Active pulmonary tuberculosis: when to suspect it

Causes and risk

High risk here means recent exposure to a person with a case of infectious TB; a history of a positive test result for Mycobacterium tuberculosis; HIV infection; injection or noninjection drug use; foreign birth and immigration within 5 years from a region of high incidence; residents and employees of high-risk congregate settings; membership in a medically underserved, low-income population; or a medical risk factor, which includes diabetes mellitus, prolonged corticosteroid and other immunosuppressive therapy, chronic renal failure, certain hematological malignancies and carcinomas, weight more than 10 percent below ideal body weight, silicosis, gastrectomy or jejunoileal bypass.

Among people with a positive tuberculin skin test, presumed infected with Mycobacterium tuberculosis, the risk factors for development of active tuberculosis fall into two groups. High risk: acquired immunodeficiency syndrome, human immunodeficiency virus infection, transplantation related to immune-suppressant therapy, silicosis, chronic renal failure requiring hemodialysis, carcinoma of head and neck, recent TB infection within 2 years, and abnormal chest x-ray with fibronodular disease. Moderate risk: tumour necrosis factor alpha inhibitors, diabetes mellitus of all types, treatment with glucocorticoids at ≥15mg/d prednisone, and young age when infected, 0-4 years.

Transmission itself is by inhalation of droplet nuclei of 1-5 microns aerosolized by coughing, sneezing or talking, and the air in a room occupied by a person with pulmonary tuberculosis may remain infectious for approximately 30 minutes even after his or her absence. What raises the risk of transmission is respiratory (pulmonary or laryngeal) disease, respiratory secretions that are acid-fast bacteria smear positive, presence of cough, HIV infection, atypical manifestations of disease, delayed diagnosis, cough-inducing procedures such as sputum induction or bronchoscopy, incorrect, ineffective or no therapy, delayed treatment, inadequate ventilation, duration of exposure and proximity to the infectious patient, overcrowding, absence of sunlight and high humidity.