Latent tuberculosis infection

Prior exposure to TB leading to persistent latent tuberculosis, usually contained within lung granulomas
Goal is to identify those who are at increased risk of developing active TB and would benefit from treatment to prevent future reactivation
Use the TST in 3D calculator and the BCG World Atlas for risk estimation
Standard prescription is 4 months of rifampin 10 mg/kg/day (up to 600 mg); counsel patient on side effects and monitor liver enzymes weekly to start

Background

One quarter to one third of the world population has LTBI (estimated at 1.7 billion people)
More prevalent in the same countries as active tuberculosis, and is highest in South-East Asia, Pacific, and African regions
More common in older patients who would have been exposed when active tuberculosis was more prevalent

Done routinely in tuberculosis-endemic countries
Commonly causes an elevated scar as site of inoculation (often on the deltoid)
- Compared to smallpox, which forms a crater
Receipt of the BCG vaccine affects interpretation of the tuberculin skin test

Sn 90%, Sp >95
Lower specificity after BCG vaccination, which can cause false positives
- Especially if received after age 5 years
- Also if received after age 1 year, or received multiple times

Sn 95%, Sp >95%
Doesn't crossreact with BCG (uses ESAT-6 and CFP-10) but can crossreact with other non-tuberculous mycobacteria (Mycobacterium intracellulare, Mycobacterium chelonae, and Mycobacterium fortuitum)
Preferred for those who have received BCG after infancy
May be more useful in patients who are unlikely to follow up in 48 hours, or who need urgent immunosuppression and need a faster result
QuantiFERON-TB Gold Plus (QFT-Plus) likely has better PPV than TST in a low-prevalence population

Is it truly positive?
- Consider IGRA
- BCG vaccine can be considered a cause of false positive when
  - vaccine given after 12 months of age, and
  - patient has no risk factors, and
  - either Canadian-born non-Aboriginal, or not from endemic country
Rule out active TB
- signs/symptoms
- CXR or CT chest
- Sputum x3 if coughing or cavitary lesions
Evaluate risk of reactivation treatment
- INH 300 daily x9 mo with pyridoxine
- baseline liver enzymes and vision testing

Standard regimen (9INH) 1
- Nine months of isoniazid with daily vitamin B6
Alternative shorter courses:
- 4RIF (10 mg/kg [600 mg maximum]): not yet in guidelines, but likely preferred. Slightly higher risk of hepatitis.
- 6INH
- 3-4INH/RMP

TST in 3D online TBST/IGRA Interpreter
BCG World Atlas, which has a listing of every country's BCG vaccination policies