Mycobacterium Tuberculosis (TB)

Risk Factors

Tuberculosis (TB) risk is significantly elevated in certain populations and circumstances. Key risk factors include:

• Housing conditions: Homelessness and crowded living situations, such as prisons
• Geographic factors: Immigration from or travel to developing nations
• Occupational exposure: Healthcare workers and others in medical settings
• Personal contacts: Close interaction with known TB patients

Clinical Presentation

TB typically presents with a combination of respiratory and systemic symptoms that develop gradually over time:

Respiratory symptoms: Persistent cough, hemoptysis (blood in sputum), and dyspnea (shortness of breath)

Systemic symptoms: Weight loss, fatigue, night sweats, fever, and cachexia (muscle wasting)

Sweating occurs because TB is a chronic inflammatory condition that elevates body temperature, triggering the body's cooling response. It is thought that when cortisol levels decrease at night, this allows for unregulated temperature control, causing night sweats due to unchecked temperature elevation.

Physical findings: Hypoxia, tachycardia, lymphadenopathy, abnormal lung examination findings

• Primary
  • Ghon focus: Small area of consolidation, usually in the mid-lower lobes.
  • Hilar lymphadenopathy: Enlargement of nearby lymph nodes is very common.
  • Ghon complex = Ghon focus + affected hilar lymph nodes.
  • Ranke complex = Calcified Ghon complex (indicates healed primary TB).
  • Pleural effusion: Often unilateral.
  • Miliary pattern (in disseminated disease): Tiny 1–3 mm nodules diffusely spread throughout both lungs—looks like "millet seeds."
• Secondary
  • upper lobe cavitary lesions typically
• Location: Apical and posterior segments of the upper lobes, or superior segments of lower lobes (oxygen-rich areas).
  • Cavitary lesions: Thick-walled cavitations, often with no air-fluid level (unless superinfected).
• Nodular infiltrates: Patchy areas of consolidation.
• Fibrosis and scarring: In chronic or healed TB.
• Volume loss and traction bronchiectasis in longstanding disease.

Duration: Symptoms typically persist for more than 3 weeks, distinguishing TB from acute respiratory infections

Diagnosis

Active Disease

Best initial test: Sputum acid-fast stain

• Important note: This test is not specific to TB and can detect other mycobacteria

Most accurate test: Mycobacterial culture of sputum

• Limitation: Results can take several weeks to obtain

Imaging findings:

• Chest X-rays typically show cavitary infiltrates in the upper lobes
• The Gohn complex may be present, characterized by calcifications in one or more nearby lymph nodes

Special considerations:

• Patients with HIV or primary tuberculosis may present differently, showing lower lobe infiltrates without cavitation
• This occurs because the lower lobes have greater volume and are more susceptible to primary infection
• Reactivation tuberculosis, in contrast, typically affects the upper lobes

Tuberculin Skin Test (TST)

The purified protein derivative (PPD) test involves injecting purified TB proteins under the skin, which triggers a type IV hypersensitivity reaction. Interpretation varies based on risk factors:

≥ 5 mm induration: Positive for patients with HIV, other significant risk factors, close TB contacts, or chest X-ray evidence of TB

≥ 10 mm induration: Positive for homeless individuals, prisoners, residents of developing nations, IV drug users, patients with chronic illnesses, residents of healthcare or correctional facilities, and healthcare workers

≥ 15 mm induration: Positive for everyone else not listed above (general population)

Important Testing Considerations

• Immunocompromised patients with latent TB infection may have negative tuberculosis skin tests due to inadequate immune response
• BCG vaccination can cause false-positive PPD reactions, though this effect typically diminishes within 5 years of vaccination

Treatment

Active Disease

Initial phase (2 months) - 4 for 2

• Four-drug regimen: INH (isoniazid), pyrazinamide, rifampin, and ethambutol

Continuation phase (4 months) - 2 for 4

• Two-drug regimen: INH and rifammin

Administer vitamin B6 (pyridoxine) with INH to prevent peripheral neuropathy

Latent Disease

For patients with a positive PPD test but no signs or symptoms of active disease:

• Treatment: INH monotherapy