If you have a positive Tuberculin (TB) skin test, do you get a chest x-ray or a blood test, such as an Interferon-Gamma Release Assay (IGRA)?

Management of Positive TB Skin Test

After a positive TB skin test, a chest X-ray should be performed to exclude active tuberculosis before considering blood tests or other diagnostic procedures. 1

Diagnostic Algorithm for Positive TB Skin Test

• A chest radiograph is indicated for all persons with a positive tuberculin skin test to exclude active pulmonary TB 1
• If the chest X-ray is normal and no symptoms consistent with active TB are present, the patient may be considered for treatment of latent tuberculosis infection (LTBI) 1
• If the chest X-ray shows abnormalities or the patient has symptoms suggestive of TB, further diagnostic evaluation including sputum examination is required 1

Interpretation of Positive TB Skin Test Results

• For high-risk individuals (HIV-infected, recent contacts of TB cases, immunocompromised), ≥5 mm induration is considered positive 2
• For moderate-risk individuals (healthcare workers, residents of congregate settings), ≥10 mm induration is considered positive 2
• For low-risk individuals with no known risk factors, ≥15 mm induration is considered positive 2

Role of Chest X-ray in TB Screening

• Chest radiography has high sensitivity for detecting manifestations of active TB 1
• Typical radiographic findings suggestive of TB include upper lobe infiltration (particularly with cavitation), patchy or nodular infiltrates in the apical or subapical posterior upper lobes 1
• The radiographic presentation may be atypical in HIV-infected patients, who may have infiltrates in any lung zone or mediastinal/hilar adenopathy 1
• In immunocompromised hosts, particularly those with low CD4 counts, CT should be considered as chest radiographs may be deceptively normal 1

Role of Blood Tests (IGRAs)

• Interferon-Gamma Release Assays (IGRAs) are not typically used after a positive TB skin test 3, 4
• IGRAs are primarily used as an alternative to the TB skin test for initial screening, not as a confirmatory test after a positive skin test 1, 2
• In BCG-vaccinated individuals, IGRAs may be preferred over skin testing for initial screening due to less cross-reactivity 1

Special Considerations

• Pregnant women with a positive tuberculin skin test should have chest radiographs (with appropriate shielding) as soon as feasible, even during the first trimester of pregnancy 1
• Children younger than 5 years of age should have both posterior-anterior and lateral chest radiographs 1
• For individuals with chest radiographic findings suggestive of prior, healed TB infections, three consecutive sputum samples should be obtained for AFB smear and culture 1
• HIV-infected persons with respiratory symptoms should have sputum specimens submitted for mycobacterial examination, even if the chest radiograph is normal 1

Common Pitfalls to Avoid

• Relying on blood tests (IGRAs) to confirm a positive skin test result rather than proceeding directly to chest X-ray 2, 4
• Failing to consider the patient's risk factors when interpreting skin test results 2
• Misinterpreting a boosted reaction as a new infection in serial testing programs 1, 2
• Assuming that a previous BCG vaccination is the cause of a positive skin test without appropriate follow-up 1, 2
• Neglecting to perform a chest X-ray in asymptomatic individuals with positive skin tests 1

Follow-up After Chest X-ray

• If chest radiographs are normal and no symptoms consistent with active TB are present, treatment for LTBI should be considered 1
• If radiographic or clinical findings are consistent with TB, further studies including bacteriologic examinations should be performed 1
• Routine follow-up chest films are not required for asymptomatic individuals with positive skin tests after the initial evaluation 1