Anaphylaxis Workup

Updated: Feb 22, 2017
  • Author: S Shahzad Mustafa, MD; Chief Editor: Michael A Kaliner, MD  more...
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Approach Considerations

Because anaphylaxis is primarily a clinical diagnosis, laboratory studies are not usually required and are rarely helpful. However, if the diagnosis is unclear, especially with a recurrent syndrome, or if other diseases need to be excluded, some limited laboratory studies are indicated.


Lab Studies

Certain laboratory studies may be ordered in specific situations.

Histamine and tryptase assessment

If a patient is seen shortly after an episode, plasma histamine or urinary histamine metabolites, or serum tryptase measurements may be helpful in confirming the diagnosis. [2]

Plasma histamine levels rise within 10 minutes of onset but fall again within 30 minutes. Urinary histamine levels are generally not dependable, as this test can be affected by diet and by bacteria in the urine. Urinary histamine metabolites measurement is a better test but is not generally available.

Serum mature tryptase (previously called beta-tryptase) levels peak 60-90 minutes after the start of an episode and may persist for as long as 5 hours. The estimated positive predictive value of tryptase elevations in 259 subjects with anesthesia-associated anaphylaxis is 92.6%, and the estimated negative predictive value of normal tryptase levels is 54.3%. Serial tryptase measurements might improve diagnostic sensitivity, but further investigation is needed.

Basal levels of total and mature tryptase between episodes of anaphylaxis can be helpful to rule out systemic mastocytosis. Patients with mastocytosis constitutively produce large quantities of alpha-tryptase, while individuals with anaphylaxis from other causes have normal levels of alpha-tryptase at baseline between episodes of anaphylaxis. During anaphylaxis, a ratio of total tryptase (alpha + mature) to mature tryptase of 20 or greater is consistent with mastocytosis, whereas a ratio of 10 or less suggests anaphylaxis of another etiology.

Recent reports concerning insect sting anaphylaxis suggest closer scrutiny to baseline total tryptase levels, especially in patients who experienced hypotension during anaphylaxis, might be appropriate. [60, 61, 62] Higher baseline tryptase concentrations (>11.4 µg/L) may indicate mastocytosis or a monoclonal mast cell disorder (eg, c-kit mutation) and may require bone marrow biopsy and cytogenetic analysis for further evaluation. [61, 62]

Detecting the rise of histamine or tryptase levels can be difficult, and some patients might have a rise in one but not the other. In one emergency department study evaluating patients with acute allergic reactions, 42 of 97 had elevated histamine while 20 had elevated tryptase levels. [63] No correlation was demonstrated between the levels of tryptase and histamine.

Other potentially useful biomarkers are being studied, and these include platelet-activating factor (PAF), bradykinin, chymase, mast cell carboxypeptidase A3, dipeptidyl peptidase I, IL-33 and other cytokines, leukotrienes, and prostaglandins. [64] Low levels of the PAF acetylhydrolase have been reported in fatal anaphylaxis, and failure of this enzyme to inactivate PAF may help identify individuals at risk of severe or even fatal anaphylaxis. [65]

5-Hydroxyindoleacetic acid levels

If carcinoid syndrome is considered, urinary 24-hour 5-hydroxyindoleacetic acid levels should be measured.


Skin Testing and In Vitro IgE Tests

Skin testing, in vitro IgE tests, or both may be used to determine the stimulus causing the anaphylactic reaction (eg, food allergy, medication allergy [particularly penicillin], or insect bite or sting).

Testing for food allergy

If the patient’s medical history and physical examination findings suggest a possible association with food ingestion, percutaneous (puncture) food allergen–specific skin tests and/or in vitro–specific IgE tests (eg, radioallergosorbent assay test [RAST] or ImmunoCAP IgE tests [Phadia AB; Uppsala, Sweden]) can be performed, with an understanding that both false-positive and false-negative results may occur. In the absence of a suggestive clinical history, the rate of false-positive results has been reported to be roughly 50% for both skin tests and in vitro–specific IgE tests. In vitro–specific IgE testing can have roughly a 95% positive predictive value if values are over certain cutoff levels, which vary with the individual food in question.

Conversely, the negative predictive value of skin testing has been reported to be about 95% (it may not be reliable for fresh fruits/vegetables or crustaceans because of the lack of labile allergenic proteins in commercial extracts).

Thus, the detectable presence of specific IgE in the skin or serum can confirm a clinical suspicion formed by compatible patient history and examination. However, undetectable specific IgE occasionally occurs in patients with food allergy and therefore further evaluation (eg, physician-supervised oral food challenge is necessary in cases in which the history is highly suggestive before informing a patient that he or she is not allergic to a suspected food and may ingest it). The double-blind, placebo –controlled food challenge is considered the criterion standard for diagnosis, but a physician-supervised single-blind or an open-food challenge may be considered diagnostic in certain circumstances. These areas are reviewed in depth in the 2010 NIAID-sponsored expert panel report on the diagnosis and management of food allergy in the United States. [66]

These guidelines additionally recommend that the following not be used for a diagnosis, either individually or in combination: intradermal tests, measurement of total serum IgE, and the atopy patch test. [66]

Many panallergens (eg, profilins, chitinases, lipid transfer proteins, tropomyosin) can add to the confusion, as foods may share pathogen-related proteins with nonfood allergens. Intradermal skin testing and IgG RAST tests have no role in the diagnosis of food allergy.

Testing for medication allergy

If the patient’s history suggests a penicillin etiology and the reagents are available, skin testing for penicillin should be performed with the appropriate positive and negative controls. Penicillin G and major determinant (Pre-Pen; ALK-Abelló, Inc; Round Rock, Tex) are commercially available for skin testing. Of note, although the minor determinant mix (MDM) of penicillin comprises only 5% of penicillin metabolites, the MDM has also been implicated in anaphylaxis. The MDM, however, is not available commercially and generally only available at research centers. If the MDM can be used in conjunction with the commercially available penicillin skin testing, the negative predictive value increases from 97% to closer to 99%.

If penicillin skin testing yields positive results, one must use an alternative antibiotic or pursue a desensitization protocol to penicillin. If skin testing yields negative results, most clinicians recommend giving the first dose of penicillin orally and in an allergist’s office. This can be done immediately following the negative penicillin skin testing.

If penicillin skin testing yields positive results, myriad desensitization protocols are available for penicillin (and numerous other medications). Informed consent must be obtained. Desensitization protocols must be performed in a setting equipped to quickly handle anaphylaxis (generally in an intensive care unit). Protocols generally involve administering 8-12 escalating doses of the medication orally or intravenously every 20-30 minutes while observing the patient for pruritus, flushing, urticaria, dyspnea, hypotension, or any other manifestations of anaphylaxis. If no manifestations are observed, the patient can receive the full dose of the medication.

Importantly, although there are commercially available reagents for penicillin skin testing, skin testing for other beta-lactam antibiotics or the majority of other medications should be considered experimental because the haptenic determinants are unknown. Skin testing with the parent drug may be beneficial if the results are positive, but a negative result does not exclude the potential for severe clinical reactivity.

Testing for suspected insect bite or sting

If the patient’s history suggests an insect sting, allergen-specific skin testing to Hymenoptera venoms should be performed. As noted in the 2011 Joint Task Force update on insect hypersensitivity practice parameters, however, if those tests remain negative after 6 weeks in a patient with a serious reaction, then further testing can include in vitro IgE tests. [46] Skin testing and in vitro IgE testing should be performed 4-6 weeks following the episode of anaphylaxis to improve the sensitivity of the diagnostic test. Skin testing for imported fire ant hypersensitivity should be performed using whole-body extracts since venom preparations for the imported fire ant are not commercially available.

Patients’ ability to identify the type of flying insect is unreliable (eg, many confuse yellow jackets and bees), generally mandating testing for all flying Hymenoptera. However, exceptions to this mandate can be made for patients whose stings were accompanied by sterile pustule formation within 24 hours (pathognomonic for fire ant sting) or for whom an impaled stinger and abdominal remnant were found at the sting site (the honeybee eviscerates itself as it stings). In these cases, testing may be limited to fire ant and honeybee allergen-specific IgE, respectively.

Skin testing and in vitro IgE testing should be performed 4-6 weeks following the episode of anaphylaxis to improve the sensitivity of the diagnostic test.

Testing for causes of IgE-independent reactions

Because these reactions are not mediated through IgE, skin testing has no role in their diagnosis. No other diagnostic tests help assess the risk of recurrent IgE-independent reactions.