What is the recommended initial test for diagnosing pertussis?

Diagnostic Testing for Pertussis

For patients with suspected pertussis, a nasopharyngeal aspirate or Dacron swab of the nasopharynx for culture is the recommended initial test to confirm the presence of Bordetella pertussis, as isolation of the bacteria is the only certain way to make a definitive diagnosis. 1

Clinical Diagnosis

• Suspect pertussis when a patient has a cough lasting >2 weeks without another apparent cause, accompanied by paroxysms of coughing, post-tussive vomiting, and/or an inspiratory whooping sound 1
• A clinical case is defined as an acute illness with cough persisting for >2 weeks associated with post-tussive vomiting, typical whooping sound, or severe paroxysms 1

Diagnostic Testing Algorithm

First-Line Testing: Nasopharyngeal Culture

• Collect nasopharyngeal aspirate or use a Dacron swab of the nasopharynx for culture 1
• Culture is 100% specific but has variable sensitivity (30-60% in practice) 1
• Sensitivity decreases after:
  • 2+ weeks of cough illness
  • Antimicrobial treatment
  • Previous pertussis vaccination 1
• Culture requires 1-2 weeks for definitive negative results 1
• Despite limitations, culture remains essential for antimicrobial susceptibility testing and molecular subtyping 1

Second-Line Testing: PCR

• PCR has increased sensitivity (80-100%) and shorter turn-around time compared to culture 1
• PCR is 2-3 times more likely than culture to detect B. pertussis when classic symptoms are present 1
• CDC endorses PCR for diagnosis only when the clinical case definition is also met (>2 weeks of cough with paroxysms, inspiratory "whoop," or post-tussive vomiting) 1
• Limitations of PCR:
  • No FDA-licensed PCR test kit
  • No standardized protocols or reagents
  • Variable quality control across laboratories 1
  • PCR may remain positive for weeks after treatment initiation 2

Serologic Testing

• Not recommended as first-line testing 1
• Requires paired sera (acute and convalescent) to demonstrate a fourfold increase in IgG antibodies against pertussis toxin 1
• First sample should be taken within 2 weeks of cough onset, second sample 3-4 weeks later 1
• High specificity (99%) but moderate sensitivity (63%) 1
• Limited clinical utility as patients often delay seeking care 1
• No serologic method has been validated or approved for diagnostic use in the United States 1

Not Recommended

• Direct Fluorescent Antibody (DFA) testing is not recommended by CDC due to low sensitivity (10-50%) and potential for misinterpretation 1

Timing Considerations

• Begin testing as early as possible in the course of illness 1
• Culture sensitivity drops significantly after 3 weeks of cough (only 1-3% sensitive) 1
• PCR remains positive longer than culture and can detect B. pertussis for up to 3 weeks after treatment initiation 2

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 1

Treatment Considerations

• Do not delay treatment while awaiting test results 1
• Early treatment (within first 2 weeks) will rapidly clear B. pertussis from the nasopharynx and decrease coughing paroxysms 1
• Patients should be isolated for 5 days after starting antibiotic therapy 1

Remember that diagnostic yield is highest when testing is performed early in the course of illness, before antibiotics are started, and with proper specimen collection technique 1.