Preferred Initial Test for Suspected Pertussis
Collect a nasopharyngeal specimen (using a Dacron swab or aspirate) for PCR testing as the preferred initial diagnostic test when pertussis is clinically suspected (cough >2 weeks with paroxysms, post-tussive vomiting, or inspiratory whoop). 1
Diagnostic Algorithm
Clinical Case Definition First
Before ordering any test, confirm the patient meets clinical criteria: 1
- Cough lasting >2 weeks AND
- At least one of: paroxysms of coughing, post-tussive vomiting, or inspiratory whooping sound 1
Critical caveat: The CDC endorses PCR for diagnosis only when the clinical case definition is met—do not order PCR for non-specific cough without these features. 1
Specimen Collection Technique
- Use Dacron (or rayon) nasopharyngeal swabs—these do not inhibit PCR amplification and are validated for both PCR and culture 1
- Collect from the posterior nasopharynx, not the anterior nares, as B. pertussis colonizes the posterior nasopharyngeal mucosa 1
- Nasopharyngeal aspirates provide even higher yield than swabs (30/33 vs 26/33 positive in comparative testing) and allow sample splitting for multiple assays 1
Why PCR is Preferred Over Culture
| Test Feature | PCR | Culture |
|---|---|---|
| Sensitivity | 80-100% | 30-60% |
| Turnaround time | 24-48 hours | 1-2 weeks |
| Specificity | High | 100% |
| Performance after antibiotics | Maintained (2-3× more sensitive) | Dramatically reduced |
| Performance after 2+ weeks of cough | Maintained | Significantly reduced (1-3% after 3 weeks) |
PCR is 2-3 times more likely than culture to detect B. pertussis when classic symptoms are present, making it the superior initial test. 1
Timing Considerations
- Begin testing as early as possible in the illness course 1
- PCR maintains sensitivity even in patients who have:
Do not delay treatment while awaiting test results—start antibiotics when pertussis is clinically suspected, as early treatment (within first 2 weeks) rapidly clears the organism and decreases coughing paroxysms. 1
What NOT to Order
- Single-sample serology (IgG antibody): The CDC does not endorse this for routine diagnosis because it cannot differentiate recent infection from remote infection or vaccination response 2
- Paired sera (acute and convalescent): Requires fourfold rise in titer and takes weeks, providing only retrospective diagnosis too late to guide acute management 1, 2
- Blood culture: Has no role—B. pertussis does not cause bacteremia and remains localized to the respiratory tract 1
When Culture Still Matters
Despite PCR being preferred, culture remains essential for: 1
- Antimicrobial susceptibility testing
- Molecular subtyping for public health surveillance
Consider sending both PCR and culture if the specimen is collected early (<2 weeks of cough) and the patient has not yet received antibiotics, as culture provides 100% specificity and public health value. 1
Confirmed Diagnosis Criteria
The CDC defines a confirmed case as: 1
- Isolation of B. pertussis from nasopharyngeal culture, OR
- Clinical case definition met + positive PCR, OR
- Clinical case definition met + epidemiologic linkage to a confirmed case
Infection Control
Isolate patients for 5 days after starting antibiotic therapy regardless of test results. 1