Tuberculosis: Latent TB

Latent Tuberculosis Risk Stratification, Diagnostic Testing Pearls, and Evidence-Based Treatment Regimens

Fibonacci Infectious Disease

by Richard Strongwater, MD

and Jay A. Zazlow, MD MPH

Reviewed by Harish Moorjani, MD

Latent tuberculosis is an asymptomatic, nontransmissable infection with Mycobacterium tuberculosis, carrying a 5 to 10% lifetime risk of progressing to active disease. One half of this risk of progression to active disease occurs within the first two years after infection.

Risk factors for progression to active disease- immunodeficiency, recent exposure to tuberculosis, and chronic kidney disease requiring dialysis, children younger than five years.

High-risk groups in the United States for contracting latent TB- recent immigrants from high-incidence countries (Africa, Asia, eastern Europe, Central America, and South America), health care professionals, persons living or working in institutional settings, and homeless persons. Also extended travel in high-incidence countries and illicit drug use.

Additional risk factors for progression to active disease- patients with silicosis or cancer of head, neck, or lung, and presence of abnormalities on CXR consistent with healed fibrotic changes from past M. tuberculosis infection. Asians and Native Hawaiians and other Pacific Islanders living in U.S. have highest rates of active disease.

Symptoms of Active infection- 3 weeks or more cough, hemoptysis, weight loss, fever, night sweats

Diagnostic Evaluation

Blood test vs skin test

Interferon-gamma release blood assays (IGRAs) and tuberculin skin tests (TSTs) can identify M. tuberculosis infection but cannot differentiate between active and latent forms. Active TB should be diagnosed by the presence of symptoms or signs on radiography, AFB smears or by culture.

IGRA versus TST: IGRA is recommended over a TST in individuals who are at least five years of age and in patients who are likely to have M. tuberculosis infection.

If active pulmonary TB infection is suspected: AFB smear can be performed. Typically, three specimens are tested. Note that a negative result does not exclude active pulmonary TB.

If extrapulmonary TB is suspected: Collect specimens from those sites for mycobacterial culture.

The QuantiFERON-TB gold assay is a blood test used to detect latent tuberculosis (TB) infection (LTBI). It is the most commonly used interferon-gamma release assay (IGRA) used to diagnose infection with Mycobacterium tuberculosis. In patients infected with M tuberculosis, white blood cells release interferon-gamma when exposed to M tuberculosis antigens.

The QuantiFERON-TB gold blood test has a sensitivity of 70%-90% and a specificity of 95% for LTBI. It is not designed to diagnose active tuberculosis, which requires microbiologic testing (i.e., sputum acid-fast bacilli and culture).

One practical advantage of IGRAs over the TB skin test (TST) is that the patient does not need to return for a second visit for test interpretation. Unlike the TST, the IGRA is not affected by Bacillus Calmette-Guerin (BCG) vaccination status. The PPD has a specificity as low as 60% in individuals from countries where BCG vaccination is used. Algorithms are available to confirm a positive PPD using the QuantiFERON-TB gold test in low-risk individuals suspected of having a false-positive PPD (e.g., those having BCG vaccination, serial testing, or cross-reactivity from other nontuberculous mycobacterium strains).

IGRA confirmation of positive TST results may persuade reluctant patients with latent TB to consider treatment.

Diagnostic Evaluation – Pearls to Know [1]

Tuberculin skin test (TST)- 80 -95% sensitive, 60 - 80% specific (based on delayed hypersensitivity to TB antigens) is more sensitive, less specific- positive if induration of 15 mm or more, especially in persons without risk factors. Positive if 10 mm or more in patients with higher risk. Positive if 5 mm or more in immunocompromised patients and those exposed to active TB.

Interferon-gamma release assay (IGRA) AKA Quantiferon-TB Gold In-Tube test or T-Spot TB test-enzyme-linked immunosorbent assays- more specific (greater than 95%) and less sensitive (70-91%) than the TST

IGRA preferred, TST acceptable- in groups with historically low rates of returning for TST result interpretation (homeless, drug users). Persons who have received BCG vaccination. False-positive results are more likely if the interval between BCG vaccination and use of the TST is less than 10 years.*

TST preferred, IGRA acceptable- children younger than five years

Either test acceptable- persons who have occupational exposure to Mycobacterium tuberculosis and who are undergoing periodic screening. And recent contacts with known or suspected active tuberculosis.

Screening after exposure to active TB- Use either the TST or IGRA, but either test may not show conversion for eight to 12 weeks in infected individuals. "Window prophylaxis" should be initiated in exposed high-risk patients. If repeat testing at 12 weeks is negative, consider stopping medications (except further clinical evaluation is indicated for immunocompromised patients).

*Despite previous recommendations that tuberculin skin test (TST) results be interpreted without regard for BCG vaccination status, about 20% of positive TST results in this population may represent false positives rather than latent or active TB infection.

Other Pearls [1]

Two-step TST- Repeat skin test in one to three weeks after initial negative test (this stimulates a reaction or booster effect in persons infected with M. Tuberculosis in the distant past.

IGRA test antigens (blood test) are not shared by BCG or environmental mycobacteria (except Mycobacterium marinum, M. szulgai, and M. kaansaii)

TST contains ESAT-6 and CFP-10 antigens, which may boost IGRA results

Treatment and Recommended Follow-Up [3]

Neither the TST or the IGRA can distinguish between latent and active disease. At risk patients with positive TB results should be offered treatment after active disease is ruled out. Any Xray abnormalities should be followed with three sputum AFB smears to exclude active TB.

Five approved treatment regimens for latent TB infection

1. Standard nine-month INH regimen- reduces TB activation risk by 90%

2. Six-month INH regimen- reduces risk by 60-80%

3. Four-month daily rifampin regimen- reduces risk by 60% but has less liver toxicity

4. Three-month, 12-dose regimen of INH and rifapentine (Priftin), given once weekly under direct observation- as effective as nine months of INH in persons 12 years and older

5. Three-month, daily isoniazid plus rifampin. This regimen is recommended for:

   • Children and adults of all ages who are HIV-negative

   • Patients with HIV taking some combinations of antiretroviral therapy (ART)

For further information on indications and dosing see Treatment for Latent Tuberculosis. CDC Tuberculosis. April 17, 2025

Sources

1. Adapted from American Family Physician- June 1, 2014. Holly Hartman-Adams et al.

2. CDC: MMWR 12/16/05 54(RR15) 49-55.

3. Treatment for Latent Tuberculosis. CDC Tuberculosis. April 17, 2025.

4. Paul E. Sax. One-Month Treatment for Latent Tuberculosis. NEJM Voices. April 2, 2026.