,
cottonseed, fireweed, yellowdock, Russian thistle, deer fly, Rhizopus spp., Candida
albicans, black fly, horse fly, and black ant (1, 2, 5, 6, 7, 8).
Several studies comparing reactions in normal horses and horses with a variety of
allergic skin and respiratory diseases have concluded that normal horses will have one
or more positive reactions, but that ‘allergic’ horses will have a significantly greater
number of positive reactions; although the significance of reactions to individual
allergens has been hard to interpret (1, 7, 8, 11, 12, 13, 14).
Possible causes of false positive and false negative results are summarized in a table 2,
extracted from Equine Dermatology by Scott and Miller (1).
ACVSC Proceedings Dermatology Chapter Science Week 2005 27