QuantiFERON®-TB Gold is a test that is now being widely used in the United States for the detection of Mycobacterium tuberculosis infection. This enzyme-linked immunosorbent assay (ELISA) test detects the release of interferon-gamma (IFN-g) in fresh heparinized whole blood from sensitized persons when it is incubated with mixtures of synthetic peptides simulating two protein that are present in M. tuberculosis ESAT-6 and CFP-10 (1). Non tuberculous mycobacteria have generally been associated with immunosuppression. Amongst this genus, Mycobacterium kansasii differs from the other members, in that it has the ability to cause disease in otherwise healthy and immunocompetent individuals (2 ). We present a case of a patient who had a positive QuantiFERON®-TB Gold test but later grew M.kansasii in his bronchoalveolar lavage fluid.
An 85 year old Caucasian male presented to the Veterans Affairs Hospital in Louisville with the complaints of hemoptysis and fatigue for the past one week. This was associated with low grade fever, decreased appetite and weight loss for the last two weeks. Physical examination revealed bilateral rhonci on chest auscultation. Laboratory data revealed a white blood cell count of 11.8x 103/mm3 and hemoglobin of 13.5 gm/dL. A Chest Computerized tomography revealed diffuse bilateral alveolar infiltrates. Sputum smear and subsequently the cultures were reported negative for bacteria and acid fast bacilli. The patient was treated for a possible community acquired pneumonia with moxifloxacin 400mg once a day for ten days. On a follow up visit two weeks later he continued to complain of hemoptysis and fatigue. At this visit a Quantiferon- TB Gold test was performed and was reported to be positive. Repeat sputum cultures were done which were again reported to have no growth for bacteria or acid fast bacilli. A bronchoscopy was also preformed. No endobronchial lesion was seen and a bronchoalveolar lavage was collected. The lavage fluid was positive for Mycobacterium kansasii. He was started on ethambutol, isoniazid and rifampacin. His symptoms improved considerably with treatment. On a follow up visit one month later his hemoptysis had completely resolved and his appetite improved.
M. kansasii pneumonia is clinically and radiographically indistinguishable from classical pulmonary tuberculosis, which could reflect similarities in pathogenesis (2 ). At the genetic level these two organisms share two peptides -early secretory antigenic target–6 (ESAT-6) and culture filtrate protein-10 (CFP-10). Since the U.S. Food and Drug Administration (FDA) approval in 2005, the QuantiFERON®-TB Gold has been gaining acceptance as an aid for diagnosing Mycobacterium tuberculosis infection. One of the advantages of using this test instead of the Tuberculin Skin Test is that the proteins that are targeted by the QuantiFERON®-TB Gold that is the ESAT 6 and CFAP-10 are absent from all Bacille Calmette-Guérin (BCG) vaccine strains and from commonly encountered nontuberculous mycobacteria. However these proteins are also present in M. kansasii, M. szulgai, and M. marinum. It is therefore possible that these bacteria could cause a false positive result as occurred in our patient. We performed a Medline search from 2005 to this date and there have been no report of a positive QuantiFERON®-TB Gold test in a patient with M.kansasii infection.
Quantiferon-TB Gold is a useful test to detect M.tuberculosis infection; however the physician should be aware of its limitations. Infections with some non tuberculous mycobacteria may cause a positive result as well.
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