Tuberculosis - Pulmonary Disease Background

Last updated: 22 July 2026
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Introduction

Pulmonary tuberculosis (PTB) is a case of tuberculosis (TB) that involves the lung parenchyma. Miliary TB is considered pulmonary tuberculosis since lung lesions are also seen. TB in the pleura, mediastinal, and/or hilar lymph nodes with no evidence of abnormalities in the chest X-ray is considered extrapulmonary TB (EPTB). Patients presenting with both PTB and EPTB are classified as a case of pulmonary tuberculosis.

A TB suspect is anyone who has signs or symptoms suggestive of TB (eg ≥2 weeks productive cough and/or abnormal chest X-ray). Definite TB is considered in patients with culture or molecular tests positive for Mycobacterium tuberculosis or in patients with at least one sputum smear positive for acid-fast bacilli (AFB). TB cases are also classified based on the disease's anatomical site, bacteriological results (including drug resistance), previous treatment history, and patient’s human immunodeficiency virus (HIV) status.

Epidemiology

Approximately 1.7 billion people are estimated to have been infected with M tuberculosis.  The burden of tuberculosis is greatest in low-income countries, particularly India and China. Pulmonary tuberculosis affects individuals of all age groups. Globally, the incidence of PTB is approximately twice as high in males as in females.

Southeast Asia accounted for 46% of global tuberculosis cases, with India, Indonesia, China, the Philippines, Pakistan, and Bangladesh among the most affected countries. The Philippines had an estimated incidence of over 500 cases per 100,000 population, while Australia and New Zealand had fewer than 10 cases per 100,000. The burden of tuberculosis (TB) in Asia is influenced by a complex interplay of social, economic, environmental, and biological factors. The prevalence of extrapulmonary tuberculosis (EPTB) continues to increase across South and Southeast Asia, particularly in Pakistan, India, Bangladesh, and Afghanistan.

Etiology

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Mycobacterium tuberculosis is a slow-growing, aerobic, nonmotile, acid-fast bacillus that has a lipid-rich cell wall and capsule that contribute to its virulence, resistance, and survival. As a facultative intracellular pathogen, it survives and replicates within macrophages, eventually causing their destruction and promoting disease spread.

Pathophysiology

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Tuberculosis is primarily transmitted from person to person through inhalation of airborne droplet nuclei released by individuals with active pulmonary TB, and the risk of transmission increases with prolonged exposure in enclosed spaces. Once inhaled, the bacilli may be eliminated by the host immune system, remain dormant as a latent tuberculosis infection, or progress to active disease depending on the interaction between bacterial virulence factors and the host's immune response. The pathophysiology of tuberculosis involves pulmonary inflammation, tissue destruction, and caseous necrosis caused by M tuberculosis.

Risk Factors

Risk factors for tuberculosis include residence in or immigration from high-burden countries (eg India, China, Indonesia, the Philippines, Vietnam, African countries, and Mexico); close contact with an individual with active tuberculosis; HIV infection; chronic renal failure; on immunomodulatory therapy; and poorly controlled diabetes mellitus (DM).

Classification

Patient Registration Group

The patient registration group is the basis of management for each patient.

New Patient

New patients are patients who have never had tuberculosis treatment or have taken an anti-TB regimen for <1 month. These patients are assumed to be drug susceptible unless the patient is from an area with a high prevalence of Isoniazid resistance or if the patient had active TB after contact with a documented multidrug-resistant TB (MDR-TB) patient. Patients who have MDR-TB at any time during the therapy will be considered treatment failures.

Previously Treated Patient

A previously treated patient has a history of taking anti-TB drugs for ≥1 month and is currently smear- or culture-positive again. This is further subclassified based on the outcome of their most recent treatment course:

  • Relapse: Patient has been cured or completed most recent treatment
  • Failure: Most recent treatment has failed
  • Default: Treatment was interrupted for ≥2 consecutive months

 

A previously treated patient strongly determines resistance to drugs. Timely detection of multi-drug resistance (MDR) and the start of an MDR regimen with second-line drugs give a better likelihood of cure and prevents the development and transmission of further resistance. The incidence of MDR is higher in previously treated patients than in new patients. Patients returning after defaulting or relapsing have lower MDR rates than patients who had treatment failure. Patients with prior treatment failure should be given an empirical MDR regimen while waiting for test results if conventional drug susceptibility testing (DST) was used. This prevents clinical deterioration of the patient and decreases the risk of transmission to contacts.

The National Tuberculosis Control Program (NTP) should use country-specific drug resistance data of patient groups on failure, relapse, and default to know the level of MDR. High MDR levels are noted in patients who are treated in poorly operating NTP; living with HIV; with type 2 diabetes mellitus (DM); with a history of using poor- or unknown-quality anti-TB drugs; exposed in institutions with high prevalence rates or outbreaks of MDR; with conditions associated with malabsorption or rapid-transit diarrhea; whose prior regimen included Rifampicin throughout the course; and who still have positive sputum smears at the second or third month of treatment.

The World Health Organization (WHO) classifies cases of drug-resistant tuberculosis into five categories: 

  • Isoniazid-resistant tuberculosis
  • Rifampicin-resistant tuberculosis (RR-TB)
  • MDR-TB defined as resistance to Isoniazid and Rifampicin
  • Pre-extensively drug-resistant tuberculosis (pre-XDR-TB) defined as resistance to Rifampicin and any fluoroquinolone
  • Extensively drug-resistant tuberculosis (XDR-TB) defined as resistance to Rifampicin, any fluoroquinolone, plus Bedaquiline and/or Linezolid