Interpretation of B. pertussis IgG Antibody Level of 1.93
A single B. pertussis IgG antibody level of 1.93 (units not specified, but likely U/mL) is too low to diagnose active or recent pertussis infection and cannot distinguish between past infection, remote vaccination, or waning immunity. 1
Why This Result Is Not Diagnostic
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
No FDA-licensed serologic assays exist for routine pertussis diagnosis in the United States, and single-sample serology cannot distinguish between recent infection, remote infection, or vaccination response. 1
For acute infection diagnosis, serology requires demonstration of a fourfold change in titer between acute and convalescent sera (samples taken 3-4 weeks apart), not a single value. 2, 1
What This Level Means Clinically
If this value is in U/mL, it falls far below the threshold of ≥100 U/mL that research suggests is diagnostic of recent or active B. pertussis infection. 3
Such low levels are present in the general population and likely represent either waning immunity from prior vaccination (which wanes 5-10 years after the last dose) or remote past exposure. 1, 3
IgG antibodies due to immunoblot assays have variable sensitivity and specificity and are of limited value in serological diagnosis of pertussis due to lack of quantification. 4
What You Should Do Instead
If clinical suspicion for pertussis exists (cough >2 weeks with paroxysms, post-tussive vomiting, or inspiratory whoop):
Order nasopharyngeal culture (100% specific, gold standard) or PCR testing (80-100% sensitive) immediately—these are the only two CDC-endorsed diagnostic tests. 5, 1
Do not delay treatment while awaiting test results if clinical suspicion is high; begin macrolide antibiotics immediately. 5
Isolate the patient for 5 days after starting antibiotic therapy. 5
If assessing immunity status without active symptoms:
- Consider the individual susceptible and recommend Tdap vaccination if they have not received one in the past 10 years, as immunity wanes 5-10 years after the last pertussis vaccine dose. 1
Common Pitfall to Avoid
Do not rely on single serologic values to diagnose or exclude pertussis. The only way to confirm pertussis is through nasopharyngeal culture isolation, PCR confirmation when clinical criteria are met, or epidemiologic linkage to a confirmed case. 2, 5, 1