The American Thoracic Society (ATS) and the Centers for Disease Control and Prevention (CDC) have developed standards for the diagnosis and classification of tuberculosis. The 20-page document was prepared “to provide a framework for… the diagnostic approaches to tuberculosis infection [and] disease and to present a classification scheme that facilitates management…” of the disease. The standards are endorsed by the Infectious Diseases Society of America. The statement describes diagnostic strategies for high- and low-risk patients and outlines a classification scheme for tuberculosis.
The statement is posted on the CDC Web site (http://www.cdc.gov/mmwr/) and on the ATS Web site (http://www.thoracic.org/statementframe.html). Included are sections on the epidemiology of tuberculosis, transmission of Mycobacterium tuberculosis, pathogenesis, clinical manifestations, diagnostic microbiology (including methods of collecting, handling and culturing specimens and drug susceptibility testing) and tuberculin skin testing. The following discussion summarizes the classification scheme for tuberculosis and the interpretation of tuberculin skin test reactions.
The tuberculosis guidelines state that the patient's human immunodeficiency virus (HIV) status must be known, because a different approach to diagnosis and treatment may be warranted in HIV-infected patients. The classification scheme is as follows:
0. No tuberculosis exposure, not infected.
1. Tuberculosis exposure, no evidence of infection. The tuberculin skin test is negative in such patients. The guidelines specify that the action taken in these patients depends mainly on the degree of exposure toM. tuberculosis, how recently it occurred and the patient's immune status. If a significant exposure has occurred within three months, a repeat skin test should be performed 10 weeks after the last exposure. In addition, initiation of treatment of latent tuberculosis should be considered, especially in children younger than 15 years and in patients with HIV infection.
2. Latent tuberculosis infection, no disease. This class is characterized by a positive reaction on tuberculin skin testing, but no clinical, bacteriologic or radiographic evidence of active tuberculosis. Treatment of latent tuberculosis may be indicated in some patients. The patient's treatment status should be determined as follows: never has received antituberculosis therapy, currently is receiving therapy, has completed a course of therapy or has had an incomplete course of therapy.
3. Tuberculosis, clinically active. This category includes all patients with clinically active tuberculosis whose diagnostic procedures are complete. According to the ATS/CDC guidelines, if the diagnosis is still pending, the patient should be classified as “tuberculosis suspect” (class 5). Patients with clinically active tuberculosis have clinical, bacteriologic and/or radiographic evidence of current tuberculosis, most definitively established by isolation of M. tuberculosis. The patient remains in class 3 until treatment of the current episode is completed.
Clinically active tuberculosis is also classified by location of disease—that is, pulmonary, pleural, lymphatic, bone and/or joint, genitourinary disseminated (miliary), meningeal, peritoneal and other. Bacteriologic status, with specification of the test technique (i.e., microscopy, nucleic acid amplification and culture), is also included in the definition of this class. The results of drug susceptibility testing are also included.
The ATS/CDC guidelines note that data on chest radiographic findings and tuberculin skin test reaction are needed in some situations. The findings on chest radiographs are classified as normal, abnormal, cavitary or noncavitary, and stable or worsening or improving. Tuberculin skin test reactions are classified as positive or negative, with the size of the induration noted.
4. Tuberculosis, not clinically active. This class includes patients with a positive tuberculin skin test reaction and a history of tuberculosis or abnormal stable radiographic findings. Bacteriologic studies are negative and there is no clinical and/or radiographic evidence of current disease. The guidelines state that patients in this class may never have received chemotherapy, may be receiving treatment for latent infection or may have completed a previous course of chemotherapy.
5. Tuberculosis suspect (diagnosis pending). The ATS/CDC guidelines stipulate that patients should not remain in this class for more than three months. When diagnostic studies are completed, the patient's disease should be classified according to one of the four preceding classes.
Tuberculin Skin Testing
The accompanying table outlines the guidelines for determining a positive tuberculin skin test reaction. The guidelines define skin test conversion in persons with previously negative results as an increase in the reaction size of 10 mm or more within a period of two years.
According to the ATS/CDC guidelines, studies indicate that the tuberculin purified protein derivative (PPD) test has a false-negative rate of 25 percent during the initial evaluation of patients with active tuberculosis. Factors such as the common occurrence of poor nutrition and general health, overwhelming acute illness and immunosuppression appear to account for such a high false-negative rate. Specificity has been found to be approximately 99 percent in populations without previous mycobacterial exposures or previous Calmette-Guérin bacillus vaccination, but specificity declines to 95 percent in populations in which cross-reactivity with other mycobacteria is common. In view of an M. tuberculosis infection rate of 5 to 10 percent in the general population, the tuberculin skin test has a low positive predictive value. Thus, screening in low-risk groups is not recommended, because a false-positive result is more likely than a true-positive result.
The ATS/CDC guidelines state that persons who have been in close contact with persons with infectious tuberculosis have a 25 to 50 percent chance of M. tuberculosis infection. Groups with a high prevalence of tuberculosis include persons born in countries with a high rate of tuberculosis, persons with poor access to health care, persons who live or spend time in facilities such as nursing homes, prisons, homeless shelters and drug treatment centers, and persons who inject illegal drugs. High-risk groups are identified as children younger than four years, persons with HIV infection, persons in close contact with persons with infectious tuberculosis, persons whose tuberculin skin test results have converted to positive in the past one to two years, persons whose chest radiographs suggest old tuberculosis and persons with certain medical conditions, such as diabetes mellitus, silicosis, prolonged corticosteroid therapy, immunosuppression, leukemia, Hodgkin's disease, severe kidney disease, head and neck cancer, malnutrition and certain intestinal conditions.