Latent tuberculosis infection is a condition in which a person is infected with Mycobacterium tuberculosis -complex but does not have active tuberculosis disease. People with latent tuberculosis infection are at risk of progressing to active tuberculosis. Therefore, it is essential that individuals at high risk of progression to active tuberculosis are identified through screening.
Nearly one-third of the world’s population has latent tuberculosis infection.  Risk of progression varies based on age and comorbid conditions.  The greatest risk for progression to active disease occurs within the first 2 years of infection, during which time about 5% of individuals progress to tuberculosis disease. After the first 2 years following infection, the risk of developing active disease over an individual’s lifetime is 5-10%. 
See Tuberculosis: Diagnostic Imaging and Treatment Challenges, a Critical Images slideshow, to help determine the best approach for patients with this multisystemic disease.
There is no direct test to detect the presence of latent tuberculosis infection in an individual. The screening tests for latent tuberculosis infection rely on measurements of adaptive host immune responses to the bacteria. The tuberculin skin test measures an individual’s response to a solution of Mycobacterium tuberculosis -complex antigens, known as purified protein derivative. [4, 5]
Until the early 2000s, the tuberculin skin test was the standard for screening for latent tuberculosis infection.  However, the test has limitations, including precise intradermal administration, the need for a follow-up visit to interpret the results, specific criteria for interpretation of the results, and the possibility of false-positive results with Bacillus Calmette-Guerin vaccination or other environmental mycobacteria.  Because of this, interferon-gamma release assays are gaining acceptance as an alternative screening test. [7, 8] Because interferon-gamma release assays are performed in the laboratory, requiring one blood draw and only one patient visit to obtain results, they are significantly distinct from the traditional tuberculin skin test.
Selecting Individuals to Screen
The selection of individuals for screening of latent tuberculosis infection should be based on clinical, social, and environmental risk factors. Screening should be performed with the intent to treat positive test results.
There are two main categories of people who should be screened for latent tuberculosis infection: 1) individuals at risk for exposure to persons with active tuberculosis disease, and 2) individuals with conditions or other factors associated with progression from latent tuberculosis infection to tuberculosis disease. 
Individuals at risk for exposure to persons with tuberculosis disease include the following  :
Known close contacts of a person with infectious tuberculosis disease
Immigrants from tuberculosis-endemic regions of the world (e.g., Africa, Asia, Eastern Europe, Latin America, and Russia)
Employees or residents of facilities or institutions with people who are at high risk for tuberculosis, such as hospitals, homeless shelters, correctional facilities, nursing homes, or residential facilities for patients with acquired immunodeficiency syndrome
Conditions and other factors associated with progression from latent tuberculosis infection to tuberculosis disease include the following  :
Human immunodeficiency virus infection
Recently infected with M. tuberculosis within the past 2 years
Radiographic evidence of prior healed tuberculosis
History of prior untreated or inadequately treated tuberculosis
Low body weight (10% below ideal body weight)
Infants and children under 5 years old with positive tuberculosis test
Drug abuse, including alcohol abuse and injection drug use
Chronic renal failure or on hemodialysis
Solid organ transplantation
Based on the 2012 American College of Rheumatology guidelines, individuals who are being considered for biologic agent immunosuppression, including tumor necrosis factor antagonists, should be screened prior to starting biologic therapy. 
Individuals with a prior positive test or who have had a severe reaction to tuberculin skin test in the past, including skin necrosis, blistering, ulceration, or anaphylactic shock,  should not be screened again.
Selecting a Test
There is considerable controversy regarding the appropriate test to use for tuberculosis screening. Data on interferon-gamma release assays (IGRAs) is evolving and their validity in specific patient populations is not as well established as the tuberculin skin test (TST). In general, the IGRAs have extremely high specificity (93%–99%), with more limited sensitivity (70%–90%). [14, 15]
The Centers for Disease Control and Prevention (CDC) issued updated guidelines in 2010 evaluating the role of screening for tuberculosis with IGRAs.  Overall, both IGRAs and TSTs have been endorsed. IGRAs may be used in place of TSTs whenever testing is indicated. The guidelines also indicate preferred tests in certain situations, but routine use of either TST or IGRAs is acceptable practice.
The World Health Organization (WHO) issued guidelines in 2015  that also endorsed either TSTs or IGRAs in high- and upper-middle income countries, while in low-income and other middle-income countries, the recommendation was that IGRAs should not replace TSTs.
In the United States, IGRAs are recommended for individuals who may not return for a TST reading, such as those with a history of drug abuse or who are homeless. IGRAs are also recommended for individuals who have received Bacille Calmette-Guerin vaccination. TST is the preferred test for children younger than 5 years.
The CDC recommends against routine use of simultaneous or sequential TST and IGRA for the same patient, although there are exceptions. For example, if the initial TST is negative, repeat testing with IGRA (or vice versa) can be performed if the patient is at high risk for infection, progression, poor outcome, or if there is a high clinical suspicion for active tuberculosis. A positive result from a second test increases the sensitivity for detecting tuberculosis in higher risk patients, although multiple negative tests cannot exclude a diagnosis of tuberculosis. Sequential testing also can be considered if the initial test is borderline or indeterminate.
In the setting of multiple tests, clinicians may be faced with discordant findings (i.e., one positive test and one negative test). The CDC recommends an individualized approach with careful consideration to the quality of each test, the patient’s specific response to testing (e.g., size of induration or values for antigen, positive and negative controls on the IGRA) and the risk of testing or treating a given patient.
For patients who are at low risk for infection and progression, deeming a positive result to be falsely positive is reasonable given the overall low incidence of tuberculosis in the United States. TST reactions of less than 15 mm in size in otherwise healthy, low-risk patients who have received a Bacille Calmette-Guerin vaccination and who have a negative IGRA can be considered to be a false-positive TST reaction. However, for patients who are at high risk of acquisition or progression of tuberculosis, one positive test result can be considered as evidence for infection.
Differing conclusions are presented in the guidelines developed by Canadian, U.K., and U.S. expert panels regarding the use of IGRAs.  Given the evolving data and the varied public health priorities, this assessment is not surprising. The choice of a specific test should be based on the local epidemiology of tuberculosis as well as the risk factors of each individual.
Tuberculin Skin Test
The tuberculin skin test (TST) measures an individual’s cell-mediated immune response to a solution of more than 200 M. tuberculosis complex antigens, known as purified protein derivative (PPD).
Testing equipment for the TST includes a purified protein derivative solution, tuberculin syringe, 27-gauge needle, and alcohol swabs.
PPD solution should be stored in the dark and refrigerated at 36-46°F.  To minimize reduction in potency by adsorption, the PPD solution should not be transferred from one container to another.
In the United States, 5 tuberculin units (TUs) are used, whereas in most European countries, 2 TUs are used.  Skin tests should be given immediately after the syringe is filled.
The Mantoux technique is the standard method of administration of PPD solution, in which intradermal injection of tuberculin material on the inner surface of the forearm is used.  The test is performed as followed:
Inject 0.1 mL of 5 TU PPD solution intradermally on the volar surface of the lower arm using a 27-gauge needle and tuberculin syringe.
Produce a wheal 6-10 mm in diameter.
The arm in which the test was administered is noted.
The skin test should be read 48-72 hours after administration.
The area of induration (not erythema) is measured in millimeters in the axis perpendicular to the long axis of the arm.
The immune reaction to administered antigen is a type 4 delayed (cellular) hypersensitivity reaction. T cells primed by the prior infection are recruited to the test area, where they release lymphokines leading to local vasodilatation, edema, fibrin deposition, and recruitment of other inflammatory cells leading to induration of the involved skin. 
Because this test is an indirect measurement of latent tuberculosis infection, it has certain limitations. Bacillus Calmette-Guerin vaccine, which protects infants and young children from meningeal and miliary tuberculosis,  may affect results. The vaccine is not offered in the United States, but foreign-born residents who have been vaccinated may develop a positive TST reaction. Because it is difficult to distinguish whether the reaction is a true-positive result (indicating latent tubculosis infection) or a false-positive result (indicating history of Bacillus Calmette-Guerin vaccination), individuals with known or suspected Bacillus Calmette-Guerin vaccination with a positive TST should be treated as if they have a positive test result. [21, 22]
Some individuals with latent tubculosis infection have an initial negative skin test reaction when tested years after infection because the TST becomes less sensitive over time. For individuals who have a negative initial TST, a second test should be administered (two-step testing) using the same methods and interpretation paradigm. The initial TST serves as a boost to stimulate a response.
Sensitivity and specificity of the TST is influenced by different cutoff values for positivity in different clinical settings. Table 1 shows the degree of induration required for a positive test in selected population groups. 
Table 1. Interpretation of tuberculin skin test results (Open Table in a new window)
|Degree of induration required for a positive result||Patient population|
Individuals with HIV infection
Individuals who have had close contact with a patient with infectious tuberculosis
Individuals with chest radiographs that are consistent with prior untreated tuberculosis (fibrotic changes)
Organ transplant recipients
Other immunosuppressed patients (taking the equivalent of >15 mg/day of prednisone, tumor necrosis factor antagonists, or other biologics)
Recent immigrants (within the last 5 years) from tuberculosis-endemic countries
Injection drug users
Residents or employees of congregate settings (e.g., prisons, long-term care facilities for the elderly, homeless shelters)
Mycobacteriology laboratory personnel
Children younger than 4 years
Infants, children, and adolescents who have been exposed to high-risk adults
|>15 mm||Individuals with no known risk factors for tuberculosis|
Table 2. Causes of false-positive and false-negative reactions for the tuberculin skin test (Open Table in a new window)
|False-positive reactions||False-negative reactions|
|Infection with non-tuberculosis mycobacteria||Cutaneous anergy due to a lack of an appropriate immune response (e.g., immunocompromised or suppressed individual)|
|Prior Bacillus Calmette-Guerin vaccination (although not a contraindication to tuberculin skin testing)||Recent tuberculosis infection (within 8-10 weeks)|
|Incorrect method of administration||Very old tuberculosis infection|
|Incorrect interpretation of reaction||Very young age (< 6 months old)|
|Incorrect antigen used||Recent live-virus vaccination (e.g., measles, smallpox)|
|Overwhelming tuberculosis disease|
|Certain viral illnesses (e.g., measles, chicken pox)|
|Incorrect method of administration|
|Incorrect interpretation of reaction|
|Severe metabolic or physiologic stress (e.g., sepsis, recent surgery, malnutrition, burns)|
Interferon-gamma Release Assays
Two interferon-gamma release assays (IGRAs) are currently approved by the U.S. Food and Drug Administration (FDA) in the United States:
QuantiFERON-TB Gold In-Tube test (QFT-GIT)
T-SPOT.TB test (T-Spot)
QuantiFERON-TB Gold In-Tube Test
The QFT-GIT uses three specialized blood collection tubes, each holding 1 mL of blood:
Grey: Negative control
Red: Tuberculosis antigens
Purple: Mitogen control
Immediately after blood collection, all tubes of blood must be vigorously shaken 10 times to ensure the entire inner surface of the tube has been coated with blood. Appropriate mixing is essential to ensure the antigens embedded in the tube walls interact with the blood. 
Tubes must be incubated within 16 hours at 37°C for 16-24 hours. After incubation, an enzyme-linked immunosorbent assay (ELISA) is performed on separated plasma using a specially developed QFT-GIT microwell plate. The optical density is measured and a software algorithm using FDA-approved cutoff values compares the negative control, positive control, and the antigen wells. [8, 6] A qualitative response of positive, negative, or indeterminate is generated from these values.
Indeterminate results may occur for several reasons. The positive control optical density may be below the threshold, indicating the patient’s blood did not react with the mitogen and suggesting an anergic response. The negative control optical density may exceed the threshold value, suggesting high background interferon-gamma levels.
Common clinical reasons for indeterminate results include relative anergy and immunosuppresion (specifically limiting interferon gamma production in the positive control), extremes of age, active infection, or antimycobacterial treatment. [6, 24] Technical reasons for indeterminate results include prolonged transit time after draw and before incubation, insufficient mixing, and incomplete washing of the ELISA plates. [8, 24]
For the T-Spot test, whole blood is collected in lithium heparin or sodium citrate anticoagulated tubes (or specialized Leucosep or Cell Preparation Tubes), holding 8 mL of blood. Anticoagulated or nonanticoagulated ethylenediaminetetraacetic acid tubes are inappropriate for testing and should not be used.
After blood is collected, samples should be processed within 8 hours of collection unless processed with T-Cell Xtend reagent, which extends the hold time up to 32 hours after venipuncture.
In the laboratory, peripheral blood mononuclear cells (PBMCs) are separated, washed, and counted. After preparation, isolated PBMCs are placed into a specially prepared microtiter plate and exposed to the positive control, a negative control, and two tuberculosis antigens. The positive control contains phytohemagglutinin, which nonspecifically stimulates T-cell production of interferon-gamma.
After a 16- to 20-hour incubation period, secreted interferon-gamma binds to the antibodies coating the base of the plate. A second antibody conjugated to alkaline phosphatase binds to the interferon-gamma and a final substrate is added, which is cleaved by the enzyme to form an insoluble spot on the plate. These spots are counted and then interpreted using FDA-approved interpretation criteria. [6, 25] Notably, the FDA approved a “borderline” criterion, which increases the specificity and sensitivity of the assay by reducing the false-positive and false-negative results near the breakpoint.
As with the QFT-GIT, indeterminate results can occur because of either a lack of positive control response or a high background level of interferon-gamma. Both technical and clinical factors can lead to indeterminate results, including improper testing procedures and washings, as well as patient anergy or immunosuppression.
Individuals with positive screening result need further testing to determine if positivity is due to latent tuberculosis infection or active tuberculosis disease. A clinical evaluation and chest radiograph should be performed on all patients with a positive screening test to assess for active tuberculosis disease. If the patient does not have clinical signs or symptoms, and the chest radiograph does not have findings suggestive of active tuberculosis; the patient should be diagnosed with latent tuberculosis infection and offered a treatment course to diminish the risk of progressing to active tuberculosis.