Identifying TB Infection

A person exposed to an individual with infectious TB or who has other risk factors for TB as noted above should be given a tuberculin skin test.

The Mantoux tuberculin skin test is the preferred method of skin testing. The Mantoux tuberculin skin test is the intradermal injection of purified protein derivative (PPD) of killed tubercle bacilli, usually on the inner forearm. The site is examined by a trained health care worker 48 to 72 hours after injection for induration (palpable swelling). The diameter of induration is measured and recorded; erythema or bruising is disregarded.

The criteria endorsed by the American Thoracic Society and CDC for a positive tuberculin skin-test result (Table 1) are intended to increase the likelihood that persons at high risk for TB will be candidates for preventive therapy and that persons having tuberculin reactions not caused by M. tuberculosis will not receive unnecessary diagnostic evaluation or treatment.

For each of the risk groups listed in Table 1, reactions below the cutoff point are considered negative. A negative TB skin test result does not absolutely rule out TB infection, especially in persons with TB-like symptoms, HIV infection, or AIDS. Also, because it takes 2 to 10 weeks from the time of exposure for a person to react to tuberculin, the initial skin test result of an infected contact may be falsely negative. Therefore, a repeat skin test 10 weeks post exposure is warranted.

Some persons with both HIV and TB infections may have false negative skin test reactions (anergy). Anergy refers to the inability to react to a skin test antigen even though the person is infected with the organism being tested. Several delayed-type hypersensitivity (DTH) antigens (such as tetanus toxoid, mumps or Candida) administered by the Mantoux technique have been used in an attempt to determine anergy status. Recent CDC recommendations, however, note several factors that limit the usefulness of anergy skin testing. These include problems with standardization and reproducibility, the low risk for TB associated with a diagnosis of anergy, and the lack of apparent benefit of preventive therapy for groups of anergic HIV-infected persons. Therefore, the use of anergy testing in conjunction with PPD testing is no longer routinely recommended for screening programs for M. tuberculosis infection conducted among persons infected with HIV in the United States.

Persons with latent TB infection should be evaluated for HIV risk behaviors and offered counseling and HIV-antibody testing if such behaviors are present.

Many foreign countries still use BCG as part of their TB control programs, especially for infants. In persons vaccinated with BCG, sensitivity to tuberculin is highly variable, depending upon the strain of BCG used and the group vaccinated. There is no reliable method of distinguishing tuberculin reactions caused by BCG from those caused by natural infections. A reaction to tuberculin in a person with a history of BCG vaccination is more likely to be due to infection with M. tuberculosis if:

• the induration is large
• the person was vaccinated a long time ago
• the person is a recent contact of a person with infectious TB
• there is a family history of TB
• the person comes from an area where TB is common
• chest radiograph findings show evidence of previous TB. In a BCG-vaccinated person who has any of the preceding risk factors, a positive tuberculin reaction probably indicates infection with M. tuberculosis. Such persons should be evaluated for isoniazid preventive therapy after disease has been ruled out.

Table 1 Summary of interpretation of tuberculin skin-test results

An induration of >5 mm is classified as positive in the following: Persons who have had recent close contact with persons who have active TB; Persons who have human immunodeficiency virus (HIV) infection or risk factors for HIV infection but unknown HIV status (e.g., injecting drug users); Persons who have fibrotic chest radiographs consistent with healed TB. An induration of >10 mm is classified as positive in all persons who do not meet any of the above criteria, but who belong to one or more of the following groups having high risk for TB: Injecting-drug users known to be HIV seronegative; Persons who have other medical conditions that have been reported to increase the risk for progressing from latent TB infection to active TB. These medical conditions include diabetes mellitus, conditions requiring prolonged high-dose corticosteroid therapy and other immunosuppressive therapy (including bone marrow and organ transplantation), chronic renal failure, some hematologic disorders (e.g., leukemias and lymphomas), other specific malignancies (e.g., carcinoma of the head or neck), weight loss of <10% below ideal body weight, silicosis, gastrectomy, jejunoileal bypass; Residents and employees of high-risk congregate settings: prisons and jails, nursing homes and other long-term facilities for the elderly, health-care facilities (including some residential mental health facilities), and homeless shelters; Foreign-born persons recently arrived (i.e., within the last 5 years) from countries having a high prevalence or incidence of TB; Some medically underserved, low-income populations, including migrant farm workers and homeless persons; High-risk racial or ethnic minority populations, as defined locally; Children <4 years of age or infants, children, and adolescents exposed to adults in high-risk categories. An induration of >15 mm is classified as positive in persons who do not meet any of the above criteria.

Identifying TB Disease

If the skin test result is positive or if symptoms suggestive of TB are present (e.g., productive and prolonged cough, fever, chills, loss of appetite, weight loss, fatigue, or night sweats), a chest radiograph should be obtained to help rule out active pulmonary TB. The chest radiograph may also be used to detect the presence of fibrotic lesions suggestive of old, healed TB or silicosis.

Acid-fast bacilli (AFB) smears and cultures should be performed on sputum specimens of all persons who have symptoms of TB or whose chest radiograph suggests TB. A positive AFB smear is an indication for beginning treatment for TB. However, a positive AFB smear may also indicate the presence of nontuberculous mycobacteria. A positive culture for Mycobacterium tuberculosis is the only definitive proof of TB disease.

Health care providers of HIV-infected persons should be aware of atypical patterns of TB disease in these persons. Extra-pulmonary TB is more common. Also, pulmonary TB may present in an unusual manner (e.g., in the lymph nodes or in the lower part of the lungs).

All persons with TB infection or TB disease should be offered counseling and HIV-antibody testing, because medical management may be altered in the presence of HIV infection.