Diagnosing Pertussis: A Structured Clinical Approach
Suspect pertussis in any patient with cough lasting ≥2 weeks accompanied by paroxysmal coughing, post-tussive vomiting, or inspiratory whooping, and initiate macrolide antibiotics immediately without waiting for laboratory confirmation. 1, 2
Clinical Diagnosis Algorithm
Step 1: Assess Key Clinical Features
In adults with acute (<3 weeks) or subacute (3-8 weeks) cough:
- Rule OUT pertussis if: The patient has fever (sensitivity 81.8% for absence of fever) OR the cough is not paroxysmal (sensitivity 93.2% for paroxysmal cough) 3, 1
- Rule IN pertussis if: Inspiratory whooping is present (specificity 77.7%) OR post-tussive vomiting occurs (specificity 79.5%) 3, 1
- Paroxysmal cough alone has high sensitivity but low specificity, so its absence effectively excludes pertussis 3
In children with acute cough (<4 weeks):
- Post-tussive vomiting is moderately helpful (sensitivity 60%, specificity 66%) but less reliable than in adults 3, 1
- The presence of fever does NOT exclude pertussis in children, unlike adults 4
- Assess for the classic triad: paroxysmal cough, post-tussive vomiting, and inspiratory whoop 1, 4
Step 2: Recognize Disease Phases
The clinical presentation evolves through three distinct phases 1:
- Catarrhal phase (1-2 weeks): Nonspecific symptoms including coryza, intermittent cough, sneezing, minimal fever—clinically indistinguishable from common respiratory infections 1
- Paroxysmal phase (4-6 weeks): Hallmark paroxysmal cough with post-tussive vomiting and inspiratory whoop 1
- Convalescent phase (2-6+ weeks): Gradual improvement with decreasing cough frequency, though nonparoxysmal cough may persist for months 1
Step 3: Consider Special Populations
Critical pitfall: Do not dismiss pertussis in vaccinated adolescents and adults—the illness presents with milder symptoms and the classic whoop is frequently absent in previously vaccinated individuals 1, 5
Infants require heightened suspicion: They may present atypically with apneic spells and minimal cough rather than classic whooping 1
Timing matters for infectivity: Patients are most contagious during the catarrhal stage and first 3 weeks after cough onset, when symptoms are least specific 1
Laboratory Confirmation Strategy
Preferred Diagnostic Test: PCR
Order nasopharyngeal PCR as the first-line confirmatory test 2, 5:
- PCR has superior sensitivity (80-100%) compared to culture (30-60%) 2
- PCR is 2-3 times more likely than culture to detect Bordetella pertussis when classic symptoms are present 2
- Results available within 24-48 hours versus 1-2 weeks for culture 2
- PCR remains reliable even after 2 days of antibiotic treatment, while culture sensitivity drops dramatically 2
Specimen collection: Use nasopharyngeal aspirate or Dacron swab (not cotton) of the nasopharynx 2
When to Use Culture
Culture remains essential despite lower sensitivity 2:
- 100% specificity when positive 3, 2
- Required for antimicrobial susceptibility testing and molecular subtyping 2
- Sensitivity decreases significantly after 2+ weeks of cough, after antimicrobial treatment, or in previously vaccinated patients 2
Critical timing: Begin testing as early as possible in the illness course; culture sensitivity drops to only 1-3% after 3 weeks of cough 2
Serology: Limited Role
Serology has significant limitations 3, 2:
- Requires paired acute and convalescent sera showing fourfold rise in IgG or IgA antibodies to pertussis toxin or filamentous hemagglutinin 3
- Results available too late (weeks) to guide acute management 2
- Cannot differentiate between recent infection, remote infection, or vaccination response 2
- Not useful in infants 6
Confirmed Diagnosis Criteria
A confirmed diagnosis requires one of the following 3, 2:
- Isolation of B. pertussis from nasopharyngeal culture, OR
- Clinical case definition met with PCR confirmation, OR
- Compatible clinical picture with epidemiologic linkage to a confirmed case
Treatment Protocol
Antibiotic Therapy
Initiate macrolide antibiotics immediately when pertussis is clinically suspected—do not wait for laboratory confirmation 1, 2, 4:
- Azithromycin is the preferred agent 4, 5
- All children and adults with confirmed or probable pertussis should receive treatment 3, 1
- Isolate patients for 5 days from the start of antibiotic treatment 3, 1, 2, 4
Timing and Efficacy
Early treatment within the first 2-3 weeks diminishes coughing paroxysms and prevents disease spread 3, 1, 2:
- Treatment beyond 3 weeks after cough onset may be offered but the patient is unlikely to respond clinically 3, 1
- Antibiotics eliminate the organism from the nasopharynx and prevent transmission even if started late 1
Ineffective Therapies
Do not offer the following—they provide no benefit 3:
- Long-acting β-agonists
- Antihistamines
- Corticosteroids
- Pertussis immunoglobulin
Vaccination History Assessment
Verify vaccination status for all patients 4:
- All children should receive complete DTaP primary series followed by a single-dose DTaP booster in early adolescence 3, 1
- Adults up to age 65 should receive TDap vaccine according to CDC guidelines 3, 1
- Ensure household contacts, especially infants, are up to date on pertussis vaccination 4
Key Clinical Pitfalls to Avoid
Do not wait for laboratory confirmation to start treatment—antibiotics should be initiated based on clinical suspicion to prevent transmission 1, 2
Do not rely on physical examination findings—the exam is often surprisingly unremarkable between coughing episodes; diagnosis relies on observing or eliciting the characteristic paroxysmal cough pattern 1
Do not dismiss pertussis based on vaccination history—waning immunity means vaccinated individuals can still contract pertussis, though with milder presentations 1, 5
Do not use PCR indiscriminately—PCR should only be ordered when the clinical case definition is met (>2 weeks of cough with paroxysms, inspiratory whoop, or post-tussive vomiting) 2
Do not rely on leukocytosis with lymphocytosis—this classic finding is frequently absent and should not be used for diagnosis 1