ELISA Test for Pertussis
The ELISA (enzyme-linked immunosorbent assay) for pertussis is a serological test that detects IgG and IgA antibodies against pertussis toxin (PT) to diagnose Bordetella pertussis infection, but it requires paired acute and convalescent sera to demonstrate a fourfold rise in antibody titers and is not FDA-approved or validated for routine diagnostic use in the United States. 1, 2
What the Test Measures
The ELISA detects antibodies against specific B. pertussis antigens, most importantly:
- Pertussis toxin (PT) - the only antigen specific to B. pertussis 3
- Filamentous hemagglutinin (FHA) - less specific, shared with other Bordetella species 1, 3
- Pertactin and fimbriae - also measured but less commonly 1
ELISAs should use purified, non-detoxified PT as the antigen and express results quantitatively in International Units per milliliter (IU/ml). 3, 4
How the Test is Interpreted
Paired Sera Testing (Standard Approach)
The most accepted serologic criterion requires paired acute and convalescent serum samples demonstrating a fourfold increase in IgG or IgA antibodies to PT or FHA. 1
- First sample: Collect within 2 weeks of cough onset 1, 5
- Second sample: Collect 3-4 weeks after the first sample 1, 5
- Interpretation: A fourfold rise is consistent with recent B. pertussis infection 1, 2, 1
Single Serum Testing (Limited Use)
- A single high-titer specimen compared to reference values is highly suggestive of recent infection when accompanied by compatible clinical symptoms 1, 5
- Diagnostic cutoffs: Single-sample serology using IgG-anti-PT between 50-120 IU/ml can be used 3
- Critical limitation: Non-rising titers may represent past infection or previous immunization rather than active disease 1, 5
- Cannot be validly interpreted for one year after acellular pertussis (aP) vaccination 3
Test Performance Characteristics
When used for documenting community outbreaks:
ELISAs using PT as antigen show specificity of 81-93%, while those using mixed antigens have unacceptably low specificity of 51-59%. 4
Major Clinical Limitations
Timing Issues
Paired sera titers have limited clinical usefulness because patients often delay seeking medical care and paired samples cannot be obtained in time to guide treatment decisions. 1 Results typically become available late in the course of illness. 2
Regulatory Status
No serologic method for pertussis diagnosis has been validated and approved for diagnostic use in the United States. 1 The CDC does not endorse serology for laboratory confirmation of pertussis cases. 2
Role of IgA Testing
IgA-anti-PT should only be used with indeterminate IgG-anti-PT levels or when a second sample cannot be obtained. 3 IgA antibody assays show sensitivities of only 53-73% and specificities of 67-94%, indicating limited diagnostic value. 4
Recommended Diagnostic Approach
For definitive diagnosis, nasopharyngeal culture remains the gold standard (100% specific), and PCR is preferred over serology for rapid diagnosis (80-100% sensitivity, 88-100% specificity). 1, 6, 5
Order paired acute and convalescent sera only for presumptive diagnosis when culture or PCR cannot be obtained or are negative, recognizing this provides Grade C evidence with intermediate benefit. 1
Common Pitfalls to Avoid
- Do not rely on single serum samples unless titers are very high and clinical picture is compatible 1, 5
- Do not use ELISA results from the year following aP vaccination - they cannot distinguish vaccine response from infection 3
- Do not use assays with mixed antigens - they lack specificity 3, 4
- Do not delay antibiotic treatment while awaiting serologic confirmation - treat based on clinical suspicion 1, 5
- Avoid micro-agglutination, immunoblots, complement fixation, or indirect immunofluorescence - these methods have inadequate sensitivity, specificity, or cannot be quantified 3