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author:("Kop, joanna")
1.  Detection of Colonization by Carbapenemase-Producing Gram-Negative Bacilli in Patients by Use of the Xpert MDRO Assay 
Journal of Clinical Microbiology  2013;51(11):3780-3787.
Detecting colonization of patients with carbapenemase-producing bacteria can be difficult. This study compared the sensitivity and specificity of a PCR-based method (Xpert MDRO) for detecting blaKPC, blaNDM, and blaVIM carbapenem resistance genes using GeneXpert cartridges to the results of culture with and without a broth enrichment step on 328 rectal, perirectal, and stool samples. The culture method included direct inoculation of a MacConkey agar plate on which a 10-μg meropenem disk was placed and plating on MacConkey agar after overnight enrichment of the sample in MacConkey broth containing 1 μg/ml of meropenem. Forty-three (13.1%) samples were positive by PCR for blaKPC and 11 (3.4%) were positive for blaVIM; none were positive for blaNDM. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the PCR assay for blaKPC were 100%, 99.0%, 93.0%, and 100%, respectively, compared to broth enrichment culture and sequencing of target genes. The sensitivity, specificity, PPV, and NPV of the assay for blaVIM were 100%, 99.4%, 81.8%, and 100%, respectively. Since none of the clinical samples contained organisms with blaNDM, 66 contrived stool samples were prepared at various dilutions using three Klebsiella pneumoniae isolates containing blaNDM. The PCR assay showed 100% positivity at dilutions from 300 to 1,800 CFU/ml and 93.3% at 150 CFU/ml. The Xpert MDRO PCR assay required 2 min of hands-on time and 47 min to complete. Rapid identification of patients colonized with carbapenemase-producing organisms using multiplex PCR may help hospitals to improve infection control activities.
PMCID: PMC3889767  PMID: 24006011
2.  Evaluation of the Analytical Performance of the Xpert MTB/RIF Assay ▿ †  
Journal of Clinical Microbiology  2010;48(7):2495-2501.
We performed the first studies of analytic sensitivity, analytic specificity, and dynamic range for the new Xpert MTB/RIF assay, a nucleic acid amplification-based diagnostic system that detects Mycobacterium tuberculosis and rifampin (RIF) resistance in under 2 h. The sensitivity of the assay was tested with 79 phylogenetically and geographically diverse M. tuberculosis isolates, including 42 drug-susceptible isolates and 37 RIF-resistant isolates containing 13 different rpoB mutations or mutation combinations. The specificity of the assay was tested with 89 nontuberculosis bacteria, fungi, and viruses. The Xpert MTB/RIF assay correctly identified all 79 M. tuberculosis isolates and correctly excluded all 89 nontuberculosis isolates. RIF resistance was correctly identified in all 37 resistant isolates and in none of the 42 susceptible isolates. Dynamic range was assessed by adding 102 to 107 CFU of M. tuberculosis into M. tuberculosis-negative sputum samples. The assay showed a log-linear relationship between cycle threshold and input CFU over the entire concentration range. Resistance detection in the presence of different mixtures of RIF-resistant and RIF-susceptible DNA was assessed. Resistance detection was dependent on the particular mutation and required between 65% and 100% mutant DNA to be present in the sample for 95% certainty of resistance detection. Finally, we studied whether assay specificity could be affected by cross-contaminating amplicons generated by the GenoType MTBDRplus assay. M. tuberculosis was not detected until at least 108 copies of an MTBDRplus amplicon were spiked into 1 ml of sputum, suggesting that false-positive results would be unlikely to occur.
PMCID: PMC2897495  PMID: 20504986
3.  Rapid Detection of Mycobacterium tuberculosis and Rifampin Resistance by Use of On-Demand, Near-Patient Technology▿ † ‡  
Journal of Clinical Microbiology  2009;48(1):229-237.
Current nucleic acid amplification methods to detect Mycobacterium tuberculosis are complex, labor-intensive, and technically challenging. We developed and performed the first analysis of the Cepheid Gene Xpert System's MTB/RIF assay, an integrated hands-free sputum-processing and real-time PCR system with rapid on-demand, near-patient technology, to simultaneously detect M. tuberculosis and rifampin resistance. Analytic tests of M. tuberculosis DNA demonstrated a limit of detection (LOD) of 4.5 genomes per reaction. Studies using sputum spiked with known numbers of M. tuberculosis CFU predicted a clinical LOD of 131 CFU/ml. Killing studies showed that the assay's buffer decreased M. tuberculosis viability by at least 8 logs, substantially reducing biohazards. Tests of 23 different commonly occurring rifampin resistance mutations demonstrated that all 23 (100%) would be identified as rifampin resistant. An analysis of 20 nontuberculosis mycobacteria species confirmed high assay specificity. A small clinical validation study of 107 clinical sputum samples from suspected tuberculosis cases in Vietnam detected 29/29 (100%) smear-positive culture-positive cases and 33/39 (84.6%) or 38/53 (71.7%) smear-negative culture-positive cases, as determined by growth on solid medium or on both solid and liquid media, respectively. M. tuberculosis was not detected in 25/25 (100%) of the culture-negative samples. A study of 64 smear-positive culture-positive sputa from retreatment tuberculosis cases in Uganda detected 63/64 (98.4%) culture-positive cases and 9/9 (100%) cases of rifampin resistance. Rifampin resistance was excluded in 54/55 (98.2%) susceptible cases. Specificity rose to 100% after correcting for a conventional susceptibility test error. In conclusion, this highly sensitive and simple-to-use system can detect M. tuberculosis directly from sputum in less than 2 h.
PMCID: PMC2812290  PMID: 19864480
4.  Sensitivity and Specificity of the ViroSeq Human Immunodeficiency Virus Type 1 (HIV-1) Genotyping System for Detection of HIV-1 Drug Resistance Mutations by Use of an ABI PRISM 3100 Genetic Analyzer 
Journal of Clinical Microbiology  2005;43(2):813-817.
The ViroSeq human immunodeficiency virus type 1 (HIV-1) genotyping system is an integrated system for identification of drug resistance mutations in HIV-1 protease and reverse transcriptase (RT). Reagents are included for sample preparation, reverse transcription, PCR amplification, and sequencing. Software is provided to assemble and edit sequence data and to generate a drug resistance report. We determined the sensitivity and specificity of the ViroSeq system for mutation detection using an ABI PRISM 3100 genetic analyzer with a set of clinical samples and recombinant viruses. Twenty clinical plasma samples (viral loads, 1,800 to 10,500 copies/ml) were characterized by cloning and sequencing individual viral variants. Twelve recombinant-virus samples (viral loads, approximately 2,000 to 5,000 copies/ml) were also prepared. Eleven recombinant-virus samples contained drug resistance mutations as 40% mixtures. One recombinant-virus sample contained an insertion at codon 69 in RT (100% mutant). Plasma and recombinant-virus samples were analyzed using the ViroSeq system. Each sample was analyzed on three consecutive days at each of three testing laboratories. The sensitivity of mutation detection was 99.65% for the clinical plasma samples and 99.7% for the recombinant-virus preparations. The specificity of mutation detection was 99.95% for the clinical samples and 100% for the recombinant-virus mixtures. The base calling accuracy of the 3100 instrument was 99.91%. Mutations in clinical plasma samples and recombinant-virus samples were detected with high sensitivity and specificity, including mutations present as mixtures. This report supports the use of the ViroSeq system for identification of drug resistance mutations in HIV-1 protease and RT genes.
PMCID: PMC548107  PMID: 15695685

Results 1-4 (4)