Summary. In most U.S. populations, screening for TB is done to identify infected persons at high risk for TB disease who would benefit from preventive therapy and to identify persons with TB disease who need treatment. Screening should be done in groups for which rates of TB are substantially higher than for the general population. Clinicians should tuberculin test high-risk persons as part of their routine evaluation. Institutional screening is recommended for the staff of health care facilities, as well as for the staff and residents of long-term care institutions where TB cases are found or the case rates of TB are high. The Mantoux tuberculin skin test is the preferred method of screening for TB infection.
In most U.S. populations, screening is done to identify infected persons at high risk of developing TB disease who would benefit from preventive therapy and to identify persons with TB disease who need treatment. Therefore, all screening activities should be accompanied by a plan for follow-up care for persons with TB infection or disease.
The preferred method of screening for TB infection is the Mantoux tuberculin skin test. Screening for TB infection should be done in well-defined groups for which rates of TB are substantially higher than for the general population. Groups that are not at high risk for TB should not be screened routinely, because they are more likely to have false-positive reactions and they may be inappropriately treated.
In general, high-risk groups that should be screened for infection include:
In some circumstances, screening for TB disease with chest radiographs or sputum smears may be more appropriate than screening for infection with the tuberculin skin test. For example, chest radiography may be the best method in jails or homeless shelters, where the risk of transmission is high but the time required to give skin tests to large numbers of transient persons and to read results makes screening for infection impractical.
Screening for TB infection or disease should always be carried out in consultation with the health department. Facilities such as drug treatment programs or long-term care facilities should screen high-risk groups only when appropriate follow-up measures can be provided either by that facility or by the health department.
Clinicians should identify patients who are in a high-risk category, and they should tuberculin test these persons as part of their routine evaluation. In particular, persons with certain medical conditions known to increase the risk for TB disease (see Transmission and Pathogenesis) should also be tuberculin tested, and their tuberculin skin test status should be clearly noted on their medical record. Persons with a positive reaction should be evaluated for TB disease and, if disease is ruled out, considered for preventive therapy.
Health care workers should be included in a TB screening and prevention program. In addition, screening is recommended for the staff of long-term care facilities who (1) may be exposed to TB patients on the job (e.g., staff of correctional facilities) or (2) would pose a risk to large numbers of susceptible persons if they developed infectious TB (e.g., staff of child care centers or AIDS hospices). Such persons should be tuberculin tested upon employment and thereafter at intervals determined by the risk of transmission in that facility. This screening is done for two reasons:
Mantoux tuberculin skin testing is the standard method of identifying persons infected with M. tuberculosis. Multiple puncture tests should not be used to determine whether a person is infected.
The Mantoux test is performed by giving an intradermal injection of 0.1 ml of purified protein derivative (PPD) tuberculin containing 5 tuberculin units (TU) into either the volar or dorsal surface of the forearm. The injection should be made with a disposable tuberculin syringe, just beneath the surface of the skin, with the needle bevel facing upward. This should produce a discrete, pale elevation of the skin (a wheal) 6 mm to 10 mm in diameter.
To prevent needlestick injuries, needles should not be recapped, purposely bent or broken, removed from disposable syringes, or otherwise manipulated by hand. After they are used, disposable needles and syringes should be placed in puncture-resistant containers for disposal. Institutional guidelines regarding universal precautions for infection control (e.g., the use of gloves) should be followed.
The reaction to the Mantoux test should be read by a trained health care worker 48 to 72 hours after the injection. If a patient fails to show up for the scheduled reading, a positive reaction may still be measurable up to 1 week after testing. However, if a patient who fails to return within 72 hours has a negative test, tuberculin testing should be repeated.
The area of induration (palpable swelling) around the site of injection is the reaction to tuberculin. The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis). Erythema (redness) should not be measured. All reactions should be recorded in millimeters of induration, even those classified as negative. If no induration is found, "0 mm" should be recorded.
A tuberculin reaction of 5 mm or greater of induration is classified as positive in the following groups:
A tuberculin reaction of 10 mm or greater of induration is classified as positive in persons who do not meet the preceding criteria but who have other risk factors for TB. These include:
A tuberculin reaction of 15 mm or greater of induration is classified as positive in persons with no known risk factors for TB. Determining whether those persons identified as positive are candidates for preventive therapy is discussed in Preventive Therapy.
In general, these guidelines for interpreting skin test reactions should also be applied to persons who may have occupational exposure to TB (e.g., health care workers and staff of nursing homes, drug treatment centers, or correctional facilities). Thus, the appropriate cutoff for defining a positive reaction depends on the employee's individual risk factors for TB. However, the appropriate cutoff also depends on the prevalence of TB in the facility. In facilities where the risk of exposure is very low, 15 mm or greater may be an appropriate cutoff for employees with no other risk factors. In facilities where TB patients receive care, 10 mm or greater may be an appropriate cutoff for employees with no other risk factors.
The tuberculin skin test is a valuable tool, but it is not perfect. Several factors can affect the skin test reaction: for example, infection with mycobacteria other than M. tuberculosis (nontuberculous mycobacteria) and vaccination with bacille Calmette-Guerin (BCG). These factors can lead to false-positive reactions (a positive skin test reaction in a person not infected with M. tuberculosis). See BCG Vaccination for further information on interpreting tuberculin reactions in persons with a history of BCG vaccination. Other factors, such as anergy, can lead to false-negative reactions.
The absence of a reaction to the tuberculin test does not rule out the diagnosis of TB disease or infection. In immunosuppressed persons, delayed-type hypersensitivity responses such as tuberculin reactions may decrease or disappear. This condition, known as anergy, may be caused by many factors, such as HIV infection, overwhelming miliary or pulmonary TB, severe or febrile illness, measles or other viral infections, Hodgkin's disease, sarcoidosis, live-virus vaccination, and the administration of corticosteroids or immunosuppressive drugs. On average, 10% to 25% of patients with TB disease have negative reactions when tested with a tuberculin skin test. Approximately one third of patients with HIV infection and more than 60% of patients with AIDS may have skin test reactions of <5 mm even though they are infected with M. tuberculosis.
Anergy is detected by administering at least two other delayed-type hypersensitivity antigens, such as tetanus toxoid, mumps, or Candida, by the Mantoux technique. This should be done in conjunction with tuberculin skin testing. Persons who have a reaction of 3 mm or greater to any of the antigens (including PPD) are not anergic. In addition, persons who have a positive reaction to tuberculin should be considered infected with M. tuberculosis, regardless of their reaction to the other antigens. The results of anergy testing should be recorded in millimeters of induration, not simply as positive or negative.
If anergy is demonstrated, the probability of infection should be assessed, and persons whose risk of exposure is judged to the high (known contacts of persons with infectious TB or persons from a group with a high prevalence of TB infection) should be evaluated for preventive therapy.
Although low CD4+ T-lymphocyte counts (200/microliter or less) and anergy seem closely correlated, anergy can occur in patients with a relatively high CD4+ T-lymphocyte count. In addition, reactivity to PPD or control antigens may be present at very low CD4+ T-lymphocyte levels. Currently, CD4+ T-lymphocyte testing is not recommended as a substitute for anergy testing.
In some people who are infected with M. tuberculosis, delayed-type hypersensitivity to tuberculin may wane over the years. When these people are skin tested many years after infection, they may have a negative reaction. However, this skin test may stimulate (boost) their ability to react to tuberculin, causing a positive reaction to subsequent tests. This boosted reaction may be misinterpreted as a new infection.
Two-step testing is used to distinguish boosted reactions and reactions due to new infection. If the reaction to the first test is classified as negative, a second test should be done 1 to 3 weeks later. A positive reaction to the second test probably represents a boosted reaction. On the basis of this second test result, the person should be classified as previously infected and cared for accordingly. This would not be considered a skin test conversion. If the second test result is also negative, the person should be classified as uninfected. In these persons, a positive reaction to any subsequent test is likely to represent new infection with M. tuberculosis (skin test conversion). Two-step testing should be used for the initial skin testing of adults who will be retested periodically, such as health care workers.