in 85-90% of cultures that contain M. tuberculosis. Culture- and smear-negative cases of suspect TB are treated empirically on the basis of clinical suspicion and lack of an alternative diagnosis.

PCR-based diagnostics provide the diagnosis of TB and, to a lesser extent, extrapulmonary TB, rapidly and with greater sensitivity and specificity compared with sputum smears. Such diagnostics are expensive, however, and offer fewer advantages in cases of paucibacillary (that is, having few bacilli) TB.

TB presents as pulmonary disease in 80% of reported cases in the United States (CDC 2005c). Similarly, pulmonary TB accounted for 70.7% of cases among hospitalized active-duty US Army personnel from 1980 to 1996 (Table 5.6). The difference in age distribution between the civilian population and the military population probably accounts for much of the 9% difference in the proportion of pulmonary TB between the two groups.

The most common symptoms of pulmonary TB are cough that produces purulent sputum for at least 2 weeks, night sweats, weight loss, and anorexia. Hemoptysis and pleurisy also may occur. Half of patients with pulmonary TB are afebrile, and one-fifth lack pulmonary symptoms altogether.

Signs of consolidation may be present on physical examination. Chest radiographs most frequently show opacities localized to apical and posterior segments of the upper lobes and the superior (dorsal) segment of the lower lobes. Early cavities may be present; these typically are thin-walled and surrounded by opacities, and 10% have air-fluid levels. TB may present atypically in some patients, particularly diabetics, immunocompromised people, and people with HIV infection. In such cases, chest radiographic findings are variable, ranging from dense lobar or segmental consolidation to atelectasis, large-mass lesions, or cavities.

About 20% of reported cases of active TB occur outside the lungs in such regions as the lymph nodes (9%), pleura (4%), bones and joints (2%), meninges (1%), genitourinary tract (1%),