In a close contact of a patient with smear‑negative pulmonary tuberculosis, is a repeat sputum acid‑fast bacilli (AFB) direct smear required before labeling the contact as having latent tuberculosis infection (LTBI)?

Evaluation of Close Contacts of Smear-Negative Pulmonary TB

No, a repeat sputum AFB smear is not required before labeling a close contact as having latent tuberculosis infection (LTBI). In fact, by definition, LTBI patients should have negative sputum examinations—if sputum AFB smears or cultures are positive, the patient has active TB disease, not latent infection 1.

Understanding the Distinction Between LTBI and Active TB

LTBI represents a state of persistent immune response to Mycobacterium tuberculosis antigens without clinically active disease. 1 The key diagnostic criteria are:

• Positive immunologic test (TST or IGRA) 1, 2
• Negative evaluation for active disease, including negative sputum examination 1
• Asymptomatic status with normal or stable chest radiographs 1

Active TB disease, in contrast, produces positive sputum cultures because bacteria are actively multiplying and present in respiratory secretions 1.

Proper Evaluation Protocol for Close Contacts

Initial Assessment

For close contacts of smear-negative pulmonary TB patients, the evaluation should proceed as follows:

1. Perform tuberculin skin test (TST) or IGRA at initial contact 3
2. Obtain chest radiograph to exclude active pulmonary TB 3, 1
3. Assess for TB symptoms: cough, fever, night sweats, weight loss 1

When Sputum Testing IS Indicated

Sputum examination is NOT indicated for most persons being considered for treatment of LTBI 3, 1. However, there are critical exceptions:

• Chest radiographic findings suggestive of prior, healed TB require three consecutive sputum samples on different days for AFB smear and culture 3, 1
• HIV-infected persons with respiratory symptoms require sputum examination, even if chest radiograph appears normal 3, 1
• Any patient with symptoms suggestive of active TB (cough, fever, night sweats, weight loss) requires full evaluation for active disease before any treatment decisions 1

When Sputum Testing is NOT Needed

If the chest X-ray is normal and no symptoms exist, sputum testing is not needed, and the patient is a candidate for LTBI treatment 1. Routine sputum testing in asymptomatic individuals with positive TST/IGRA and normal chest X-rays is unnecessary and wasteful 1.

Timing of Repeat Testing

Contacts with a negative initial tuberculin skin test should be retested approximately 8 to 12 weeks after the first test unless the initial skin test was performed more than 8 weeks after the contact's last exposure to the index patient 3. This window period accounts for the time required for tuberculin skin test results to become positive after infection (8-10 weeks) 3.

Window Period Management for High-Risk Contacts

During the 8-12 week window period between first and second skin tests, the following contacts with initially negative results should receive treatment for LTBI after TB disease has been ruled out by clinical examination and chest radiograph 3:

• Contacts younger than 5 years (highest priority for those younger than 3 years) 3
• Contacts with HIV infection or who are otherwise immunocompromised 3

Common Pitfalls to Avoid

Do not confuse the evaluation of a contact with the evaluation of the source case. The source case (the patient with smear-negative PTB) may require repeat sputum specimens to confirm their diagnosis, but this is separate from evaluating their contacts 3.

Do not order sputum AFB smears on asymptomatic contacts with normal chest radiographs. This wastes resources and may lead to false-positive results that complicate management 1.

Do not delay LTBI treatment in high-risk contacts waiting for a second TST. Children under 5 and immunocompromised individuals should begin treatment during the window period if initial evaluation excludes active disease 3.