Diagnosis of Active Tuberculosis: TB Gold Test and Chest Imaging
A positive TB Gold test (IGRA) alone does not indicate active tuberculosis; chest imaging and additional diagnostic tests are necessary to confirm the diagnosis of active tuberculosis. 1
Understanding TB Gold Test Results
The TB Gold test (QuantiFERON-TB Gold or IGRA) is designed to detect tuberculosis infection, but cannot distinguish between latent TB infection and active TB disease:
- IGRAs have higher specificity (92-97%) compared to tuberculin skin tests (56-95%) 2
- They have less cross-reactivity with BCG vaccination or exposure to non-tuberculous mycobacteria 2
- A positive TB Gold test indicates TB infection but requires further evaluation to determine if the infection is active or latent 1
Diagnostic Algorithm for Suspected TB
Step 1: Initial Assessment
- Consider TB in patients with persistent respiratory symptoms (cough ≥2-3 weeks), especially with fever, night sweats, weight loss, or hemoptysis 1
- Assess risk factors: close contact with active TB patients, immigration from TB-endemic countries, HIV infection, immunocompromised status 1
Step 2: Imaging
- Chest radiography (X-ray) is the first-line imaging test for suspected TB with high sensitivity for detecting manifestations of active TB 2, 1
- Radiographic findings suggestive of active TB include:
Step 3: Bacteriological Confirmation
- Collect at least 3 sputum specimens on different days for:
- AFB smear microscopy (rapid but limited sensitivity)
- Mycobacterial culture (definitive diagnosis)
- Nucleic acid amplification testing (NAAT) 1
- Smear-negative results do not exclude TB diagnosis; approximately 60% of culture-positive TB patients have positive AFB smears 2
Step 4: Additional Testing for Equivocal Cases
- CT is appropriate when chest X-ray findings are equivocal/non-diagnostic or in immunocompromised patients with high clinical suspicion but negative chest X-ray 2, 1
- CT has higher specificity than chest X-ray and can detect:
- Centrilobular nodules
- "Tree-in-bud" appearance
- Cavitation
- Upper lobe/superior segment lower lobe involvement 1
Important Clinical Considerations
Special Populations
- HIV-infected patients may present with atypical radiographic findings:
- Infiltrates in any lung zone rather than typical apical cavitary disease
- Mediastinal/hilar adenopathy
- Rarely, normal chest radiograph 2
- Immunocompromised patients may benefit from CT when clinical suspicion is high despite unrevealing chest radiograph 2
Diagnostic Pitfalls
- Beware of false negatives: Advanced immunosuppression may cause false-negative results in all immunologically-based tests for TB infection 2
- Avoid misinterpreting latent TB as active disease: The rationale for chest imaging after a positive TB test is to distinguish latent TB from active TB, as these are managed differently 2
- Don't rely solely on TB Gold test: In one study, among contacts who developed active TB, 49% initially had negative QFT-G results 3
Infection Control
- Patients with suspected TB should be placed in respiratory isolation until determined non-infectious 1
- Isolation should continue until three consecutive negative sputum smears are obtained and clinical improvement is demonstrated 1
Conclusion of Active TB Diagnosis
A definitive diagnosis of active TB requires:
- Clinical symptoms consistent with TB
- Radiographic findings suggestive of active TB
- Microbiological confirmation through positive culture for M. tuberculosis (gold standard) or positive NAAT with clinical correlation 1
Remember that a positive TB Gold test alone is insufficient for diagnosing active TB; chest imaging and microbiological confirmation are essential components of the diagnostic process.