From the Guidelines
A patient with active tuberculosis who is acid-fast bacillus (AFB) negative on sputum smear is generally considered to have low infectiousness, but they are not completely non-infectious. These patients are approximately 10-20 times less infectious than those who are AFB smear-positive 1. However, they can still transmit TB, particularly to close contacts with prolonged exposure. For infection control purposes, patients with suspected or confirmed active TB should be placed in airborne isolation regardless of their AFB smear status until they have received effective anti-TB therapy for at least 2 weeks, show clinical improvement, and ideally have three consecutive negative AFB smears 1. Healthcare workers should use N95 respirators when caring for these patients. The lower infectiousness in AFB-negative cases is due to the smaller bacterial load in respiratory secretions, which makes the bacteria less likely to be aerosolized and transmitted during coughing, speaking, or other respiratory activities. Despite the negative smear, culture may still be positive, confirming the presence of viable mycobacteria that can potentially cause infection in others. Key factors to consider when assessing infectiousness include the extent of illness, presence of cavitary pulmonary disease, degree of positivity of sputum AFB smear results, frequency and strength of cough, likelihood of infection with multidrug-resistant organisms, and nature and circumstances of contact between the infected person and exposed contacts 1. In congregate settings, more stringent criteria for determining absence of infectivity should be followed, including three consecutive AFB-negative sputum smears 1. It's also important to note that patients who remain in hospitals or reside in congregate settings are subject to different criteria for infectiousness, and all patients with suspected or proven multidrug-resistant TB should be subjected to these more stringent criteria for absence of infectivity 1. Overall, while AFB-negative patients with active TB are considered to have low infectiousness, they still pose a transmission risk, particularly in high-risk settings or with prolonged exposure, and should be managed with appropriate infection control measures.
From the Research
Infectiousness of AFB Negative Patients with Active TB
- The infectiousness of a patient with active tuberculosis (TB) who is acid-fast bacillus (AFB) negative is a concern in the management of TB 2.
- A study found that AFB smear-negative patients were more likely to have false-negative T-SPOT.TB results, which could lead to delayed diagnosis and treatment 2.
- However, the use of Mycobacterium tuberculosis PCR can help rule out active disease and reduce the need for isolation in AFB smear-negative patients 3.
Risk Factors for Negative AFB Results
- Female sex, age, AFB smear-negative, and HIV coinfection are associated with negative T-SPOT.TB results 2.
- Chronic cough and heavy AFB staining are risk factors for inappropriate administration of unnecessary anti-TB treatment in patients with AFB smear-positive sputum 4.