What is the recommended initial test for a pertussis (whooping cough) workup?

Diagnostic Testing for Pertussis

For patients with suspected pertussis, a nasopharyngeal swab for PCR testing should be the initial diagnostic test of choice due to its superior sensitivity (80-100%) compared to culture (25-50%) and rapid turnaround time. 1, 2

Understanding Pertussis Testing Options

PCR Testing (First-Line)

• Advantages:

  • 2-3 times more sensitive than culture 1
  • Results available within hours (vs. 1-2 weeks for culture) 2
  • Can detect B. pertussis even after antibiotic treatment has begun 2
  • Sensitivity: 61-65%, Specificity: 88-98% 2

• Collection technique:

  • Use a Dacron (polymer) swab of the nasopharynx 1, 3
  • Proper specimen collection is critical for accuracy 2

Culture (Second-Line)

• Advantages:

  • Gold standard for specificity (100%) 2
  • Definitive confirmation of B. pertussis 1

• Limitations:

  • Low sensitivity (25-50%) 1
  • Requires specialized media 1
  • Results take 1-2 weeks 2
  • Significantly affected by prior antibiotic use 3

Serologic Testing (Not First-Line)

• Not standardized or FDA-approved for diagnostic use in the United States 1
• Most useful in epidemiologic studies rather than individual diagnosis 1
• Requires paired sera (acute and convalescent) taken 3-4 weeks apart 1
• Limited clinical utility due to timing issues and inability to distinguish between recent infection and vaccination 1

Testing Algorithm

1. Initial presentation with suspected pertussis:

   • Collect nasopharyngeal specimen using Dacron swab for PCR testing 1, 3
   • Consider collecting a second swab (calcium alginate) for culture if available 3

2. Timing considerations:

   • Optimal timing: Within first 2-3 weeks of cough onset 1
   • PCR sensitivity decreases as illness progresses 2

3. Interpretation of results:

   • Positive PCR with clinical symptoms: Confirms pertussis 1
   • Negative PCR with strong clinical suspicion: Consider culture or serologic testing 4

Clinical Pearls and Pitfalls

• PCR limitations: No FDA-licensed PCR test kit exists; laboratories must develop and validate their own tests, leading to variability in performance 1, 2

• False positives: PCR may detect B. pertussis DNA in asymptomatic carriers or after the infectious period has passed 2

• Clinical correlation: CDC recommends PCR confirmation only when the case meets clinical case definition (≥2 weeks of cough with paroxysms, inspiratory "whoop," or post-tussive vomiting) 1

• Vaccination impact: Recently vaccinated individuals may have lower PCR positivity rates compared to pre-vaccinated or post-vaccinated individuals 5

• Antibiotic interference: Prior antibiotic treatment significantly reduces culture sensitivity but has less impact on PCR results 3

• Do not use DFA: Direct fluorescent antibody testing has poor sensitivity (10-50%) and is not recommended by the CDC 1

Special Considerations

• Infants: PCR is particularly useful in pre-vaccinated infants, where clinical presentation may be atypical 5

• Brief resolved unexplained events (BRUE): Consider pertussis testing in infants with BRUE, as pertussis can cause gagging, gasping, and color change followed by respiratory pause 1

• Outbreak settings: Lower threshold for testing and consider both PCR and culture 1

By following this evidence-based approach to pertussis diagnosis, clinicians can rapidly identify cases, initiate appropriate treatment, and implement infection control measures to prevent transmission.