The 3 C's of Pertussis
The three classic clinical features of pertussis are: paroxysmal Cough, post-tussive (after Cough) vomiting, and Convalescent stage—though more commonly clinicians refer to the three characteristic symptoms as paroxysmal cough, posttussive vomiting, and inspiratory whoop (the "whooping" sound). 1, 2
Clinical Presentation Framework
The classic presentation of pertussis involves three distinct phases of illness, not to be confused with the "3 C's" of characteristic symptoms 1:
The Three Phases of Pertussis
Catarrhal phase (1-2 weeks): Nonspecific symptoms including coryza, intermittent cough, sneezing, lacrimation, and minimal fever—clinically indistinguishable from minor respiratory infections 1, 3
Paroxysmal phase (4-6 weeks): The hallmark stage characterized by the three key clinical features: paroxysmal cough, posttussive vomiting, and inspiratory whoop 1
Convalescent phase (2-6 weeks or longer): Gradual improvement with decreasing frequency of coughing bouts, though nonparoxysmal cough can persist for months 1
The Three Characteristic Symptoms (The True "3 C's")
In Adults
For adults with acute or subacute cough, assess specifically for paroxysmal cough, posttussive vomiting, inspiratory whooping, and absence of fever to rule in or out pertussis. 1, 2
Paroxysmal cough: High sensitivity (93.2%) but low specificity (20.6%)—meaning its absence makes pertussis unlikely 1, 2
Posttussive vomiting: Low sensitivity but high specificity (77.7%)—when present, strongly suggests pertussis 1, 2
Inspiratory whoop: Low sensitivity but high specificity (79.5%)—when present, strongly suggests pertussis 1, 2
In Children
For children with acute cough (<4 weeks), assess for the three classical characteristics: paroxysmal cough, posttussive vomiting, and inspiratory whooping. 1, 2, 4
- Posttussive vomiting in children: Moderate sensitivity (60.0%) and specificity (66.0%)—less diagnostically powerful than in adults but still clinically useful 1, 2, 4
Diagnostic Algorithm
If paroxysmal cough is absent OR fever is present → pertussis is unlikely 1, 2
If posttussive vomiting OR inspiratory whoop is present → pertussis is likely and should be confirmed with PCR testing 1, 2
Critical Pitfalls to Avoid
Do not dismiss pertussis in vaccinated adolescents and adults—the illness can be milder with absent whoop in previously vaccinated individuals 1, 3
Do not wait for laboratory confirmation to initiate treatment—antibiotics should be started when pertussis is clinically suspected to prevent transmission, even though they won't alter the clinical course if started after cough onset 1, 2
Do not overlook infants with atypical presentation—infants can present with apneic spells and minimal cough rather than classic whooping 1, 5
Recognize that patients are most infectious during the catarrhal stage and first 3 weeks after cough onset—when symptoms are least specific 1