Venom skin tests and how they are conducted


Indications. Venom skin tests are best performed by specialists experienced using the technique and interpretation of the tests. Venom skin tests are suggested in patients with a history of systemic reaction to a sting, but not in those with large local reactions or in kids with isolated diffuse urticaria.

Testing supplies and preparation. Commercial venom protein extracts are purified preparations of Hymenoptera venoms that have been standardized in order to maintain a constant and reproducible response during skin testing and immunotherapy. There are 5 venoms available for testing: honeybee, yellow jacket, yellow hornet, white-faced hornet, and wasp venom.

These products are supplied as lyophilized preparations and should be reconstituted with a unique diluent containing 0.9% saline, 0.03% human serum albumin (HSA) (which stabilizes venom proteins and prevents their adsorption to the walls of the container), and 0.4% phenol.

Following reconstitution, the full-strength venom extract is diluted in a serial fashion to achieve the concentrations required for skin testing. Of note, the manufacturer's instructions concerning the storage of the lyophilized supplies (don't freeze) and storage times before expiration ought to always be observed. Whole body extracts of Hymenoptera insects don't contain enough venom proteins for either accurate diagnosis or therapy of insect sting allergy. An essential exception will be the situation of fire ant sensitivity, which can be tested and treated using entire physique fire ant extracts.

Techniques. The regular technique for venom skin testing will be the intradermal method, beginning with an appropriately low concentration, and then increasing till a positive result is obtained or the highest concentration is accomplished. Patients with a history of severe systemic reactions should be tested initially with a puncture technique using a venom concentration of 0.001 micro g per ml before proceeding towards the intradermal technique. Sensitization to multiple venoms may be present even when there has only been a reaction to a single insect.

As a result, skin testing should be performed with a complete set of the five Hymenoptera venoms, a negative diluent control, along with a positive histamine control. The preferred location for performing venom skin tests is on the flexor surface of the forearm. Interpretation of venom skin tests is based upon the size of the wheal and erythema, and the presence of pseudopodia.

Skin test results in patients with a convincing history are generally clearly positive, but can be negative in up to 30% of patients. There are three situations in which skin tests might be negative: (i) In a patient having a strongly positive history in whom the sting has occurred in the remote past, and might represent a loss of sensitivity. (ii) During the refractory period for 3 to 6 weeks following a sting reaction. It is therefore reasonable to perform skin tests during this period if there is a seasonal need to begin immunotherapy as soon as feasible.

However, if the results are negative, the tests should be repeated in 4 to 6 weeks. (iii) Some cases of sting anaphylaxis have been said to be non-IgE mediated and might be related to subclinical mastocytosis or merely toxic mast cell hyperreleasability. You will find reports of patients with sting reactions who had negative skin tests and experienced systemic reactions to subsequent stings. Most of these patients had a positive RAST, which suggests the significance of performing a serological test for venom-specific IgE antibodies in patients with a positive history and negative skin test.

There are numerous various patterns of venom skin test sensitivity. Skin tests are positive to all three of the common vespid skin test preparations (yellow jacket, yellow hornet, white-faced hornet) in 95% of vespid allergic patients. Notably, the degree of skin test sensitivity does not correlate reliably with the degree of sting reaction.

The strongest skin tests frequently occur in patients who've had only big local reactions and have a extremely low risk of anaphylaxis, whereas some patients who have had abrupt and near-fatal anaphylactic shock show only weak skin test (or RAST) sensitivity. In reality, almost 25% of patients presenting for systemic allergic reactions to stings had been skin test positive only at the 1.0 micro g per ml concentration, demonstrating the importance of skin testing using the full diagnostic range of concentrations.

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This article was sent to us by: Paula Lappin at 01182011

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