Latent TB Infection: Diagnosis Tests for TB Infection

Latent TB Infection: Guide for Diagnosis and Treatment

Diagnosis

Tests for TB Infection

Currently, there are 2 testing methods available for the detection of M. tuberculosis infection in the United States:

• Mantoux tuberculin skin test (TST)
• Interferon-gamma release assays (IGRAs)

Tuberculin Skin Test (TST)

The TST is used to determine if a person is infected with M. tuberculosis. If a person is infected, a delayed-type hypersensitivity reaction is detectable 2 to 8 weeks after infection. The skin test is administered intradermally using the Mantoux technique by injecting 0.1 ml of 5 TU purified protein derivative (PPD) solution. The reading and interpretation of TST reactions should be conducted within 48 to 72 hours of administration.

Key Points

• Training is essential for health care providers to gain proficiency in the administration and interpretation of the TST.
• The TST should not be performed on a person who has written documentation of either a previous positive TST result or treatment for TB disease.
• Patients or family members should never measure TST results; this should only be done by a trained health care professional.
• Interpretation of the TST result is the same for persons who have had BCG vaccination because a majority of BCG cross-reactivity wanes with time.
• A TST that was not measured and recorded in millimeters (mm) of induration must be repeated.

Classification of Tuberculin Skin Test Reactions

Interpretation of TST results is based on the measurement of the reaction in millimeters, the person’s risk of acquiring TB infection, or the risk of progression to disease if infected. See the risk stratification below.

A TST reaction of ≥5 mm of induration is considered positive in:

• HIV-infected persons
• Recent contacts of a person with infectious TB disease
• Persons with fibrotic changes on chest radiograph consistent with prior TB
• Patients with organ transplants and other immunosuppressed patients (including patients taking the equivalent of ≥15 mg/day of prednisone for 1 month or more or those taking TNF-α antagonists)

A TST reaction of ≥10 mm of induration is considered positive in the following individuals:

• Recent arrivals to the United States (within last 5 years) from high-prevalence areas
• Injection drug users
• Residents or employees of high-risk congregate settings (e.g., correctional facilities, long-term care facilities, hospitals and other health care facilities, residential facilities for patients with HIV infection/AIDS, and homeless shelters)
• Mycobacteriology laboratory personnel
• Persons with clinical conditions that increase the risk for progression to TB disease
• Children younger than 5 years of age
• Infants, children, and adolescents exposed to adults in high risk categories

A TST reaction of ≥15 mm of induration is considered positive in the following individuals:

• Persons with no known risk factors for TB

Although skin testing activities should be conducted only among at-risk groups, certain individuals may be required to have testing for employment or school attendance independent of risk. CDC and ATS do not recommend a testing approach that is independent of a risk assessment.

Interferon-Gamma Release Assays (IGRAs)

IGRAs are used to determine if a person is infected with M. tuberculosis by measuring the immune response to TB proteins in whole blood. Specimens are mixed with peptides that simulate antigens derived from M. tuberculosis and controls. In a person infected with M. tuberculosis, the white blood cells recognize the simulated antigens and release interferon-gamma (IFN-γ); results are based on the amount of IFN-γ released.

There are 2 U.S. Food and Drug Administration (FDA) approved IGRAs commercially available in the United States:

• QuantiFERON ^®-TB Gold-in-Tube test (QFT-GIT)
• T-SPOT ^® TB test

Key Points

• Advantages of IGRAs include the following:
  • Requires a single patient visit to conduct the test.
  • Does not cause booster phenomenon.
  • Laboratory test not affected by health care worker perception or bias.
  • Results can be available within 24 hours.
  • Unaffected by BCG and most environmental mycobacteria.
• Limitations of IGRAs include the following:
  • Blood sample must be processed within 8-30 hours after collection.
  • Limited data exist on use in groups such as children younger than 5 years of age, persons recently exposed to TB, immunocompromised persons, and those who will be tested repeatedly (serial testing).

Interpretation of IGRA Results

The interpretation of IGRAs is based on the amount of IFN-γ released, in QFT, or on the number of cells that release IFN-γ, in T-SPOT ^®.TB. Laboratories should provide both the qualitative and quantitative results.

• Qualitative results are reported as positive, negative, indeterminate or borderline.
• Quantitative results are reported as numerical values that include a response to the TB antigen and 2 controls, nil and mitogen. Quantitative results may be useful for clinical decision making in individual cases, in combination with risk factors.

• Page last viewed: tháng một 9, 2016
• Page last updated: November 25, 2013

Content Source:

Division of Tuberculosis Elimination