Interpretation of B. pertussis IgG Antibody Result of 3.35
A B. pertussis IgG antibody level of 3.35 (units not specified, but likely IU/ml or similar) is below the diagnostic threshold and does not indicate recent pertussis infection in this 30-year-old female.
Understanding the Result
The CDC does not endorse single-sample serologic testing for routine diagnostic use of pertussis because these assays lack standardization and cannot clearly differentiate immune responses from recent disease, remote disease, or vaccination 1. However, understanding the context of this result is important:
Diagnostic Thresholds
- For acute infection diagnosis: Serology requires demonstration of a fourfold change in titer between acute (<2 weeks after cough onset) and convalescent sera (>4 weeks after acute sample) 1
- Single-sample interpretation: Research suggests that anti-PT IgG levels ≥40 IU/ml are generally considered positive for recent infection 2, while levels around 3.35 fall well below any diagnostic threshold
- This patient's result of 3.35 indicates either no recent infection, waning immunity from childhood vaccination, or no prior exposure 3
Clinical Implications
If Patient Has Prolonged Cough (>2 weeks)
Do not rely on this serology result for diagnosis. The CDC endorses only two diagnostic tests for pertussis 1, 4:
- Nasopharyngeal culture (100% specific, gold standard) 1, 4
- PCR testing (only when clinical case definition is met: >2 weeks of cough with paroxysms, inspiratory "whoop," or post-tussive vomiting) 1, 4
If Assessing Immunity Status
- This low IgG level suggests minimal to no protective immunity against pertussis 3
- Immunity wanes 5-10 years after the last pertussis vaccine dose 1
- Research shows that antibody levels significantly decrease 4-6 years after vaccination 3
- Recommendation: This patient should receive Tdap booster vaccination if she has not received one in the past 10 years 1
Important Caveats
Why This Test Has Limited Value
- No FDA-licensed serologic assays exist for routine pertussis diagnosis in the United States 1, 5
- Single-sample serology cannot distinguish between recent infection, remote infection, or vaccination response 1
- The timing of antibody response makes serology impractical for acute diagnosis—results become available late in the illness course 1
When Serology Might Be Useful
- Epidemiologic surveillance and outbreak investigations (not individual diagnosis) 6
- Paired sera testing (acute and convalescent samples showing fourfold rise) can confirm infection retrospectively 1
Recommended Action
For this 30-year-old female with a B. pertussis IgG of 3.35:
- If she has prolonged cough with paroxysms, obtain nasopharyngeal culture or PCR (if clinical criteria met) rather than relying on serology 4
- If assessing immunity status, consider her susceptible and recommend Tdap vaccination 1
- Do not delay treatment if pertussis is clinically suspected while awaiting test results 4