Investigations
1st investigations to order
mast cell tryptase
Test
Measure blood mast cell tryptase concentrations in people aged:
16 years and over during or soon after resuscitation[51][52][Evidence C]
Under 16 years during or soon after resuscitation only if the trigger is not food-related.[51][52]
Do not delay initial resuscitation or treatment of anaphylaxis to obtain tryptase levels.[32][52]
Ideally obtain three samples using a serum or clotted blood test tube:[32][52][78]
As soon as possible after resuscitation, then
1 to 2 hours after the start of symptoms (but no later than 4 hours), then
After 24 hours or at the allergy clinic to provide baseline levels (some people have a raised baseline level).
Record accurately on the test tube the times of:[32]
The start of symptoms
The time of blood sampling when managing the acute problem.
In practice, mast cell tryptase testing is not necessary for recurrent episodes of anaphylaxis when diagnosis of anaphylaxis is definite.
Result
May be elevated; can range from insignificantly elevated to levels above 100 nanograms/mL
Serum mast cell tryptase levels are normally undetectable (<1 nanogram/mL) in healthy people who have not had anaphylaxis in the preceding hours
Even in clinically obvious food-induced anaphylaxis, tryptase levels may not be elevated, even if serum was collected in a timely manner[51][79]
Elevation of tryptase levels may also exist in non-anaphylactic conditions, such as systemic mastocytosis
12-lead ECG
Test
Usual investigation for a medical emergency.
Result
Non-specific ST ECG changes are common post-adrenaline (epinephrine) and with anaphylaxis. May show myocardial ischaemia even with normal coronary arteries
blood gases
Test
Usual investigation for a medical emergency.
Progressive metabolic acidosis can develop, indicating poor tissue oxygenation and perfusion. In practice, a venous blood gas will provide sufficient data in most cases.
Result
Elevated lactate
urea and electrolytes
Test
Urea and electrolytes may not always be necessary as a blood gas will usually give creatinine and creatinine clearance values. If they are required, they are normal in the initial phase of anaphylaxis, unless comorbidity is present.[32]
Result
Normal in initial phase of anaphylaxis unless comorbidity present
Investigations to consider
chest x-ray
Test
Request a chest x-ray only after time-critical interventions have been administered.
In practice, a chest x-ray is required only if significant chest signs are present indicating a possible alternative diagnosis, or if the diagnosis is uncertain.
Result
May show hyperinflation if bronchoconstriction; interstitial fluid
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