Contagiousness of Tuberculosis

Who Is Contagious

The Centers for Disease Control and Prevention recommends that individuals with active pulmonary or laryngeal disease that is untreated or inadequately treated are considered contagious, particularly if they are coughing, have positive acid-fast bacilli (AFB) sputum smears, or have cavitary disease on chest radiograph 1, 2, 3
The highest risk for transmission comes from individuals who have pulmonary or laryngeal TB with cough or undergoing cough-inducing procedures, positive AFB sputum smears, cavitation on chest radiograph, or no treatment or inadequate treatment 1, 2, 3, 4, 5

The American Thoracic Society recommends that after starting standard multidrug therapy, infectiousness drops dramatically within the first 2 days, primarily due to isoniazid killing viable bacteria 6
The bacterial load in sputum decreases by >90% (10-fold) within 2 days of starting treatment, and >99% (100-fold) by days 14-21 due to rifampin and pyrazinamide effects 4, 6
For patients with drug-susceptible TB being discharged home, they can be considered non-infectious after 2-3 weeks of standard multidrug therapy if they have negligible likelihood of multidrug-resistant TB, received standard multidrug anti-TB therapy for 2-3 weeks, complete adherence to treatment, clinical improvement, and all close contacts identified, evaluated, and started on treatment if indicated 4, 6

The Centers for Disease Control and Prevention recommends that children with typical primary TB lesions are usually not contagious because they rarely produce forceful coughs, rarely develop cavitary lesions, and often have negative sputum AFB smears 2, 5
However, children can be infectious if they have laryngeal or extensive pulmonary involvement, pronounced cough, positive sputum AFB smears, cavitary TB on chest radiograph, or no therapy, just started therapy, or inadequate therapy 2, 5, 7
The World Health Organization recommends that if you have suspected or confirmed multidrug-resistant TB, you remain contagious much longer—potentially weeks to months—and require the stricter criteria of three consecutive negative sputum smears before being considered non-infectious 1, 6, 4

Active pulmonary or laryngeal tuberculosis is required for a person to be contagious; extrapulmonary disease alone is generally not transmissible. 8
A positive acid‑fast bacilli (AFB) sputum smear denotes the highest level of contagiousness. 8
Cavitary lesions identified on chest radiography markedly increase the risk of transmission. 8
Coughing or any procedure that aerosolizes respiratory secretions (e.g., sputum induction) elevates the likelihood of spreading infection. 8

Children younger than 10 years are rarely contagious even when they have active disease because they seldom generate forceful coughs, rarely develop cavitary lesions, and often have negative sputum smears. 8
Isolation is not required for children with typical primary TB lesions unless they exhibit laryngeal involvement, extensive pulmonary disease, pronounced cough, a positive sputum smear, or cavitary disease. 8

The CDC recommends defining the exposure window for contact investigations as the period from 10 weeks before the most recent negative tuberculin skin test (PPD) through 2 weeks before the first positive test, to approximate when infection occurred. 9