What tests are used for tuberculosis (TB) screening?

Tuberculosis Screening Tests

For tuberculosis (TB) screening, a combination of symptom evaluation, chest radiography, and either tuberculin skin test (TST) or interferon-gamma release assay (IGRA) should be used, with IGRA being preferred in BCG-vaccinated individuals due to its higher specificity. 1

Primary Screening Methods

Symptom Evaluation and Risk Assessment

• Initial screening should include assessment for:
  • Presence of TB symptoms (cough, hemoptysis, fever, night sweats, weight loss, chest pain, shortness of breath, fatigue)
  • TB exposure history (household contacts, travel to endemic areas)
  • Risk factors for progression to active TB (HIV, immunosuppression, diabetes)

Radiographic Screening

• Chest X-ray to rule out active TB
• Any abnormality on chest radiography combined with TB symptoms offers the highest sensitivity (90%) and negative predictive value (99.8%) for ruling out active TB 1

Immunologic Testing

Two main tests are available:

1. Tuberculin Skin Test (TST)

   • Traditional method requiring intradermal placement of tuberculin purified protein derivative
   • Reading required 48-72 hours after placement
   • Limitations:
     • False positives in BCG-vaccinated individuals
     • False negatives in immunocompromised patients
     • Requires patient return for reading

2. Interferon-Gamma Release Assays (IGRAs)

   • Blood tests that measure interferon-gamma release after stimulation with TB antigens
   • Two commercial options:
     • QuantiFERON-TB Gold (QFT-G)
     • T-SPOT.TB
   • Advantages:
     • No cross-reactivity with BCG vaccination or most non-tuberculous mycobacteria 1
     • Single patient visit
     • More specific than TST
     • Better correlation with TB risk in immunocompromised hosts 2

Testing Strategy Recommendations

General Population

• Either TST or IGRA can be used in high-income and upper middle-income countries with TB incidence less than 100 per 100,000 1
• IGRA is preferred in BCG-vaccinated individuals 1

Special Populations

Healthcare Workers

• Baseline screening with individual risk assessment plus either TST or IGRA 1
• Postexposure testing for those with negative baseline tests
• Serial screening no longer routinely recommended unless specific risk factors present 1

Immunocompromised Patients

• Dual testing strategy (both TST and IGRA) recommended for:
  • Patients on immunosuppressive therapy 1
  • Patients in countries with medium or high TB prevalence 1
  • Those at high risk for progression to active TB 1

Patients Requiring Biologic Therapy

• Screen before initiating anti-TNF therapy, JAK inhibitors, or other biologics 1
• For inflammatory bowel disease patients, use combination of clinical risk stratification, chest X-ray, and IGRA 1
• For rheumatologic conditions, dual testing may improve diagnostic yield 3

Interpretation Considerations

• Positive results on either test should prompt evaluation for active TB before LTBI diagnosis
• Negative results in high-risk individuals should be interpreted with caution
• For contacts of active TB cases, negative initial tests should be repeated 8-10 weeks after last exposure 1
• Agreement between TST and IGRA is only moderate (kappa values 0.33-0.50) 3, 4, 5

Common Pitfalls to Avoid

1. Relying solely on TST in BCG-vaccinated individuals
2. Failing to repeat testing 8-10 weeks after exposure in contacts with initial negative results
3. Not considering immunosuppression when interpreting negative results
4. Overlooking the need to rule out active TB before diagnosing LTBI
5. Not collecting blood for IGRA within the required timeframe (must be processed within 12 hours) 1

By following these evidence-based recommendations, clinicians can optimize TB screening to identify latent infection and prevent progression to active disease, ultimately reducing TB-related morbidity and mortality.