Testing for allergy
Testing for mediators in the blood identifies the type of reaction but not the causative agent (due to poly-pharmacy).
1. Mast cell tryptase: Degranulation of mast cells results in the release of tryptase. Normal tryptase values in the blood are <13 ng/ml. Although non-specific for anaphylaxis, an increase in tryptase levels does indicate mast cell activation. False-positive results may occur from hypoxia, trauma etc.
Blood samples for testing should be drawn:
(i) immediately after management of the reaction and
(ii) 1-2 hours later.
The reasoning behind this temporal assay is that serum mast cell tryptase has a half-life of 90 minutes and peak concentrations occur 15-120 minutes after the allergic reaction.
Blood samples have to be drawn in an EDTA tube and the samples may be frozen prior to transfer to a laboratory.
2. Histamine: The use of histamine levels to detect an allergic reaction is largely impractical due to the rigorous criteria involved in storing and processing blood samples. False-positive reactions can occur from non-specific reactions.
3. IgE antibody: Due to non-availability of tests which are IgE specific to anaesthetic drugs, this test has limited practical value.
When allergic reactions involving anaesthetic agents are involved, skin tests may be performed to detect the possible causative agent. Clinical information is valuable to the immunologist performing the test. This consists of the following:
1. Type of surgery and anaesthesia
2. List of all drugs used prior to and during the procedure (includes skin disinfectants)
3. Temporal relationship between clinical events and drug administration
4. Degree of cardiovascular collapse or depression
5. Results of other tests if performed (tryptase etc.)
Skin testing for allergic reactions is usually performed 6 weeks after the reaction occurred. Prior to testing, antihistaminic drugs should have been discontinued for at least 24 hours. Other drugs that will suppress responses during skin testing are H2 receptor blockers, cough suppressants, antidepressants and topical steroids. By contrast, drugs such as β-blockers, non-steroidal anti-inflammatory drugs and opioids, including codeine, will enhance the response during skin testing.
It is important that all non-depolarising neuromuscular blocking agents (NMBAs) are tested, although, clearly, only one agent would have been part of the anaesthetic regimen. This is because there is a 70% chance of cross-reactivity among NMBAs.