What is the recommended initial screening test for tuberculosis (TB)?

TB Screening Test Recommendation

Either an interferon-gamma release assay (IGRA) or tuberculin skin test (TST) is acceptable as the initial screening test for tuberculosis, with IGRA preferred in patients with prior BCG vaccination, those on immunosuppressive therapy, or when return for TST reading is unlikely. 1, 2

Initial Screening Test Options

Both testing modalities are endorsed by the American Thoracic Society, Infectious Diseases Society of America, and CDC as acceptable first-line screening tools 1:

• IGRA (blood test): Measures interferon-gamma release from T cells in response to M. tuberculosis-specific antigens 3
• TST (Mantoux skin test): Intradermal injection of purified protein derivative (PPD) requiring return visit in 48-72 hours for reading 1, 4

When to Prefer IGRA Over TST

IGRA should be your first choice in these specific situations 2:

• Patients with history of BCG vaccination (IGRA is not confounded by BCG, while TST produces false positives) 1, 3
• Patients already on immunosuppressive therapy 2
• Situations where patient return for TST reading within 48-72 hours is unlikely 2
• Healthcare workers and other populations requiring serial testing (avoids booster phenomenon confusion) 3

The key advantage: IGRA has superior specificity (0.95-1.0) compared to TST in BCG-vaccinated populations because it uses antigens absent from BCG vaccine 3, 5.

TST Remains Acceptable When

• IGRA is unavailable or cost-prohibitive 1
• Patient can reliably return for reading 2
• No prior BCG vaccination 1

Critical technical point: TST must produce a visible "wheal" at injection; if significant leakage occurs or injection is subcutaneous, repeat immediately at another site 4. Measure induration (not erythema) transversely; ≥5 mm is positive for any contact or high-risk individual 1.

Interpretation Cutpoints for Positive Results

Use ≥5 mm induration for TST in these populations 1:

• All TB contacts (regardless of exposure duration) 1
• HIV-infected persons 2
• Patients on immunosuppressive therapy 2
• Recent immigrants from high-burden countries 2
• Healthcare workers 1

The traditional ≥10 mm cutpoint is outdated for targeted screening; modern guidelines universally recommend ≥5 mm for at-risk populations 1, 5.

Confirmatory Testing Strategy

For asymptomatic low-risk persons with positive initial test, perform a second confirmatory test (either IGRA or TST); consider infected only if both are positive 1, 2. This approach reduces false-positive results in low-prevalence populations where consequences of unnecessary treatment may outweigh benefits 1.

Conversely, for high-risk contacts or immunocompromised patients, a single positive test is sufficient to proceed with evaluation 1, 2.

Post-Exposure Testing Timeline

Perform initial test immediately when exposure is recognized, then repeat at 8-10 weeks post-exposure if initially negative 2, 6. This accounts for the immunologic window period—the time required for detectable immune response to develop 1, 6.

Critical pitfall: Never assume a negative test during the first 8 weeks post-exposure rules out infection 6. For children <5 years and immunocompromised contacts, initiate window prophylaxis immediately after excluding active disease, even with negative initial testing 1, 6.

Next Steps After Positive Screening

• Obtain chest radiograph to distinguish latent TB infection from active disease 2, 7
• Perform symptom evaluation for cough, fever, night sweats, weight loss, hemoptysis 2
• If chest X-ray is abnormal or symptoms present, collect sputum for acid-fast bacilli smear and culture 1, 7
• Report all confirmed cases to local/state health departments 7

Common Pitfalls to Avoid

• Do not skip testing in BCG-vaccinated persons—history of BCG is not a contraindication to screening; use IGRA preferentially 1, 2, 4
• Do not repeat testing in persons with documented prior positive results—instead, perform symptom screening only 2
• Do not use two-step testing for contact investigations—this is for baseline occupational screening only, not post-exposure evaluation 1
• Do not delay prophylaxis in high-risk contacts (children <5, HIV-infected, immunocompromised) while waiting for 8-10 week repeat test 1, 6