STEPTRACKERRESPIRATORY | PATHOLOGY + MICROBIOLOGY | HIGH-YIELD REVIEW

Tuberculosis

Mycobacterium tuberculosis: a Th1 granuloma that contains the infection — until immunity falls and the apex cavitates.

FOCUS ON
Primary TB makes a Ghon complex (subpleural focus + hilar node) and usually goes latent. Secondary TB reactivates at the oxygen-rich apex and cavitates. Know the IL-12 → IFN-γ → TNF granuloma loop, the extrapulmonary sites, PPD vs IGRA, and the side effects of each RIPE drug.
STAGE / TOPIC
DETAIL
MECHANISM
CLUE
Organism & risk
Acid-fast, aerobic bacillus with mycolic-acid wall; airborne droplet nuclei.
Cord factor blocks phagolysosome fusion; sulfatides help survival
HIV, endemic-country birth, prison, homelessness, diabetes, silicosis, anti-TNF, steroids
Primary TB
Ghon focus (subpleural, lower upper lobe / upper lower lobe) + hilar node = Ghon complex; calcified = Ranke complex.
Th1 granulomas contain the bacilli
Usually silent → latent infection (PPD/IGRA ⁺)
Progressive primary TB
Infection keeps spreading after first exposure.
Weak cell-mediated immunity
Young children, HIV
Secondary (reactivation) TB
Apical upper-lobe lesions with caseation and cavities.
High O₂ tension at apex; caseous necrosis erodes into bronchi → spread
Fever, night sweats, weight loss, hemoptysis
Miliary TB
Countless millet-seed granulomas in lungs, liver, spleen, kidney, meninges, marrow.
Hematogenous dissemination
HIV, infants
Extrapulmonary TB
Meningitis (basilar, CN palsies), Pott disease (thoracic spine, gibbus, psoas abscess), adrenals (Addison), kidney (sterile pyuria), cervical nodes (scrofula), pericardium (constriction), ileocecal gut.
Seeding during primary bacteremia
CSF: lymphocytes, ↑ protein, ↓ glucose
Diagnosis
PPD (cutoff 5/10/15 mm by risk) or IGRA; chest x-ray; sputum AFB smear, culture (gold standard), NAAT.
PPD = type IV hypersensitivity
BCG → false ⁺ PPD (use IGRA) · anergy (HIV, sarcoid, steroids) → false ⁻
Treatment
Active: rifampin + isoniazid + pyrazinamide + ethambutol for 2 months, then RIF + INH for 4 months. Latent: rifampin 4 months, INH 9 months, or INH + rifapentine.
Multidrug to prevent resistance
Directly observed therapy

Mechanism — the TB granuloma

Inhaled bacilli
→
Alveolar macrophageBacilli multiply (cord factor)
→
IL-12 → Th1
→
IFN-γEpithelioid cells, giant cells
→
Caseating granulomaHeld by TNF

Contained → latent. Immunity falls (HIV, anti-TNF, age) → reactivation at the apex or dissemination.

Primary vs secondary · drug toxicities

FEATURE
PRIMARY
SECONDARY
When
First exposure
Reactivation (or reinfection)
Site
Mid-lung subpleural + hilar node
Apices of upper lobes
Lesion
Ghon complex
Cavitation
Symptoms
Usually none
Fever, sweats, weight loss, hemoptysis
Isoniazid
Neuropathy (give B6), hepatitis, drug-induced lupus
Rifampin
Orange body fluids, P450 induction, hepatitis
Pyrazinamide
Hyperuricemia/gout, hepatitis
Ethambutol
Optic neuritis, red-green color loss

How it's tested

COMMON CONFUSIONPrimary TB is not in the apex. The Ghon focus sits in the middle zones; apical cavities mean reactivation.
THE TRAPBefore starting infliximab or another TNF blocker, screen for latent TB. TNF keeps granulomas intact, and blocking it reactivates infection.
YOUR APPROACHExposure risk → primary vs reactivation vs miliary → site → test (PPD/IGRA vs sputum) → RIPE and its toxicities.

If you see… think…

Calcified subpleural nodule + calcified hilar node → Ranke complex (healed primary TB)
Immigrant with night sweats, weight loss, hemoptysis, apical cavity → reactivation TB
About to start infliximab → screen for latent TB (IGRA)
HIV patient with tiny nodules throughout both lungs → miliary TB
Thoracic vertebral collapse + psoas abscess → Pott disease
Basilar meningitis, CN palsies, low CSF glucose, lymphocytes → TB meningitis
Positive PPD in a BCG-vaccinated person → confirm with IGRA
Orange urine and tears on therapy → rifampin
Numb feet on TB therapy → isoniazid — add pyridoxine
Sterile pyuria with hematuria → renal TB