The key to the early diagnosis of TB is a high index of suspicion. Diagnosis is not difficult in persons belonging to high-risk populations who present with typical symptoms and a classic chest radiograph showing upper-lobe infiltrates with cavities (Fig. 173-6). On the other hand, the diagnosis can easily be missed in an elderly nursing-home resident or a teenager with a focal infiltrate. Often, the diagnosis is first entertained when the chest radiograph of a patient being evaluated for respiratory symptoms is abnormal. If the patient has no complicating medical conditions that cause immunosuppression, the chest radiograph may show typical upper-lobe infiltrates with cavitation (Fig. 173-6). The longer the delay between the onset of symptoms and the diagnosis, the more likely is the finding of cavitary disease. In contrast, immunosuppressed patients, including those with HIV infection, may have “atypical” findings on CXR—e.g., lower-zone infiltrates without cavity formation.
Chest radiograph showing a right-upper-lobe infiltrate and a cavity with an air-fluid level in a patient with active tuberculosis. (Courtesy of Dr. Andrea Gori, Department of Infectious Diseases, S. Paolo University Hospital, Milan, Italy; with permission.)
The several approaches to the diagnosis of TB require, above all, a well-organized laboratory network with an appropriate distribution of tasks at different levels of the health care system. Besides clinical assessment and radiography, screening and referral are the principal tasks at the peripheral and community levels. Diagnosis at a secondary level (e.g., a traditional district hospital in a high-incidence setting) can be accomplished nowadays through real-time automated nucleic acid amplification technology (e.g., the Xpert MTB/RIF assay, which also allows detection of drug resistance) or through traditional AFB microscopy, where new tools have not yet been introduced. At a tertiary level, additional technology is necessary, including molecular tests, rapid culture, and DST.
NUCLEIC ACID AMPLIFICATION TECHNOLOGY FOR TUBERCULOSIS
Several test systems based on amplification of mycobacterial nucleic acid have become available in the past few years and are now the preferred first-line diagnostic tests. These tests are progressively replacing smear microscopy, as they ensure rapid confirmation of all types of TB. One system that permits rapid diagnosis of TB with high specificity and sensitivity (approaching that of liquid culture) is the fully automated, real-time nucleic acid amplification technology known as the Xpert MTB/RIF assay. Xpert MTB/RIF can simultaneously detect TB and rifampin resistance in <2 h and has minimal biosafety and training requirements. Therefore, it can be housed in nonconventional laboratory settings as long as a stable and uninterrupted power supply can be assured. The WHO recommends its use worldwide as the first-line diagnostic test in all adults and children with signs or symptoms of active TB. Given the test’s high sensitivity, the WHO also recommends its use as the initial diagnostic test for people living with HIV in ...