at least 10% of ideal body weight, gastrectomy and jejunoileal bypass, radiation therapy, treatment with tumor-necrosis-factor inhibitors, immunosuppression associated with organ transplantation, and corticosteroid therapy. In many cases the stimulating factor is unknown.
To mitigate the risk of LTBI’s becoming active TB, such infections are treated with isoniazid for 9 months. Completing the treatment regimen reduces the risk of active TB by 70-90% (CDC 2000b), but asymptomatic people frequently fail to comply with the regimen.
If a chest x-ray picture is taken during initial TB infection, it often shows features of a condition called primary TB: patchy alveolar opacities in the middle- and lower-lung fields, common with unilateral hilar adenopathy. Occasionally, patients with primary TB have fever, nonproductive cough, dyspnea, and—rarely—erythema nodosum. Compression by enlarged lymph nodes may lead to upper- or middlelung collapse. Primary TB generally resolves without treatment. In some patients, however, the immune system cannot contain the infection, and active disease develops, as discussed below. Patients who recover from primary TB (including pleural disease)—particularly those with prior pleuritis—remain at risk for recurrence of active TB.
Historically, a distinction has been made between primary TB occurring at the time of initial TB infection and the more typical adult manifestation of disease, called reactivation TB, developing later. Yet the overlapping temporal and clinical features of the two forms often blur the distinctions between them. One reason for the apparent overlap is uncertainty as to when the primary infection occurred. Therefore, to be consistent with US diagnostic standards, the committee’s discussion of active TB below pertains to both primary and reactivation TB (CDC 2000a).
The standard approach to diagnosing active TB is through an AFB smear of expectorated sputum. The presence of such mycobacteria as M. tuberculosis in a bodily secretion or tissue specimen can be visually confirmed with the so-called acid-fast test, which exploits the unique properties of the mycobacterial cell envelope. Cells in a specimen are first stained with red carbol fuchsin, then washed with an acidic alcohol solution. The wash decolorizes almost all organisms except mycobacteria because mycobacterial cell envelopes contain mycolic acid, high-molecular-weight lipids, and waxes that prevent the wash from penetrating the cell.
About half of patients with newly diagnosed pulmonary TB have AFB-positive smears. In addition to establishing the likely diagnosis, AFB-positive smears signal highly infectious cases that must be managed through strict isolation. Smears are more likely to be negative in patients with minimal TB or noncavitary TB.
Cultures are performed on such specialized media as Lowenstein-Jensen (an egg-based media), Middlebrook 7H10 (an agar-based media), and Middlebrook 7H102 (a liquid-based media) (CDC 2000a). Using a combination of solid and liquid media will yield positive results