What workup is done to rule out active tuberculosis (TB)?

Diagnostic Workup to Rule Out Active Tuberculosis

The most effective workup to rule out active tuberculosis (TB) includes symptom screening, chest radiography, and collection of at least three sputum samples for acid-fast bacilli (AFB) smear microscopy, mycobacterial culture, and nucleic acid amplification testing. 1

Initial Assessment

Symptom Screening

• Screen for TB-suggestive symptoms:
  • Cough lasting ≥2-3 weeks (primary symptom)
  • Fever
  • Night sweats
  • Weight loss
  • Hemoptysis
  • Chest pain
  • Shortness of breath
  • Fatigue 2, 1

Risk Assessment

• Identify high-risk individuals:
  • Close contacts of persons with known/suspected TB
  • Foreign-born persons from high TB incidence areas
  • Residents/employees of congregate settings (prisons, homeless shelters)
  • Immunocompromised individuals (HIV, organ transplant recipients, anti-TNF therapy)
  • Patients with silicosis, diabetes, chronic renal failure, or malnutrition 1

Diagnostic Testing Algorithm

Step 1: Chest Radiography

• Obtain chest X-ray to look for:
  • Upper-lobe infiltration with cavitation
  • Patchy or nodular infiltrates in apical/subapical posterior upper lobes
  • Infiltrates in superior segment of lower lobe
  • Hilar/mediastinal adenopathy 1
• Note: HIV-infected patients may present with atypical radiographic findings or even normal chest X-rays 1

Step 2: Sputum Collection and Testing

• Collect at least 3 early morning sputum samples on separate days 1
• For patients unable to produce sputum, consider:
  • Sputum induction
  • Bronchoscopy with bronchoalveolar lavage 1
• Submit samples for:
  1. AFB smear microscopy (rapid results but limited sensitivity)
  2. Mycobacterial culture (gold standard, essential for definitive diagnosis)
  3. Nucleic acid amplification testing (NAAT) like GeneXpert MTB/RIF 1

Step 3: Additional Testing for Latent TB

• Interferon-gamma release assay (IGRA) is preferred over tuberculin skin test (TST) for individuals ≥5 years old due to:
  • Higher specificity (92-97% vs 56-95%)
  • Less cross-reactivity with BCG vaccination
  • No interference from most non-tuberculous mycobacteria 1, 3
• TST interpretation:
  • ≥5 mm induration is positive for high-risk individuals (HIV+, recent TB contacts)
  • ≥10 mm for moderate-risk individuals (recent immigrants, healthcare workers)
  • ≥15 mm for low-risk individuals 2

Interpretation of Results

Active TB is ruled out when:

• No TB-suggestive symptoms are present
• Chest radiograph shows no abnormalities consistent with active TB
• Three consecutive sputum samples are negative for AFB smear and culture 2, 1, 4

Active TB is likely when:

• Positive AFB smear or culture from sputum
• Positive NAAT with clinical and radiographic findings consistent with TB
• Chest radiograph showing typical TB lesions with compatible symptoms 1

Special Considerations

Immunocompromised Patients

• May present with atypical clinical and radiographic findings
• Lower threshold for diagnostic testing
• Consider additional imaging (CT scan) if chest X-ray is normal but suspicion remains high 1

Infection Control

• Implement respiratory isolation for patients with suspected TB until:
  • Three consecutive negative sputum smears are obtained AND
  • Clinical improvement is demonstrated 1, 4
• Provide surgical masks to patients with suspected TB 1

Common Pitfalls to Avoid

• Relying solely on chest X-ray for diagnosis (can miss cases, especially in HIV patients)
• Collecting inadequate sputum samples (quantity, quality, or number)
• Failing to report suspected or confirmed TB cases to local health departments
• Discontinuing isolation prematurely before obtaining adequate negative samples 1, 3

Remember that both suspected and confirmed cases of TB must be reported to local or state health departments 3.