What is the rate of conversion from latent tuberculosis (TB) to active tuberculosis (TB)?

Rate of Conversion from Latent TB to Active TB

The lifetime risk of progression from latent tuberculosis infection (LTBI) to active TB disease is estimated to be 5-15%, with the majority of cases developing within the first 5 years after initial infection. 1

General Conversion Rates

The risk of progression from LTBI to active TB varies significantly based on several factors:

• Overall lifetime risk: 5-15% for immunocompetent individuals 1, 2, 3
• Early progression risk: Highest during the first 2 years after infection 4, 3
• First-year risk: 12.9 cases per 1,000 person-years in the first year after infection 4
• Years 1-7 after infection: 1.6 cases per 1,000 person-years 4

Risk Factors That Increase Conversion Rates

The likelihood of progression from LTBI to active TB is significantly influenced by various risk factors:

Host Factors

• HIV infection: 35-162 cases per 1,000 person-years 4
  • In the US, HIV-positive individuals with positive tuberculin tests: 45 cases per 1,000 person-years 4
• Age: Children under 5 years and adolescents/young adults have higher risk 4
• Weight: Persons >15% underweight have 2.6-fold higher risk compared to those within 5% of standard weight 4
• Silicosis: 68 cases per 1,000 person-years 4
• Radiographic findings consistent with prior TB: 2.0-13.6 cases per 1,000 person-years 4
• Injection drug use:
  • HIV-positive: 76 cases per 1,000 person-years 4
  • HIV-negative: 10 cases per 1,000 person-years 4

Medical Conditions

• Chronic renal failure on hemodialysis: 10-25 times greater risk than general population 4
• Diabetes mellitus: 2-4 times greater risk, especially with insulin-dependent or poorly controlled diabetes 4
• Immunosuppressive therapy: Particularly anti-TNF treatment 4
• Organ transplantation: Significantly increased risk 4

Protection from Previous Infection

Interestingly, individuals with LTBI who are reinfected have approximately 79% lower risk of developing active TB compared to those experiencing their first infection 5. This suggests that prior infection confers some degree of protection against progression following reinfection.

Timing of Progression

The pattern of progression from LTBI to active TB follows a distinct timeline:

• In a British study of schoolchildren with tuberculin conversion, 54% of those who developed TB did so within the first year, and 82% within the first 2 years after infection 4
• Among recent immigrants from high TB-burden countries, the risk of progression is highest during the first several years after arrival 4

Clinical Implications

Understanding these conversion rates has important implications for TB control strategies:

• Targeted testing and treatment of LTBI should focus on high-risk groups 4
• Recent contacts of infectious TB cases should be prioritized for screening and preventive treatment 4
• The risk-benefit analysis of LTBI treatment is most favorable for those at highest risk of progression 1

Common Pitfalls in Assessing Risk

• Failing to recognize that the risk of progression is highest in the first 2 years after infection
• Underestimating the dramatically increased risk in immunocompromised individuals
• Not considering that certain populations (recent immigrants, institutionalized persons) may have higher rates of recent infection and therefore higher progression risk
• Overlooking the need to rule out active TB before diagnosing LTBI and initiating preventive treatment

Understanding these conversion rates is essential for developing effective strategies to identify and treat LTBI, particularly in high-risk populations, to prevent progression to active disease and reduce TB transmission.