TB Risk Calculator
Interpret tuberculin skin test (TST) and interferon-gamma release assay (IGRA) results based on risk factors. Distinguish between latent TB infection and active TB disease, and get management recommendations.
TB Testing Assessment
Risk Factors for TB Infection
Symptoms of Active TB
Understanding Tuberculosis Testing
TST vs IGRA
Tuberculin Skin Test (TST)
- • Requires two visits (placement and reading)
- • Can be affected by BCG vaccination
- • Less specific (more false positives)
- • Cheaper and more widely available
- • Boosting phenomenon in serial testing
IGRA (Blood Test)
- • Single visit (blood draw)
- • Not affected by BCG vaccination
- • More specific (fewer false positives)
- • More expensive, requires laboratory
- • Preferred for BCG-vaccinated individuals
Latent TB vs Active TB
Latent TB Infection (LTBI)
- • No symptoms
- • Not contagious
- • Positive TST or IGRA
- • Normal chest X-ray
- • Negative sputum cultures
- • 5-10% lifetime risk of activation
Active TB Disease
- • Symptomatic (cough, fever, weight loss)
- • Highly contagious (pulmonary TB)
- • Positive TST/IGRA (usually)
- • Abnormal chest X-ray
- • Positive sputum AFB/culture
- • Requires multi-drug therapy (4 drugs)
Who Should Be Tested for TB?
- •Close contacts of active TB cases
- •People with HIV or other immunosuppression
- •Recent immigrants from high-prevalence countries
- •Healthcare workers and laboratory personnel
- •Residents and employees of high-risk settings (prisons, shelters, nursing homes)
- •People with symptoms suggestive of TB
Frequently Asked Questions
What does a positive TB test mean?
A positive TST or IGRA means you have been infected with TB bacteria at some point. It does NOT mean you have active TB disease. Most people with positive tests have latent TB infection (LTBI), meaning the bacteria are dormant and you're not contagious. A chest X-ray and symptom evaluation help distinguish LTBI from active disease.
Does BCG vaccination affect TB testing?
Yes, BCG (tuberculosis vaccine) can cause a false-positive TST, especially if received recently. The effect diminishes over time but can persist for years. IGRA tests are NOT affected by BCG vaccination and are preferred for people who received BCG, especially those vaccinated after infancy or who received multiple doses.
Should I treat latent TB infection?
Yes, in most cases. Treatment of LTBI reduces the risk of developing active TB by 60-90%. Treatment is especially important for people at high risk of progression: HIV-positive, recent TB contacts, recent converters, immunosuppressed, or with fibrotic changes on chest X-ray. Treatment involves 3-9 months of antibiotics depending on the regimen chosen.
Can latent TB become active?
Yes, about 5-10% of people with LTBI develop active TB disease during their lifetime, with the highest risk in the first 2 years after infection. The risk is much higher (up to 10% per year) in immunocompromised individuals, particularly those with HIV. This is why LTBI treatment is recommended for high-risk individuals.
How is active TB treated?
Active TB requires multi-drug therapy to prevent resistance. Standard treatment is RIPE therapy: Rifampin, Isoniazid, Pyrazinamide, and Ethambutol for 2 months (intensive phase), followed by Rifampin and Isoniazid for 4 months (continuation phase). Total treatment is 6 months for pulmonary TB, longer for extrapulmonary TB or drug-resistant cases.
Is TB still common in the United States?
TB incidence in the U.S. is low (~2-3 cases per 100,000) compared to global rates, but it remains a concern. Most U.S. cases occur in foreign-born individuals from high-prevalence countries. High-risk groups include homeless individuals, people with HIV, healthcare workers, and those in correctional facilities. Globally, TB remains a leading cause of death from infectious disease.
What is an indeterminate IGRA result?
An indeterminate IGRA means the test could not be interpreted, usually due to inadequate immune response (both positive and negative controls failed). This can occur in severely immunosuppressed patients, technical errors, or improper specimen handling. The test should be repeated. If persistently indeterminate, consider TST or clinical assessment.
How is TB transmitted?
TB is spread through airborne droplets when a person with active pulmonary TB coughs, sneezes, speaks, or sings. Prolonged, close contact is usually needed for transmission. TB is NOT spread through casual contact, sharing food/drinks, or touching surfaces. Only people with active pulmonary or laryngeal TB are contagious. LTBI is not contagious.