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| | Besides laboratory inaccuracies, there are other factors which may influence test results: age, season during which the patient is tested, prior or current corticosteroid therapy. | | Besides laboratory inaccuracies, there are other factors which may influence test results: age, season during which the patient is tested, prior or current corticosteroid therapy. |
| − | Intradermal Skin Testing
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| − | Intradermal skin tests (IST) detect the ability of allergens injected intradermally to cause mast cell degranulation leading to a subsequent wheal and flare reaction. Aqueous allergens are used for testing. Allergens are selected based upon the region in which the patient lives. Tree, weed, and grass pollens as well as molds, house dust mites, insects and epidermals should be included in the test. In theory, testing should be performed utilizing the highest concentration of extract that does not cause an irritant reaction. Recommended test strengths for allergens are as follows: 1000 PNU/ml for pollens, 1:50,000 W/V for house dust mites, 250-500 PNU for epidermals and 1000 PNU/ml for insects.
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| − | Aqueous allergen extracts lose potency over time, particularly when in their more dilute form for testing. Diluted allergens should not be kept for longer than 2 weeks. They should be stored at 4C. Prior to testing, allergens should be allowed to reach room temperature. Once testing is completed, allergens should be returned to the refrigerator as soon as possible.
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| − | Test results can be affected or inhibited by numerous factors: medications, sedatives, stress, etc.
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| − | Antihistamines: Hydroxyzine has been shown to inhibit skin test reactivity, and it is presumed that other antihistamines will have the same effect. A withdrawal period of 10 days is recommended, and longer for antihistamines with longer half-lives (ceterizine, etc).
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| − | Tricyclic Antidepressants: Histamine reactivity is affected by doxepin in people for up to 11 days.
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| − | Glucocorticoids: Withdrawal of topical glucocorticoids for 3 weeks has been suggested, oral glucocorticoids a minimum of 3 weeks, and repositol steroids for a minimum of 8 weeks. However, the optimal times have not definitively been established. Patients that have been receiving glucocorticoid therapy for extended periods of time may require a longer withdrawal period, and some dogs may have positive results with a shorter withdrawal period.
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| − | Inflamed or infected skin makes IST difficult. Ideally, patients should be treated for their pyoderma or Malassezia dermatitis prior to testing.
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| − | Sedation is recommended for IST as testing is more easily performed. Sedatives that have been demonstrated to not affect test results include xylazine HCL, Medetomidine, and Tiletamine/zolazepam. Diazepam, oxymorphone, acepromazine and propofal may all adversely affect test results. The lateral thorax is routinely used for testing. The site is clipped with a #40 blade.
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| − | Intradermal injections are made with 26-27 g needles. Sharp needles ensure less bruising and easier injections. Positive (histamine phosphate 1:100,000) and negative (0.9% buffered saline) control injections are made to help ensure test accuracy. Injections are made utilizing approximately 0.05-1.0 ml solution. Injections are made based on "bleb" size. The injection blebs should be of approximately the same size at each injection site. Reactions are most commonly evaluated subjectively on a scale of 0-4 (compared to the positive and negative controls). Reactions are read at approximately 15 minutes. False positive reactions may occur with irritants, or allergens used at too high a concentration. False negatives may occur as well with drug interference, host factors, time of year, and improper technique.
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| | ===Other Tests=== | | ===Other Tests=== |