What is the diagnostic test for pertussis?

Diagnostic Testing for Pertussis

The CDC recommends collecting a nasopharyngeal aspirate or Dacron swab for culture as the gold standard diagnostic test, with PCR endorsed as an additional option when the clinical case definition is met (cough >2 weeks with paroxysms, inspiratory "whoop," or post-tussive vomiting). 1

Clinical Case Definition

Before ordering diagnostic tests, confirm the patient meets clinical criteria:

• Acute cough persisting >2 weeks accompanied by at least one of the following: 2, 1
  • Paroxysms of coughing
  • Post-tussive vomiting
  • Inspiratory whooping sound

Diagnostic Testing Algorithm

First-Line Testing: Culture

• Collect nasopharyngeal aspirate or use a Dacron swab (not cotton) of the nasopharynx for culture 2, 1
• Culture is 100% specific and remains the only certain way to make a definitive diagnosis 2, 1
• Critical limitation: sensitivity is only 30-60% in practice and drops to 1-3% after 3 weeks of cough 1
• Sensitivity decreases with: 1
  • Duration >2 weeks of cough
  • Prior antimicrobial treatment
  • Previous pertussis vaccination
• Culture requires 1-2 weeks for definitive results 1
• Despite limitations, culture remains essential for antimicrobial susceptibility testing and molecular subtyping 1

PCR Testing

• The CDC endorses PCR only when the clinical case definition is also met (>2 weeks cough with characteristic features) 1
• PCR has significantly higher sensitivity (80-100%) compared to culture 2, 1
• PCR is 2-3 times more likely than culture to detect B. pertussis when classic symptoms are present 1
• PCR provides shorter turnaround time than culture 1
• Important caveat: The 2006 ACCP guidelines rated PCR as grade I (conflicting benefit) due to lack of standardized, validated techniques at that time 2, but the CDC now endorses PCR when clinical criteria are met 1
• PCR remains positive 1-7 days longer than culture after starting erythromycin treatment 3

Timing Considerations

• Begin testing as early as possible in the illness course 1
• Optimal window: within first 2-3 weeks of cough onset 2, 1
• After 3 weeks, culture sensitivity drops dramatically to 1-3% 1
• PCR maintains better sensitivity than culture in later stages 4, 5

Serologic Testing (Not Recommended for Routine Diagnosis)

• The CDC does not endorse single-sample serologic testing for routine diagnostic use 6
• Serology cannot differentiate between recent infection, remote infection, or vaccination response 6
• No FDA-licensed serologic assays exist for routine pertussis diagnosis in the United States 6
• If serology is used, paired sera (acute and convalescent) showing fourfold rise in IgG or IgA antibodies to pertussis toxin or filamentous hemagglutinin can confirm infection retrospectively 2, 6
• First sample should be taken within 2 weeks of cough onset, second sample 3-4 weeks later 2
• Paired sera have 99% specificity but only 63% sensitivity 2

Confirmed Diagnosis Criteria

A confirmed case requires one of the following: 2, 1

• Isolation of B. pertussis from nasopharyngeal culture, OR
• Clinical case with PCR confirmation, OR
• Clinical case with epidemiologic linkage to a confirmed case

Critical 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
• Isolate patients for 5 days after starting antibiotic therapy 1
• Recommended treatment: erythromycin 40-50 mg/kg/day in children, 1-2 g/day in adults for 2 weeks 2

Common Pitfalls to Avoid

• Avoid relying solely on culture after 2-3 weeks of symptoms - sensitivity becomes extremely poor 1
• Do not use cotton swabs - use Dacron or polyester swabs for optimal specimen collection 2
• Do not order single-sample serology for acute diagnosis - it cannot distinguish infection from vaccination 6
• Do not wait for test results to initiate treatment in clinically suspected cases 1
• Avoid PCR testing in patients who don't meet clinical criteria - CDC endorses PCR only when clinical case definition is met 1