How do I differentiate between influenza and pertussis in a patient?

Differentiating Influenza from Pertussis

Laboratory testing is essential to reliably differentiate influenza from pertussis, as clinical symptoms alone are insufficient for accurate diagnosis—influenza typically presents with acute onset of fever, cough, and myalgia lasting <7 days, while pertussis is characterized by prolonged cough lasting ≥2 weeks with paroxysms, post-tussive vomiting, or inspiratory whoop. 1, 2

Key Clinical Distinguishing Features

Duration of Illness

• Influenza: Acute illness typically lasting <7 days, with symptoms improving during the convalescent phase 1
• Pertussis: Prolonged cough lasting ≥2 weeks (14 days or more) is the fundamental diagnostic criterion that differentiates pertussis from acute respiratory infections 1, 2

Fever Patterns

• Influenza: Fever is a prominent feature, particularly in children and adults with typical presentations 1
• Pertussis: High fever is uncommon during the catarrhal phase; presence of fever actually makes pertussis less likely in adults 1, 2

Cough Characteristics

• Influenza: Non-paroxysmal cough that develops acutely with other systemic symptoms 1
• Pertussis: Paroxysmal cough (series of coughs without intermediate inspiration) is the most common manifestation, often accompanied by post-tussive vomiting (53% of adults) 1, 2

Associated Symptoms

• Influenza: Myalgia, acute onset, systemic symptoms; positive predictive value of fever plus myalgia is 41% in vaccinated older persons 1
• Pertussis: Inspiratory whoop (though frequently absent in previously vaccinated adolescents and adults), post-tussive vomiting, and cough disturbing sleep (52% of adults) 1, 3, 2

Disease Phases

Pertussis Has Three Distinct Phases:

• Catarrhal phase (1-2 weeks): Coryza and intermittent cough, high fever uncommon 1
• Paroxysmal phase (4-6 weeks): Spasmodic cough, post-tussive vomiting, inspiratory whoop 1
• Convalescent phase (2-6 weeks to months): Gradual symptom improvement 1

Influenza Lacks These Distinct Phases:

• Acute onset with peak symptoms early in illness course 1

Laboratory Diagnosis

For Influenza:

• Nasopharyngeal or nasal specimens have higher yields than throat swabs 1
• Rapid influenza diagnostic tests (RIDTs) can detect influenza antigens within 15 minutes 1
• RT-PCR, viral culture, and immunofluorescence assays are available diagnostic options 1
• Testing is particularly important because clinical diagnosis based on symptoms alone has limited accuracy, with positive predictive values ranging from 30-88% depending on age and population 1

For Pertussis:

• Nasopharyngeal aspirate or Dacron swab for culture is the definitive diagnostic method 3
• PCR testing should be considered, though not universally standardized 3
• Confirmed diagnosis requires positive culture or PCR for Bordetella pertussis, or clinical definition plus epidemiologic linkage to a confirmed case 3, 2

Critical Pitfalls to Avoid

Common Diagnostic Errors:

• Not considering pertussis in the differential diagnosis of prolonged cough, especially in adolescents and adults 3, 2
• Waiting >2 weeks to consider pertussis diagnosis is an error 2
• Assuming absence of whoop rules out pertussis: The characteristic whoop is frequently absent in previously vaccinated individuals 3, 2
• Relying solely on clinical symptoms for influenza: Only 44-51% of hospitalized adults with laboratory-confirmed influenza had typical ILI symptoms 1

Age-Specific Considerations:

• Young children (<5 years) are less likely to experience typical influenza symptoms; only 64-70% with confirmed influenza had both fever and cough 1
• Older adults (≥60 years): Clinical case definitions for influenza have poor positive predictive value (30-53%) 1
• Adults with pertussis: Often present with prolonged, non-distinctive cough as the only manifestation, with the characteristic whoop usually absent 2

Practical Clinical Algorithm

1. Assess duration: If cough <2 weeks with acute onset and fever, consider influenza first 1, 2
2. Assess duration: If cough ≥2 weeks with paroxysms, post-tussive vomiting, or whoop, suspect pertussis 1, 3, 2
3. Check fever pattern: Prominent fever favors influenza; absence of fever favors pertussis 1, 2
4. Obtain laboratory confirmation: Do not rely on clinical diagnosis alone for either condition 1, 3
5. Consider epidemiology: Check for influenza activity in the community and pertussis exposure history 1, 3, 2