Page 1808 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 128: Anaphylactic and Anaphylactoid Reactions 1277

hydroxyindoleacetic acid can be measured if one is considering flush- by administering .01 to .05 mL of more diluted allergen intracutane-
ing due to the carcinoid syndrome. The measurement of various ously using a 26- to 27-gauge needle. A test is positive if the diameter
gastrointestinal vasopeptides, for example, substance P, neurokinins, of the wheal of the test allergen is ≥3 mm of the control. Skin tests may
vasointestinal polypeptide, pancreastatin is available as well. These be useful when drugs are suspected. 132,133 Skin testing to latex products
measurements may be useful to rule out the presence of a vasoactive may also be helpful. Latex-specific serum IgE antibodies can also be
134
peptide secreting tumor. Octreotide assisted CT scanning is also useful measured.
in this regard. Plasma-free metanephrine and urinary vanilmandelic Allergen-specific serum IgE antibody is an alternative to the skin test.
acid are employed if one is considering a paradoxical response to a The serum-specific IgE assay was originally performed using a radio-
pheochromocytoma. activity detecting procedure, and was referred to as radioallergsorbent
(RAST) tests. The RAST has been replaced with a second generation
LABORATORY TESTINGS FOR TRIGGERS assay employing an enzyme-linked detection agent (ELISA). In general,
OF ANAPHYLAXIS the ELISA assay is less sensitive but more specific than the intradermal
test. 45,135,136
Confirmation of the trigger for anaphylaxis (Table 128-8) should Challenge or provocation tests are the gold standard to confirm the
be evaluated carefully through meticulous history and selected skin causative agent. But challenge tests have definite limitations because of
test or allergen-specific IgE test or if necessary challenge testing with the inherent risk of provoking anaphylaxis and the time and resources
particular allergen if the risk is reasonably acceptable. Ideally this step involved in carrying out this procedure.
should involve an allergist/immunologist and can be performed in an In the future, in vitro tests might be helpful to distinguish between
outpatient setting after the patient is discharged from the hospital. Skin sensitization without risk of clinical reactivity versus sensitization
tests ideally should be done 3 to 4 weeks after the anaphylactic episode with risk of clinical reactivity. Examples of these tests include the
for the mast cell to recover and avoid a false-negative skin test. In measurement of basophil reactivity by the measurement of cell surface
84
contrast, testing for serum allergen-specific IgE antibody can be done markers after in vitro incubation with allergen, assessment of sensi-
137
at any time after the anaphylactic episode. Hemodilution secondary to tization by using recombinant allergens, peptide microassay-based
138
intravascular volume replacement during an anaphylactic episode can immunoassays to map IgE and IgG4 binding to sequential allergen
affect testing of serum-specific IgE because of the dilutional effect on epitopes, 138-140 and the assessment of allergen-specific cytokine or
circulating IgE. So it is ideal to repeat the skin test or the in vitro test chemokine production. 84
128
3 to 4 weeks after the anaphylactic event in the context of a convincing
history. It is important to remember that any positive skin test or in TREATMENT
vitro test does not conclusively establish the causative agent or establish
a diagnosis of anaphylaxis. It does establish sensitization to the sub- Since anaphylaxis is the most severe and potentially fatal form of the
stance tested. The test must be interpreted in light of the history. 84,129 immediate hypersensitivity reactions, immediate evaluation and treat-
For example, 60% of young people have a positive skin prick test to ment are critical to improve to chances of survival. The principles of
one or more foods, yet most of those with positive tests have never therapy for an acute attack of anaphylaxis have been summarized in
experienced anaphylaxis due to food. In addition, although positive Table 128-9. The medications and other agents used in the treatment of
130
skin tests and increased allergen-specific IgE levels correlate with an anaphylaxis have been summarized in Table 128-10.
of these tests do not necessarily correlate with the risk of future anaphy- ■ GENERAL EMERGENCY MEASURES
increased probability of clinical reactivity to a specific agent, the results
lactic episodes or with the severity of such episodes. 84,131 Skin testing is 1. Identify the inciting antigen and mechanism of exposure in order to
a two-step procedure involving the prick or puncture test followed by stop or limit its absorption.
the intradermal test if necessary. The prick test involves introducing the
allergen into the epidermis by means of a puncture. The allergens are 2. Prompt evaluation of the respiratory and cardiovascular systems,
applied to the skin along with a positive control (eg, histamine) and a with activation of rapid response system in the hospital setting or
negative control (saline). The immediate reaction (wheal and flare) is the 911 system in the community setting.
read at 15 to 20 minutes. A test is positive if the diameter of the wheal 3. Quick evaluation of airway patency and administration of supple-
of the test allergen is ≥3 mm of the control. Intradermal testing is done mental oxygen, up to 100%. If the anaphylactic reaction has

TABLE 128-8 Confirmation of a Potential Trigger for Anaphylaxis TABLE 128-9 Principles of Therapy in Anaphylaxis
Allergen skin tests Immediate action
Percutaneous (prick or puncture) Identify and remove the inciting antigen
Intradermal (intracutaneous) for selected allergens Assess the respiratory and cardiovascular systems
Allergen-specific serum IgE levels Establish an airway and provide supplemental oxygen
Quantitative ELISAs Establish good intravenous access
Allergen challenge tests Primary pharmacologic treatment
Foods or medications Epinephrine treatment (IM)
Exercise Intravenous Fluids (crystalloids or colloids)
Cold Secondary or adjuvant treatment
Workup of patients with idiopathic anaphylaxis H and H antagonists
1 2
Serum baseline total tryptase level Vasopressors
Evidence for urticaria pigmentosa Corticosteroids
Bone marrow biopsy Glucagon

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