What is the first recommended test for suspected tuberculosis (TB) in a patient without a cough and a normal chest X-ray (CXR)?

Diagnostic Testing for Tuberculosis in Patients Without Cough and Normal Chest X-ray

For patients with suspected tuberculosis who don't have cough and have a normal chest X-ray, either an Interferon-Gamma Release Assay (IGRA) or Tuberculin Skin Test (TST) should be performed as the first diagnostic test. 1

Initial Testing Approach

Primary Testing Options

• IGRA (Interferon-Gamma Release Assay)

  • Preferred in BCG-vaccinated individuals
  • Examples: QuantiFERON-TB Gold, T-SPOT.TB
  • No need for return visit to read results

• TST (Tuberculin Skin Test)

  • Also known as Mantoux test
  • Requires proper placement technique to produce a "wheal"
  • Must be read 48-72 hours after placement
  • Induration (not erythema) is measured for interpretation 2

Interpretation of Results

• Positive IGRA or TST with ≥5mm induration indicates TB infection 3
• A history of BCG vaccination is not a contraindication to TST testing 2

Testing Algorithm for Suspected TB Without Respiratory Symptoms

1. Initial test: Either IGRA or TST

   • Choose IGRA for BCG-vaccinated individuals
   • Choose TST when IGRA is unavailable or cost-prohibitive

2. If initial test is negative but TB suspicion remains high:

   • Consider repeat or dual testing (both IGRA and TST) 1
   • This strategy increases sensitivity at the expense of some specificity
   • Particularly important in immunocompromised patients 3

3. If initial test is positive:

   • Perform confirmatory testing with the alternative test (if initial was TST, perform IGRA; if initial was IGRA, consider TST) 1
   • This helps identify false-positive results

Special Considerations

High-Risk Populations

For individuals at higher risk of TB, maintain a lower threshold for testing:

• Foreign-born persons from high-incidence areas
• Close contacts of persons with known/suspected TB
• Residents/employees of congregate settings
• Immunocompromised individuals 3

Extrapulmonary TB Evaluation

If extrapulmonary TB is suspected despite normal chest X-ray:

• Cell counts and chemistries should be performed on amenable fluid specimens (pleural, cerebrospinal, ascitic, joint fluids) 1
• Adenosine deaminase levels should be measured on fluid collected from suspected sites 1

Important Caveats

• The absence of cough and normal chest X-ray does not rule out TB, especially in immunocompromised patients who may present atypically 3
• Better correlation between IGRA and TST occurs after the "window period" (approximately 2 months after exposure) 4
• Both IGRA and TST have limitations in sensitivity and specificity; negative results do not definitively exclude TB infection 5
• All suspected and confirmed cases of TB must be reported to local or state health departments 5

Follow-up Testing

If TB infection is confirmed (positive IGRA or TST) but active disease is still suspected despite normal chest X-ray:

• Consider additional imaging (CT scan, MRI) particularly for suspected extrapulmonary TB 6
• Collection of appropriate specimens from suspected sites of infection for AFB smear, culture, and nucleic acid amplification testing 3