Is There a Tdap Titer Test?
There is no standardized, FDA-licensed serologic test to assess immunity to pertussis (the "P" component of Tdap), and CDC guidelines do not endorse serologic testing for pertussis immunity. However, serologic testing for tetanus and diphtheria antibodies is available and can be used in specific clinical situations.
Pertussis Serology: Not Recommended
- No FDA-licensed serologic assay exists for routine diagnostic or immunity assessment for pertussis in the United States 1
- Single-sample serologic assays for pertussis lack standardization and cannot clearly differentiate immune responses from recent disease, remote disease, or vaccination 1
- The only pertussis diagnostic tests endorsed by CDC are culture and PCR (when clinical case definition is met), not serologic testing 1
- Because serologic correlates and minimum protective antibody titers for pertussis have not been established, the clinical relevance of pertussis antibody levels remains unknown 2
Tetanus and Diphtheria Serology: Available But Limited Use
Serologic testing for tetanus and diphtheria antibodies is available and can be considered in specific situations, particularly when vaccination history is unknown or uncertain.
When Serologic Testing May Be Appropriate:
- For adults or adolescents who probably received vaccination but cannot produce records, serologic testing for antibodies to tetanus and diphtheria toxin can be obtained to avoid unnecessary vaccination 1
- If tetanus and diphtheria antitoxin levels are each >0.1 IU/mL, previous vaccination with tetanus and diphtheria toxoid-containing vaccines is presumed 1
- In this scenario, only a single dose of Tdap is indicated 1, 3
Protective Antibody Levels:
- Tetanus: Antibody levels ≥0.1 IU/mL are considered protective 1, 4
- Diphtheria: Antibody levels ≥0.1 IU/mL are considered protective 1, 4
- For more certain, durable protection, tetanus titers ≥0.1 IU/mL and diphtheria titers ≥1.0 IU/mL are preferred 2
Clinical Approach When Vaccination History Is Unknown
The recommended approach is revaccination rather than serologic testing in most situations 1:
- For persons with no or questionable vaccination records, age-appropriate revaccination with Tdap is the recommended approach 1
- The alternative approach of serologic testing for tetanus and diphtheria (with subsequent vaccination based on results) is listed as an option only for DTaP in children, not routinely for Tdap in adults 1
- Persons with unknown or uncertain tetanus immunization histories should be considered to have had no previous doses of tetanus toxoid-containing vaccine 3
Important Clinical Pitfalls
- Do not assume prior vaccination without documentation, as this could leave patients inadequately protected against tetanus, which has serious morbidity and mortality implications 3
- Do not use serologic testing to assess pertussis immunity, as no validated test exists and CDC does not endorse this practice 1
- Remember that serologic testing for tetanus and diphtheria is an alternative strategy, not the primary recommendation—direct revaccination is preferred in most cases 1
- If serologic testing is performed and shows protective levels (>0.1 IU/mL for both tetanus and diphtheria), only a single dose of Tdap is needed, not a full series 1