here - Australian College of Veterinary Scientists

Table 2: Reasons for False Negative and False Positive Reactions in
Intradermal Test Reactions
Reasons for False Negative Reactions Reasons for False Positive Reactions
Subcutaneous injections
Too little allergen
Testing with mixes
Outdated allergens
Allergens too dilute
Too small volume of allergen
injected
Drug interference (Glucocorticoids,
antihistamines, tranquilisers,
progestational compounds, any
drugs that lower blood pressure
significantly)
Anergy (testing during peak of
hypersensitivity reaction)
Inherent host factors (estrus,
pregnancy, severe stress – systemic
disease, fright, struggling)
Off-season testing (1-2m after
clinical signs have disappeared)
Histamine ‘hyporeactivity’
28
Irritant test allergens
Allergen too concentrated
Contaminated test allergens (bacteria,
fungi)
Skin-sensitising antibody only (prior
clinical or present subclinical
sensitivity – these reactions would be
more appropriately termed clinically
insignificant)
Poor technique (trauma in needle
placement, blunt needles, too large a
volume injected, air injected)
‘Irritable’ skin (large reactions seen to
all injected substances, including
saline control)
Dermatographism
Ectoparasitism (cross reactions with
house dust mite extracts)
Mitogenic allergen
As further information about skin testing in horses comes to light, more accurate or
reliable skin test results and useful assumptions about the results can be expected. At
this stage, intradermal testing is still just one of the diagnostic modalities in the workup
of allergy cases and the importance of other details in the case such as clinical
signs, history, season of testing and other test results should not be overlooked.
ACVSC Proceedings Dermatology Chapter Science Week 2005