BCG and Positive Tuberculin Testing in Older Females
In an older woman with prior BCG vaccination and a positive tuberculin skin test (TST), the result is likely a false positive, and you should confirm true tuberculosis infection using an interferon-gamma release assay (IGRA). 1
Why BCG Causes False Positives in This Population
BCG vaccination creates cross-reactivity with tuberculin antigens that persists far longer than traditionally recognized. While tuberculin reactivity from BCG "generally wanes with the passage of time," 2 recent evidence demonstrates that BCG vaccination can influence TST results up to 55 years after vaccination 3. This is particularly relevant for older women who likely received BCG decades ago.
The magnitude of false positivity is substantial:
- BCG vaccination increases false-positive TST results by approximately 13% compared to IGRA testing 4
- Among BCG-vaccinated adults with positive TST, only 30% have a positive IGRA, meaning 70% are false positives 5
Key Factors That Modify False Positive Risk
Higher likelihood of TRUE infection (not BCG effect):
- TST induration >20 mm (unlikely caused by BCG) 2
- TST induration ≥16 mm 5
- Birth in or origin from high TB-incidence country 5, 6
- Abnormal chest radiograph consistent with healed TB 5
- Recent arrival to low-incidence country 6
Higher likelihood of FALSE positive (BCG effect):
- TST induration 10-15 mm range
- Birth in low TB-incidence country
- Multiple BCG vaccinations 4
- Living in low TB-incidence geographic area 4
Recommended Confirmation Strategy
The 2017 ATS/IDSA/CDC guidelines provide clear direction: Perform an IGRA rather than relying on TST alone in BCG-vaccinated individuals (strong recommendation for those unlikely to return for TST reading; conditional recommendation for others) 1.
Step-by-Step Approach:
Obtain IGRA testing (QuantiFERON-TB Gold or T-SPOT.TB)
- This is more specific for Mycobacterium tuberculosis infection
- Not affected by BCG vaccination
- Eliminates the 70% false-positive rate seen with TST alone 5
Obtain chest radiograph to exclude active TB disease 2
- Look for: upper lobe nodules, fibrotic scars, calcified granulomas, pleural thickening
- Normal chest X-ray supports latent infection (if IGRA positive) rather than active disease
Risk stratify based on combined findings:
- IGRA positive + any TST size = Treat as true latent TB infection
- IGRA negative + TST 10-15 mm + low-risk factors = Likely BCG false positive, no treatment needed
- IGRA negative + TST >20 mm + high-risk factors = Consider repeat IGRA or clinical judgment
Critical Pitfalls to Avoid
Do not dismiss BCG vaccination based on time elapsed. The conventional CDC teaching that BCG effects wane after 10 years is outdated—effects persist for 55+ years 3. Always consider BCG history regardless of when vaccination occurred.
Do not use TST size alone to distinguish BCG from true infection. While reactions >20 mm are "not likely caused by BCG" 2, this is not absolute. The two-step approach with IGRA is more reliable 5, 6.
Do not perform sputum examination unless chest radiograph shows findings suggestive of active TB or patient has respiratory symptoms 2. Asymptomatic persons with normal chest radiographs rarely have positive sputum studies.
When IGRA is Not Available
If IGRA testing is unavailable or too costly, use clinical risk stratification:
- Treat as true positive if: TST ≥16 mm, abnormal chest X-ray, high TB-incidence country origin, or recent TB exposure
- Consider observation if: TST 10-15 mm, normal chest X-ray, low TB-incidence country origin, no known TB exposure
The guideline explicitly states that "no reliable method has been developed to distinguish tuberculin reactions caused by vaccination with BCG from those caused by natural mycobacterial infections" using TST alone 2, making IGRA the preferred confirmatory test.