Pertussis Laboratory Testing
Recommended Diagnostic Tests
The CDC endorses only two laboratory tests for confirming pertussis: culture and PCR, with PCR being the preferred test when the clinical case definition is met (>2 weeks of cough with paroxysms, inspiratory "whoop," or posttussive vomiting). 1, 2
Primary Testing Algorithm
- Collect nasopharyngeal specimens using a Dacron swab or nasopharyngeal aspirate as early as possible in the illness course 2
- PCR testing is the preferred confirmatory method due to its superior sensitivity (80-100%) and rapid turnaround time compared to culture 2, 3
- PCR is 2-3 times more likely than culture to detect B. pertussis when classic pertussis symptoms are present 1, 2
- PCR should only be used to confirm diagnosis when the clinical case definition is met: cough >2 weeks with paroxysms, inspiratory whoop, or posttussive vomiting 1
Culture Testing
- Culture remains 100% specific for pertussis but has limited practical sensitivity (30-60% in routine practice) 1
- Culture sensitivity drops dramatically after 2 weeks of cough illness and is only 1-3% sensitive after 3 weeks 1, 2
- Culture requires 1-2 weeks for definitive negative results 1
- Culture remains essential for antimicrobial susceptibility testing, molecular subtyping, and validation of other laboratory assays 1, 2
Tests NOT Recommended
- Direct fluorescent antibody (DFA) testing is NOT recommended by the CDC due to poor sensitivity (10-50%) and subjective interpretation that can result in lower specificity 1
- Serologic testing is NOT endorsed by the CDC for routine diagnostic use because single-sample assays lack standardization and cannot differentiate recent disease from remote disease or vaccination 1, 4
- Paired sera serology (requiring acute and convalescent samples with fourfold titer rise) provides results too late in the illness to be clinically useful 1
Critical Timing Considerations
- Begin testing as early as possible in the course of illness, ideally within the first 2 weeks of cough 2
- Culture sensitivity decreases significantly after antimicrobial treatment or in previously vaccinated individuals 1, 2
- Do not delay treatment while awaiting test results if pertussis is clinically suspected 2, 5
Common Pitfalls to Avoid
- Avoid using PCR alone without clinical criteria: PCR tests lack FDA-licensed standardization, and approximately 100 different PCR protocols exist with variable sensitivity and specificity 1
- Low-specificity PCR tests can result in false-positive results leading to unnecessary treatment and inappropriate chemoprophylaxis of contacts 1
- Do not rely on serology for acute diagnosis, as it is not FDA-licensed for routine diagnostic use in the United States and lacks standardization 1, 4
- Do not dismiss pertussis in vaccinated individuals, as the illness can be milder with absent whoop in previously vaccinated adolescents and adults 5
Treatment Recommendations
- Initiate macrolide antibiotics when pertussis is clinically suspected without waiting for laboratory confirmation 2, 5
- Erythromycin is effective in eliminating B. pertussis from the nasopharynx, rendering patients noninfectious 6
- Azithromycin is the preferred antibiotic for treatment or prophylaxis 3
- Early treatment within the first 2 weeks will rapidly clear B. pertussis and decrease coughing paroxysms 2
- Isolate patients for 5 days after starting antibiotic therapy 2
- Antibiotic treatment is intended to prevent transmission and does not shorten the disease course or improve symptoms if started late 5, 3
Confirmed Diagnosis Criteria
- Isolation of B. pertussis from nasopharyngeal culture, OR
- Clinical case with PCR confirmation, OR
- Clinical case with epidemiologic linkage to a confirmed case 2