Absence of BCG Scar: Clinical Significance and Management
Primary Interpretation
Absence of a BCG scar does not necessarily mean the child was not vaccinated or is unprotected, as 17-49% of vaccinated individuals may not develop a visible scar, but it does warrant tuberculin skin testing to assess immune response and potential TB infection risk. 1, 2, 3
Understanding BCG Scar Formation
Normal Vaccination Response
- BCG vaccination generally results in a permanent scar at the puncture site, typically forming after a pustule develops at 2-3 weeks, ulcerates at 6 weeks, and heals within 3 months 1
- However, scar formation is not universal: studies show 51-83% of vaccinated children develop visible scars, with significant variation by age at vaccination and time since vaccination 2, 3
- In adults, scar prevalence is higher (87%), but among children vaccinated in infancy, only 51% may have visible scars 2
Factors Affecting Scar Formation
- Vaccine dose and strain: Different BCG strains and administration methods affect scar formation rates 1
- Age at vaccination: Infants vaccinated in the first month of life show variable scar formation (83% in one study, but lower in others) 3, 4
- Perinatal factors: Nutritional status and maternal antibody interference may reduce vaccine response and scar formation 4
- Individual skin characteristics: Some individuals simply do not form visible scars despite adequate immune response 1
Clinical Implications of No Scar
Does Absence of Scar Mean No Protection?
The presence or size of a BCG scar does NOT predict whether the vaccine provided protection against TB disease 1. This is a critical distinction:
- Scar presence correlates with tuberculin reactivity (58% of children with scars are TST-positive vs. 15% without scars), but this does not equate to protection 2
- Research shows BCG-vaccinated children with scars had no better clinical protection against M. tuberculosis bloodstream infections compared to those without scars 5
- The immune response measured by tuberculin testing does not predict vaccine efficacy 1
Risk Assessment in High TB Prevalence Areas
For children from high TB prevalence regions without a BCG scar, consider:
- Increased infection risk: One study found absence of BCG scar was an independent risk factor for TB infection (OR 0.60 for vaccinated vs. unvaccinated, meaning unvaccinated had 1.67 times higher risk) 6
- Poor vaccine response: 25% of Asian children vaccinated at birth had no scar, and half of those with scars were tuberculin-negative, suggesting inadequate immune response 4
- Need for evaluation: These children require tuberculin skin testing to assess both vaccine response and potential TB infection 1
Recommended Management Algorithm
Step 1: Verify Vaccination History
- Review vaccination records to confirm BCG was actually administered 7
- Document reported vaccination date and method of administration 1
Step 2: Perform Tuberculin Skin Testing
All individuals without a BCG scar should undergo tuberculin skin testing with 5 TU PPD 1:
- Test should be performed regardless of vaccination history 1
- Results should be read at 48-72 hours and recorded in millimeters of induration 1
Step 3: Interpret TST Results
For individuals from high TB prevalence areas, use ≥10 mm induration as the threshold for positive 1, 8:
TST ≥10 mm: Consider this true M. tuberculosis infection, not BCG effect, especially if:
TST 5-9 mm: Intermediate result requiring clinical judgment based on:
TST <5 mm: Suggests either no vaccination occurred, vaccine failure, or waning immunity 1
Step 4: Consider Alternative Testing
Interferon-Gamma Release Assays (IGRAs) are preferred over TST in BCG-vaccinated individuals because they eliminate false-positive results from BCG cross-reaction 8, 7:
- IGRAs measure response to M. tuberculosis-specific antigens not found in BCG 8, 7
- Prior BCG vaccination does not cause positive IGRA results 7
- Use IGRA when TST interpretation is uncertain or to confirm TST results 8
Step 5: Management Based on Results
If TST ≥10 mm or IGRA positive:
- Perform chest radiograph to exclude active TB disease 8
- If chest X-ray normal, treat for latent TB infection per standard guidelines 1, 8
- Evaluate household contacts and close contacts for TB 8
If TST <5 mm and no TB exposure:
- Consider revaccination if indications persist and child is >1 year old 1
- Continue monitoring if ongoing TB exposure risk 1
- Include in serial testing programs if applicable (e.g., healthcare workers) 1
Critical Pitfalls to Avoid
Do Not Assume BCG Scar Absence Means No Vaccination
- Up to 49% of vaccinated children may lack visible scars 2, 4
- Absence of scar does not rule out prior vaccination or immune response 2
Do Not Attribute Positive TST to BCG in High-Risk Populations
This is the most important clinical error to avoid: Several evidence-based reasons support treating positive TST as true infection 1, 8:
- BCG-induced tuberculin reactivity wanes over time and is unlikely to persist >10 years 1
- Mean TST reaction size in BCG-vaccinated persons is often <10 mm 1
- TST conversion rates after BCG are not 100% 1
- In high TB prevalence areas, the probability of true infection far exceeds the probability of BCG-induced reactivity 1
Do Not Delay Evaluation in Symptomatic Children
- Children without BCG scars who have TB exposure require immediate evaluation regardless of vaccination history 1
- Symptoms of chronic cough, fever, or weight loss warrant chest radiograph and further workup even with negative TST 8