here - Australian College of Veterinary Scientists

‘False positive’ results in equine intradermal tests are thought to be common (see
further details under Interpretation of Results). To some extent, this may be a function of
using extracts that are too concentrated and thus induce an ‘irritant’ reaction where
true allergy is not present.
Extensive work into determination of allergen threshold concentrations for
intradermal testing in horses has not been undertaken. A recent study into insect
allergens suggests that concentrations lower than 1000pnu may be more appropriate
for horses (9), and offers some foundation for possible false positive reactions seen
previously. The recommended testing concentration varied with the allergen, but for
some allergens (particularly in the fly group) the concentration that evoked a positive
response in 25% of normal horses was 250pnu/mL or less.
Data is still being evaluated for a study the author has undertaken to determine
pollen, insect, dust mite, mould and storage mite allergen threshold concentrations in
normal horses. The aim of the study was to determine the concentration at which
90% of normal horses would score (subjectively) less than 2 on the scale of 0 to 4
commonly used by clinicians. The preliminary results suggest the following:
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Many of the pollen concentrations may need to be increased (ie more
concentrated)
Results of the insect panel testing support the findings of Morris and
Lindborg (9) that most concentrations need to be decreased from 1000pnu
Dust mite concentrations may need to be dramatically decreased (ie more
dilute)
Mould concentrations may currently be somewhat too dilute
The appropriate testing concentration of Culicoides appears to be around 1:10
000 w/v, although the manufacturer recommends substantially more dilute
concentrations than this (1:25 000 and 1:50 000)
Storage mites commonly evoke strong delayed reactions, but in light of the
immediate reading, higher concentrations than 1:10 000 may be more
suitable
It is apparent that the determination of threshold allergen concentrations is complex
and there are many possible variables. It seems unlikely that a single testing
concentration will be possible across the allergens for horses. The completed study
will also assess any variations in relation to season and skin reactivity in normal
horses. It is hoped that this data will greatly increase the reliability and accuracy of
intradermal testing in horses.
Solutions for skin testing should be made up fresh every 4 weeks, and stored in the
refrigerator (and not allowing them to freeze repetitively) to maintain appropriate
potency (16).
ACVSC Proceedings Dermatology Chapter Science Week 2005