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Infectious Disease

Pulmonary Tuberculosis

Airborne infection with Mycobacterium tuberculosis causing granulomatous lung disease. Distinct latent (LTBI) and active phases; cure requires 6+ months of multidrug therapy.

Last reviewed 3 May 2026 - MedicoMedics editorial team

Cavitary upper-lobe tuberculosis with granulomatous inflammation

Pathophysiology

Inhaled droplet nuclei reach the alveoli where macrophages engulf M. tuberculosis. The bacillus blocks phagolysosome fusion and replicates intracellularly. CD4+ Th1 cells release IFN-γ, activating macrophages to form caseating granulomas (Ghon focus). In 90% infection is contained (latent TB). Reactivation typically occurs in the upper lobes where higher O2 tension favours obligate aerobes, producing cavitary disease.

Sources: WHO Consolidated Guidelines on TB Treatment (2022 update); CDC TB Treatment Guidelines (MMWR 2022;71:285)

Clinical presentation

Active TB: chronic cough >3 weeks, haemoptysis, drenching night sweats, weight loss, low-grade fever, fatigue. Extrapulmonary sites (15-20%) include lymphadenitis (scrofula), Pott disease, tuberculous meningitis, pericarditis, adrenal (Addison) and genitourinary. Latent TB is asymptomatic with a positive TST/IGRA and normal CXR.

Sources: WHO Consolidated Guidelines on TB Treatment (2022 update); CDC TB Treatment Guidelines (MMWR 2022;71:285)

Diagnosis

Active TB: chest X-ray (upper-lobe infiltrates, cavitation, hilar lymphadenopathy, miliary pattern), 3 sputum specimens for AFB smear and culture (gold standard), Xpert MTB/RIF NAAT with rifampin resistance detection. Latent TB: interferon-gamma release assay (preferred if BCG-vaccinated) or tuberculin skin test with size cutoffs by risk. All patients need HIV testing.

Sources: WHO Consolidated Guidelines on TB Treatment (2022 update); CDC TB Treatment Guidelines (MMWR 2022;71:285)

Treatment - active TB

Intensive phase: 2 months of Rifampin, Isoniazid (+ pyridoxine), Pyrazinamide, Ethambutol (RIPE). Continuation phase: 4 months of rifampin + isoniazid. Directly observed therapy improves adherence. Monitor LFTs monthly, screen visual acuity (ethambutol optic neuritis) and check uric acid (pyrazinamide). Extend to 9 months for cavitary disease with positive 2-month culture.

Sources: WHO Consolidated Guidelines on TB Treatment (2022 update); CDC TB Treatment Guidelines (MMWR 2022;71:285)

Treatment - latent TB

Preferred short-course regimens: rifapentine + isoniazid weekly for 12 weeks (3HP), rifampin daily for 4 months (4R), or isoniazid + rifampin daily for 3 months (3HR). Isoniazid alone 6-9 months is an alternative. Always exclude active TB with CXR and symptom review before starting.

Sources: WHO Consolidated Guidelines on TB Treatment (2022 update); CDC TB Treatment Guidelines (MMWR 2022;71:285)

Sample USMLE-style MCQs

Try 10 questions on this topic. Practice hundreds more free with a trial.

Question 1

Which regimen is used during the initial 2-month intensive phase of drug-susceptible pulmonary TB?

Question 2

Which side effect requires baseline and monthly screening while a patient takes ethambutol?

Question 3

Best latent TB regimen?

Question 4

MDR-TB - preferred oral all-oral regimen?

Question 5

An HIV-positive patient has a TST induration of 6 mm. How should this be interpreted?

Question 6

Which is the best initial rapid test for confirming pulmonary TB and detecting rifampin resistance?

Question 7

Which test detects rifampin resistance rapidly?

Question 8

First-line active TB shorter regimen?

Question 9

Why does reactivation TB classically involve the lung apices?

Question 10

HIV+ patient with new TB and CD4 45. When start ART?

References

Primary guidelines and peer-reviewed sources used for this entry. Reviewed 3 May 2026 by MedicoMedics editorial team.

Frequently asked

What TST induration size is positive?

≥5 mm in HIV, close contacts, immunosuppressed or CXR fibrotic changes; ≥10 mm in high-risk populations; ≥15 mm in low-risk individuals.

Why add pyridoxine (B6) to isoniazid?

INH depletes pyridoxine and causes peripheral neuropathy; supplementing B6 25-50 mg/day prevents this.

What defines MDR-TB and XDR-TB?

MDR-TB is resistant to at least isoniazid and rifampin. XDR-TB is MDR plus resistance to any fluoroquinolone and at least one injectable second-line drug.

Can a BCG-vaccinated patient have a positive TST from vaccination?

Yes, BCG can cause false-positive TST for years, so IGRA is preferred when latent TB is being ruled in or out.

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