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Clinical Observation |

Diagnostic Usefulness of a T-cell–Based Assay for Extrapulmonary Tuberculosis

Sung-Han Kim, MD; Su-Jin Choi, MS; Hong-Bin Kim, MD; Nam-Joong Kim, MD; Myoung-don Oh, MD; Kang-Won Choe, MD
Arch Intern Med. 2007;167(20):2255-2259. doi:10.1001/archinte.167.20.2255.
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Background  Diagnosing extrapulmonary tuberculosis (E-TB) remains a challenge. A recently developed Mycobacterium tuberculosis–specific region of difference 1 gene-based assay for diagnosing tuberculosis infection showed promising results. However, the diagnostic usefulness of this assay remains to be determined compared with tuberculin skin test (TST) in patients with suspected E-TB in clinical practice.

Methods  All patients with suspected E-TB were prospectively enrolled in a tertiary care hospital during a 9-month period. In addition to the conventional tests for diagnosing E-TB, the interferon γ–producing T-cell responses to early secreted antigenic target 6 and culture filtrate protein 10 by enzyme-linked immunospot (ELISPOT) assay were performed. Final diagnosis in patients having suspected E-TB was classified by clinical category.

Results  Seventy-two patients with suspected E-TB were enrolled; 34 (47%) had immunosuppressive conditions. Of 72 patients, 32 (44%) were classified as having E-TB, including 22 with confirmed tuberculosis and 10 with probable tuberculosis, and 35 (49%) were classified as not having tuberculosis. The remaining 5 (7%) had possible tuberculosis and were excluded from the final analysis. Chronic caseating granulomas, acid-fast bacilli stain, M tuberculosis polymerase chain reaction, and cultures for M tuberculosis were positive in 22 (69%), 5 (16%), 15 (47%), and 18 (56%), respectively, of 32 patients with E-TB. The sensitivity and specificity of the TST (induration size, ≥10 mm) were 47% (95% confidence interval [CI], 29%-65%) and 86% (95% CI, 70%-95%), respectively. By comparison, the sensitivity and specificity of the ELISPOT assay were 94% (95% CI, 79%-99%; P < .001 between TST and ELISPOT) and 88% (95% CI, 72%-97%; P =.99 between TST and ELISPOT), respectively.

Conclusion  The ELISPOT assay is a useful adjunct test for diagnosing E-TB.

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Flowchart for diagnostic performance of the conventional tests in 72 patients with suspected extrapulmonary tuberculosis. Confirm indicates confirmed tuberculosis; Not, not tuberculosis; Possible, possible tuberculosis; and Probable, probable tuberculosis.

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