What laboratory tests diagnose whooping cough (pertussis)?

Laboratory Testing for Whooping Cough (Pertussis)

For suspected whooping cough, order a nasopharyngeal aspirate or Dacron swab for culture as the definitive diagnostic test, and consider PCR testing when clinical criteria are met (cough >2 weeks with paroxysms, whooping, or post-tussive vomiting) for faster results. 1

Primary Diagnostic Tests

Nasopharyngeal Culture (Gold Standard)

• Culture is the only certain way to confirm pertussis diagnosis and remains essential despite limitations 2
• Collect specimen using nasopharyngeal aspirate or Dacron swab from the posterior nasopharynx 2, 1
• Culture is 100% specific but has variable sensitivity (25-60% in practice) 2, 1
• Requires enriched media and takes 1-2 weeks for definitive results 1
• Critical timing consideration: Sensitivity drops dramatically after 2-3 weeks of cough (only 1-3% sensitive after 3 weeks) 1
• Sensitivity also decreases with prior antimicrobial treatment or previous pertussis vaccination 1
• Culture remains indispensable for antimicrobial susceptibility testing and molecular subtyping 1

PCR Testing (Rapid Alternative)

• PCR has higher sensitivity (80-100%) compared to culture and provides faster turnaround time 2, 1
• The CDC endorses PCR only when the clinical case definition is met: cough >2 weeks with paroxysms, inspiratory "whoop," or post-tussive vomiting 1
• PCR is 2-3 times more likely than culture to detect B. pertussis when classic symptoms are present 1
• Important caveat: PCR assays lack universal standardization and validation for routine clinical testing 2
• Most reliable during the first 2-3 weeks of illness (catarrhal phase) 3

Serologic Testing (Limited Role)

Paired Sera Testing

• Order paired acute and convalescent sera to demonstrate a fourfold increase in IgG or IgA antibodies to pertussis toxin (PT) or filamentous hemagglutinin (FHA) 2
• First serum sample should be collected within 2 weeks of cough onset, second sample 3-4 weeks later 2
• Reported specificity and sensitivity are 99% and 63%, respectively, for documenting community outbreaks 2
• Major limitation: Patients often delay seeking care, making paired samples impractical for clinical diagnosis 2
• Serology becomes more useful after 2-3 weeks when culture/PCR sensitivity declines 3

Single-Sample Serology (Not Recommended)

• The CDC does not endorse single-sample serologic testing for routine diagnostic use because these assays lack standardization and cannot differentiate between recent disease, remote disease, or vaccination 4
• No FDA-licensed serologic assays exist for routine pertussis diagnosis in the United States 4
• IgM testing is not mentioned as a diagnostic tool in ACCP guidelines 3

Diagnostic Algorithm

When to Test

• Begin testing as early as possible in the illness course 1
• Suspect pertussis when cough lasts >2 weeks without another apparent cause, accompanied by paroxysms, post-tussive vomiting, and/or inspiratory whooping sound 1

Testing Strategy

1. First-line: Order nasopharyngeal culture for all suspected cases 2
2. Concurrent: Add PCR if clinical criteria are met and rapid diagnosis is needed 1
3. Alternative: Consider paired sera testing for retrospective confirmation when culture/PCR are negative but clinical suspicion remains high 2

Confirmed Diagnosis Criteria

A confirmed diagnosis requires one of the following 1:

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

Critical Clinical Pitfalls

• Do not delay treatment while awaiting test results 1
• Early treatment within the first 2 weeks rapidly clears B. pertussis from the nasopharynx and decreases coughing paroxysms 1, 3
• Isolate patients for 5 days after starting antibiotic therapy 1, 3
• Culture sensitivity is highest in the first 2 weeks of illness; after 3 weeks, culture becomes nearly useless 1
• Avoid relying on single-sample serology, as it cannot distinguish between infection and vaccination 4