What is latent tuberculosis infection (LTBI)?

What is Latent Tuberculosis Infection (LTBI)?

Latent tuberculosis infection (LTBI) is characterized by the presence of immune responses to previously acquired Mycobacterium tuberculosis infection without any clinical evidence of active tuberculosis disease. 1

Key Defining Features

LTBI represents a state where individuals harbor M. tuberculosis but remain asymptomatic, non-infectious, and show no clinical or radiographic signs of active TB disease. 1 The infection persists in a dormant state, detectable only through immunological testing that demonstrates the body's adaptive immune response to mycobacterial antigens. 2, 3

Clinical Characteristics

• No symptoms or signs of active TB disease - patients are completely asymptomatic 1
• Not infectious - individuals with LTBI cannot transmit TB to others 1
• Positive immunological tests - either tuberculin skin test (TST) or interferon-gamma release assay (IGRA) demonstrates immune response to M. tuberculosis antigens 1
• Normal chest radiography - no radiographic evidence of active TB disease 1

Epidemiology and Risk

Approximately one-third of the world's population is estimated to be infected with Mycobacterium tuberculosis. 1 The vast majority of these infected persons have LTBI rather than active disease. 1

Reactivation Risk

The lifetime risk of progression from LTBI to active TB disease is 5-15%, with the majority of cases developing within the first 5 years after initial infection. 1 However, this risk varies substantially based on:

• Host immune status - immunocompromised individuals face dramatically higher reactivation risk 1, 3
• Bacterial factors - strain virulence and bacterial load 1
• Environmental factors - socioeconomic conditions and healthcare access 1

Diagnostic Approach

LTBI diagnosis relies on demonstrating immune response to M. tuberculosis antigens while excluding active TB disease. 1, 3

Testing Methods

• Tuberculin skin test (TST) or interferon-gamma release assays (IGRAs) can be used to detect LTBI 1
• Chest radiography must be performed before initiating LTBI treatment to rule out active TB disease 1

Important Caveat

Neither TST nor IGRA can distinguish between LTBI and active TB disease - they only indicate immune response to mycobacterial antigens. 4, 5 Clinical evaluation and chest imaging are essential to differentiate LTBI from active disease. 1

Clinical Significance

LTBI represents the largest reservoir for future active TB cases, accounting for approximately 80% of TB disease in low-incidence countries like the United States. 6, 7 This makes identification and treatment of LTBI a critical component of TB elimination strategies. 1

High-Risk Populations Requiring Screening

Strong recommendations exist for systematic LTBI testing and treatment in:

• People living with HIV 1
• Close contacts of pulmonary TB cases (adults and children) 1
• Patients initiating anti-TNF therapy 1
• Patients receiving dialysis 1
• Patients preparing for organ or hematological transplantation 1
• Patients with silicosis 1

Treatment Rationale

Preventive treatment of LTBI has efficacy ranging from 60-90% in preventing progression to active TB, with protection lasting up to 19 years. 1 Treatment effectively averts reactivation and reduces both individual morbidity/mortality risk and community TB transmission. 1, 3

The preferred treatment regimens are short-course rifamycin-based therapies (3-4 months) rather than longer isoniazid monotherapy (6-9 months) due to higher completion rates, better tolerability, and comparable efficacy. 6