Jorgensen J H, Salinas J R, Paxson R, Magnon K, Patterson J E, Patterson T F
Departments of Pathology, The University of Texas Health Science Center, San Antonio, Texas 78284, USA.
J Clin Microbiol. 1999 Jan;37(1):175-8. doi: 10.1128/JCM.37.1.175-178.1999.
The Gen-Probe Amplified Mycobacterium Tuberculosis Direct (MTD) test has been approved for use in the United States for the rapid diagnosis of pulmonary tuberculosis in patients with acid-fast smear-positive sputum samples since 1996. Four patients infected with human immunodeficiency virus and one chronic pulmonary-disease patient seen in our institutions with abnormal chest radiographs and fluorochrome stain-positive sputa were evaluated for tuberculosis, including performance of the MTD test on expectorated sputum samples. Three of these five patients' sputa were highly smear-positive (i.e. , more than 100 bacilli per high-power field), while two patient's sputa contained 1 to 10 bacilli per field. MTD results on sputum specimens from these patients ranged from 43,498 to 193,858 relative light units (RLU). Gen-Probe has defined values of at least 30,000 RLU as indicative of a positive test, i.e., the presence of Mycobacterium tuberculosis RNA. Four of the patients' sputum cultures yielded growth of M. kansasii within 6 to 12 days, and the fifth produced growth of M. avium only. One patient's culture contained both M. kansasii and M. avium, but none of the initial or follow-up cultures from these five patients revealed M. tuberculosis. However, subsequent cultures from three of the patients again revealed M. kansasii. During the period of this study, in which MTD tests were performed on smear-positive sputum specimens from 82 patients, four of seven patients with culture-proven M. kansasii pulmonary infections yielded one or more false-positive MTD tests. The MTD sensitivity observed in this study was 93.8%, and the specificity was 85.3%. Five cultures of M. kansasii (including three of these patients' isolates and M. kansasii ATCC 12478), and cultures of several other species were examined at densities of 10(5) to 10(7) viable CFU/ml by the MTD test. All five isolates of M. kansasii and three of three isolates of M. simiae yielded false-positive test results, with readings of 75,191 to 335,591 RLU. These findings indicate that low-level false-positive MTD results can occur due to the presence of M. kansasii, M. avium, and possibly other Mycobacterium species other than M. tuberculosis in sputum. Low-level positive MTD results of 30,000 to 500,000 RLU should be interpreted in light of these findings. It remains to be determined if the enhanced MTD test (MTD 2) recently released by Gen-Probe will provide greater specificity than that observed in this report with its first-generation test.
自1996年以来，基因探针公司的结核分枝杆菌直接扩增（MTD）试验已在美国获批，用于快速诊断痰涂片抗酸染色阳性患者的肺结核。我们机构接诊了4例感染人类免疫缺陷病毒的患者和1例慢性肺病患者，这些患者胸部X线片异常且痰荧光染色阳性，对他们进行了结核病评估，包括对咳出的痰标本进行MTD试验。这5例患者中，有3例的痰涂片为高阳性（即每高倍视野有100多个杆菌），而另外2例患者的痰涂片每视野含1至10个杆菌。这些患者痰标本的MTD结果在43498至193858相对光单位（RLU）之间。基因探针公司将至少30000 RLU的数值定义为阳性试验，即表明存在结核分枝杆菌RNA。4例患者的痰培养物在6至12天内培养出堪萨斯分枝杆菌，第5例仅培养出鸟分枝杆菌。1例患者的培养物中同时含有堪萨斯分枝杆菌和鸟分枝杆菌，但这5例患者的初始培养物或后续培养物均未发现结核分枝杆菌。然而，3例患者的后续培养物再次显示出堪萨斯分枝杆菌。在本研究期间，对82例痰涂片阳性患者的痰标本进行了MTD试验，7例经培养证实为堪萨斯分枝杆菌肺部感染的患者中有4例出现了1次或多次MTD假阳性试验。本研究中观察到的MTD敏感性为93.8%，特异性为85.3%。对5株堪萨斯分枝杆菌培养物（包括其中3例患者的分离株和堪萨斯分枝杆菌ATCC 12478）以及其他几种菌种的培养物，以10(5)至10(7)个活CFU/ml的密度进行了MTD试验。所有5株堪萨斯分枝杆菌分离株和3株猿分枝杆菌分离株中有3株产生了假阳性试验结果，读数为75191至335591 RLU。这些发现表明，由于痰中存在堪萨斯分枝杆菌、鸟分枝杆菌以及可能存在的除结核分枝杆菌以外的其他分枝杆菌菌种，可能会出现低水平的MTD假阳性结果。对于30000至500000 RLU的低水平阳性MTD结果，应根据这些发现进行解释。基因探针公司最近发布的增强型MTD试验（MTD 2）是否会比本报告中观察到的其第一代试验具有更高的特异性，仍有待确定。
