Diagnostic Tests

The tuberculin skin test remains the screening intervention of choice, despite its potential for false-negative results. Negative tuberculin skin tests on initial application have been noted in up to 25% of proven cases of active TB (9). A negative reaction to tuberculin increases with age and may be partly explained due to anergy. Moreover, the "booster effect" of skin-test reactivity to antigen increases in prevalence in the elderly population (26). Thus, it is essential that all older persons who receive a tuberculin skin test [using the standard Mantoux method with 5 tuberculin units of Tween-stabilized purified protein derivative (PPD), and results read in 48-72 h] be retested within two wk of a negative response (induration of less than 10 mm) to ensure recognition of a potentially false-negative reaction. Application of the second tuberculin skin test should ideally be accompanied by common dermal control antigens (e.g., candida, mumps) to determine the presence of cutaneous anergy. A positive booster ef-fect—and therefore a positive tuberculin skin test—is a skin test reaction of 10 mm or more and an increase in induration of 6 mm or more over the first skin test reaction (27). A positive PPD skin test after the initial placement, by the booster effect, or by conversion, or if the patient has clinical manifestations suggestive of TB, warrant a chest radiograph because 75% of all TB cases in the elderly occur in the respiratory tract. Most pulmonary TB cases in elderly patients represent reactivation disease; 1020% of cases are as a consequence of primary infection or reinfection. Although reactivation TB classically involves the upper lobes of the lung (apical and posterior segments), several studies have shown that many elderly TB patients manifest their pulmonary infection in either middle or lower lung lobes (14). Thus, clinicians should exercise caution when interpreting radiographic diagnoses of TB in older patients because of the atypical location of the infection in the lung fields.

Sputum examination for Mtb by smear and culture is indicated in all patients with pulmonary symptoms and/or have radiographic changes compatible with TB and who have not been treated with anti-TB therapy. Elderly persons unable to expectorate sputum should be considered for a more aggressive diagnostic intervention. Flexible fiberoptic bronchoscopy to obtain bronchial washings and bronchial biopsy is a feasible and valuable option to diagnose TB in persons aged 65 years and older (9).

For suspected pulmonary TB, three fresh consecutive morning sputum specimens are recommended for routine mycobacteriologic studies that include an initial smear and culture for Mtb (28). There are two basic acid-fast staining techniques: carbolfuch-

sin and fluorochrome. Both the Ziehl-Neelson (heating) and Kinyoun (phenol) methods utilize carbolfuchsin and stain the organism red against a blue or green counter-stain (e.g., methylene blue). Auramine-rhodamine dye applies the fluorochrome method in which the mycobacterial cells appear golden-yellow against a dark background. Routine mycobacterial culture methods, e.g., Lowenstein-Jensen (L-J) medium, that require up to 6 wk for the growth of Mtb have been replaced by more rapid techniques that utilize radiometric systems, specific DNA probes and the polymerase chain reaction (PCR) (29). At the National Jewish Center for Immunology and Respiratory Diseases in Denver, CO, four types of mycobacterial culture media are used: (1) 7H12 broth for radiometric detection of growth in the BACTEC TB 460 system. The growth of Mtb in this medium can be detected in 1-2 wk. (2) Inoculation of Middlebrook-Cohn 7H10 or 7H11 agar provides the opportunity to examine colonial morphology and detect mixed cultures if the specimen contains more than one species. (3) 7H11 selective media containing antibiotics to inhibit growth of nonmycobacterial contaminants can also be inoculated. (4) Inoculation on the egg-based L-J slant serves as a backup for rare Mtb isolates that may not grow on the other three media.

Nucleic acid amplification (NAA) tests such as PCR and other methods for amplifying DNA and RNA may facilitate rapid detection of Mtb. An NAA test for Mtb complex (Amplified Mtb Direct Test or MTD [Gen-Probe, San Diego, CA] was recently approved by the Food and Drug Administration for use on processed clinical specimens in conjunction with culture for respiratory specimens that are positive on smear

(29). The MTD test uses transcription-mediated amplification to detect Mtb-complex ribosomal RNA; based on the product label, the test sensitivity in clinical trials was 95.6%, and the specificity was 100%. Several other NAA tests are under commercial development, including the Roche Amplicor test, a PCR test that amplifies myco-bacterial DNA.

The rapid diagnosis of TB is especially important in the high-risk elderly population, as well as HIV-infected persons and patients with MDR-TB. Novel developments in mycobacteriology laboratory technology can be divided into three major groups— automated systems for isolation of mycobacteria in liquid media, application of the same system for rapid drug susceptibility testing, and development of new amplification methods for mycobacterial speciation and detection of drug resistance.

Histological examination of tissue from various sites such as the liver, lymph nodes, bone marrow, pleura or synovium that show the characteristic tissue reaction (caseous necrosis with granuloma formation) is also useful for the diagnosis of TB disease.6.

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