Clinical Signs of Anaphylaxis
Anaphylaxis is diagnosed when there is sudden onset (minutes to hours) of skin/mucosal involvement (hives, flushing, swollen lips/tongue) PLUS respiratory compromise (wheeze, stridor, shortness of breath) OR cardiovascular collapse (hypotension, syncope), or when two or more organ systems are involved after allergen exposure. 1
Diagnostic Framework: The NIAID/FAAN Criteria
The 2020 practice parameter endorses three validated clinical criteria—meeting ANY ONE of these three criteria confirms anaphylaxis 1:
Criterion 1: Skin/Mucosal Involvement + One Major System
- Skin/mucosal signs (generalized hives, itching, flushing, swollen lips-tongue-uvula)
- PLUS at least one of:
- Respiratory symptoms (shortness of breath, wheeze, cough, stridor, hypoxemia)
- Reduced blood pressure OR end-organ dysfunction (hypotonia/collapse, incontinence)
Criterion 2: Multi-System Involvement After Allergen Exposure
Two or more of the following occurring suddenly after exposure:
- Skin/mucosal symptoms (hives, itch-flush, angioedema)
- Respiratory symptoms (dyspnea, wheeze, stridor, hypoxemia)
- Reduced blood pressure or end-organ dysfunction (collapse, incontinence)
- Gastrointestinal symptoms (crampy abdominal pain, vomiting)
Criterion 3: Isolated Hypotension After Known Allergen
Reduced blood pressure alone after exposure to a known allergen:
- Adults: Systolic BP <90 mmHg OR >30% decrease from baseline
- Infants (1 month-1 year): Systolic BP <70 mmHg
- Children (1-10 years): Systolic BP <[70 mmHg + (2 × age)]
- Children (11-17 years): Systolic BP <90 mmHg
Most Common Clinical Presentations
Urticaria, difficulty breathing, and mucosal swelling are the most frequent symptoms 2. However, the presentation varies significantly:
- Skin symptoms are present in approximately 80-90% of cases but anaphylaxis can occur without cutaneous manifestations 1
- Cardiovascular symptoms (syncope, hypotension) are more common in drug-induced anaphylaxis 3
- Respiratory compromise is more likely than hypotension in infants and children 1
- In children, shock manifests initially as tachycardia rather than hypotension 1
Critical Pitfalls to Avoid
Do not wait for hypotension or complete criteria fulfillment before treating. The guidelines explicitly state that epinephrine administration is not limited to patients meeting full diagnostic criteria 1. For example:
- A patient developing generalized urticaria immediately after immunotherapy injection may warrant epinephrine for impending anaphylaxis
- After an insect sting, reduced blood pressure might be the only manifestation 1
Isolated urticaria without systemic involvement is NOT anaphylaxis and may respond to antihistamines alone, whereas true anaphylaxis requires immediate epinephrine 1.
Age-Specific Considerations
Pediatric patients present differently:
- Respiratory compromise predominates over cardiovascular collapse 1
- Tachycardia appears before hypotension 1
- Normal heart rate: 80-120 bpm at age 3 years; 70-115 bpm after age 3 1
Older age is a risk factor for severe or fatal reactions 3, along with coexisting asthma, cardiovascular disease, and mast cell disorders 2.
Timing and Biphasic Reactions
Symptoms typically develop within one to two hours of allergen exposure 2, though the criteria allow for "minutes to several hours" 1.
Biphasic anaphylaxis (recurrence after complete resolution) occurs in 4-5% of cases, typically within 1-78 hours after initial reaction 1. This is distinct from persistent symptoms that never fully resolve.
When Diagnosis is Uncertain
If clinical diagnosis remains unclear, serum tryptase levels (reflecting mast cell degranulation) can be obtained 2. However, diagnosis relies primarily on clinical criteria, not laboratory testing 1.