Department of Health
John J. Dreyzehner, MD, MPH, Commissioner

Latent tuberculosis infection (LTBI)

Latent tuberculosis infection is an asymptomatic state in persons who are infected with Mycobacterium tuberculosis. LTBI is detected as a result of skin testing among persons with risk factors for TB. For persons with untreated latent TB infection with intact immunity, the estimated risk of developing symptomatic tuberculosis disease is 5% to 10% over a lifetime, with about half of that risk occurring during the first year or two after infection. For persons who are immunocompromised by HIV co-infection, the risk of developing disease increases to 5% to 10% per year.

Finding LTBI provides an opportunity to treat and prevent progression to active disease (reactivation). Studies have shown that TB reactivation can be prevented with 60%-80% efficacy. Due to relatively low TB prevalence in the United States, treatment of LTBI is considered an important public health strategy to achieve TB elimination.

Purpose and Use of TB/LTBI Risk Assessment Tool

The Tennessee TB Elimination Program has developed a TB/LTBI Risk Assessment Tool (RAT) (PH-3714, RDA-150) to determine whether persons are at high or low risk for TB. This Tennessee public health form is used by local health departments.  It is used as a guide to educate the patient about key issues concerning TB/LTBI, as a flow sheet to determine a patient’s risk of TB/LTBI, as documentation that the counseling and risk assessment for TB/LTBI are done, and as a way to report the findings from the assessment.

Currently, our priority high-risk group in Tennessee is the foreign-born (i.e. immigrants, refugees and residents from high-incidence countries). In the local health department, such persons are screened for TB/LTBI using the RAT. We are actively screening and testing the foreign-born, but the RAT is also used to screen cases, contacts or persons suspected to have other TB risk factors. High-risk persons may present to any health department program such as HIV/STD clinics, WIC clinics, women’s health, etc., and all health department personnel should be trained to either screen these persons using the RAT or to refer them to the health department TB Clinic for screening.

In addition, this form should be used to screen any person (at health department or community site) for whom a health department employee is considering offering a PPD.

At employer-based community sites, you may need to screen all employees using the RAT, due to employers’ requests to avoid discrimination. In these situations, educate the employer that only those employees determined to be high-risk are advised to receive a skin test. A skin test is not necessary for employees determined to be low-risk and should not be given.

Test for LTBI

Tuberculin skin testing with intermediate strength (5 TU) purified protein derivative (PPD), administered by the Mantoux intradermal technique is the best currently available test for detecting LTBI. The test is administered by injecting 0.1 ml (5 TU) Tubersol® or Aplisol® intradermally (not subcutaneously) into the ventral surface of the forearm. 48 to 72 hours after the test is administered, the diameter of palpable or visible induration (raised swelling) should be measured transverse to the long axis of the arm and recorded in mm. Note: redness without induration should be ignored.

Who should be tested for LTBI

Tuberculin skin testing should be reserved for persons who meet some or all of the following criteria:

• Are at high risk for TB infection or disease
• If infected, are at high risk of developing active TB
• If infected, would be a candidate for treatment to prevent active TB

High-risk Groups

High-risk persons in Tennessee who should be tuberculin skin tested include:

• Close contacts of active TB cases or suspects
• Foreign-born persons from high-prevalence countries (excluding Canada, Western Europe, Australia, New Zealand and Japan)
• Residents and staff or volunteer workers in high-risk congregate settings (alcohol and drug rehabilitation or methadone maintenance centers, homeless shelters, correctional facilities, mental health facilities, and long-term care facilities)
• Persons working in health care facilities where known TB cases exist, and other cases are suspected or likely
• Persons with clinical conditions associated with progression to active TB, such as HIV infection, silicosis, diabetes mellitus, chronic renal failure/hemodialysis, gastrectomy, jejunoileal bypass, solid organ transplantation (renal, cardiac), carcinoma of the head and neck, leukemia, lymphoma, and being more than 10% underweight
• Injection drug users
• Children who meet any of these criteria or who have been in regular contact with high-risk adults
• Persons with frequent, prolonged travel to high-prevalence countries (see above)
• Other persons at high-risk for TB/LTBI as locally defined

BCG

BCG is administered to more than 80% of children in the world as part of the Extended Program on Immunization. The vaccine is given primarily in countries with rates of TB infection much higher than those found in the United States or Western Europe, and many persons who have previously received BCG may also have TB infection. Tuberculin sensitivity after BCG vaccination may range from no induration to > 19 mm. Sensitivity generally wanes with time, though it can be affected by the age at vaccination, the number of vaccinations, and the number and frequency of tuberculin skin tests since vaccination.

Persons who have received BCG should be tested for LTBI unless otherwise indicated. Induration considered positive, using appropriate criteria, should be assumed to be due to TB infection, not BCG.  Treatment should be recommended, unless contraindicated.

Children from high-TB incidence areas with a positive TST should be considered for treatment regardless of BCG since the consequences of developing active TB are potentially severe.

TST Positivity for LTBI

There are 3 cut-offs for a positive TST:  ³ 5 mm, ³ 10 mm, ³ 15mm, depending on the risk of TB infection or disease in the individual or population being tested, as detailed below.

5 mm or greater TST induration:

• HIV-infected persons
• Immunosuppressed persons (organ transplants, steroid use, etc.)
• Recent contacts of TB case
• Persons with fibrotic changes on chest X-ray consistent with old TB

10 mm or greater TST induration:

15 mm or greater TST induration:

Persons who should receive evaluation for active TB and LTBI treatment,  if TB is ruled out, regardless of TST result:

These contacts should receive a TST immediately. Even if TST is 00mm, these groups should be treated for LTBI, if active TB is ruled out. TST should be placed again 10-12 weeks after last exposure to TB case.

Retesting for LTBI

Only those persons with ongoing risk of new exposure to TB should be retested. For example, it is not unreasonable to retest a foreign-born person if he/she has developed new risk factors for TB, such as return travel to a high-incidence country.

Mandated skin-testing programs (e.g., those that were formerly conducted among teachers and foodhandlers) should be discouraged unless the targeted groups contain substantial proportions of persons at high-risk.

By definition, low-risk persons testing less than 15 mm induration are not considered TB-infected, and should be retested only if their risk status changes. Routine tuberculin testing is not recommended for populations at low-risk for TB/LTBI. However, if these persons are tested (e.g., at entry into a work site where exposure to TB is anticipated and a longitudinal tuberculin testing program is in place), a cut off of ³ 15mm is recommended. Below this cut off, a false-positive reaction may occur due to environmental mycobacteria or other factors.

Anergy testing in HIV+ persons

HIV-infected persons may have a compromised ability to react to tuberculin skin tests because of cutaneous anergy associated with progressive HIV immunosuppression. However, the usefulness of anergy testing in selecting tuberculin-negative, HIV-infected persons who might benefit from treatment of LTBI has not been demonstrated. Thus, anergy testing is no longer recommended.