Paula H. Vance

Clostridium difficile Testing in the Clinical Laboratory by Use of Multiple Testing Algorithms

ABSTRACT The incidence of Clostridium difficile infection (CDI) has risen almost 3-fold in the United States over the past decade, emphasizing the need for rapid and accurate tests for CDI. The Cepheid Xpert C. difficile assay is an integrated, closed, nucleic acid amplification system that automates sample preparation and real-time PCR detection of the toxin B gene (tcdB). A total of 432 stool specimens from symptomatic patients were tested by a glutamate dehydrogenase (GDH) assay, a toxin A and B enzyme immunoassay (EIA), the Xpert C. difficile assay, and a cell culture cytotoxicity neutralization assay (CCCN). The results of these methods, used individually and in combination, were compared to those of toxigenic culture. Results for the Xpert C. difficile assay alone showed a sensitivity, specificity, positive predictive value, and negative predictive value (NPV) of 94.4, 96.3, 84.0, and 98.8%, while the EIA alone gave corresponding values of 58.3, 94.7, 68.9, and 91.9%, respectively. An algorithm using the GDH assay and the EIA (plus the CCCN if the EIA was negative) showed corresponding values of 83.1, 96.7, 83.1, and 96.1%. The Xpert C. difficile assay was statistically superior to the EIA (P, <0.001 by Fishers exact test) and to the GDH-EIA-CCCN algorithm (P, 0.0363). Combining the GDH and Xpert C. difficile assays lowered both the sensitivity and the NPV of the Xpert assay. The GDH-EIA-CCCN procedure required, on average, 2 days to complete testing on GDH-positive results, while testing by the Xpert C. difficile assay was completed, on average, in less than 1 h. Xpert C. difficile testing yielded the highest sensitivity and NPV, in the least amount of time, of the individual- and multiple-test algorithms evaluated in this study.

Differences Among Nocardia spp. in Susceptibility to Aminoglycosides and β-Lactam Antibiotics and Their Potential Use in Taxonomy

Using disk diffusion and agar dilution methods, 109 clinical isolates of Nocardia were tested against 10 antimicrobial agents. A species difference in susceptibility was noted for cefamandole, carbenicillin, gentamicin, and kanamycin. These differences in susceptibility may be of use taxonomically as well as having some potential use clinically.

Beta-Lactamase Production and Resistance to Beta-Lactam Antibiotics in Nocardia

Although ampicillin has been suggested as a useful agent for the treatment of nocardiosis in man, little is known regarding the presence of beta-lactamase in Nocardia or its possible role in determining resistance to ampicillin and the other beta-lactam antibiotics. We have evaluated 55 isolates of Nocardia for susceptibility to five beta-lactam antibiotics and for the presence of beta-lactamase. Nocardia were resistant to penicillin G, cloxacillin, and cefazolin, but 27 and 62% were susceptible to 3.1 and 25 μg of ampicillin per ml, respectively. Almost 90% of these ampicillin-susceptible or intermediate strains were also susceptible to carbenicillin. The combination of ampicillin and cloxacillin was synergistic against many ampicillin-resistant strains. Beta-lactamase was detected in 89% of Nocardia isolates when intact cells were used and in six of six strains after cell fractionation. This beta-lactamase was most active against penicillin G and ampicillin, with lesser activity against carbenicillin and cephaloridine. These studies suggest that beta-lactamase may be present in all clinical isolates of Nocardia and that mechanisms of antimicrobial resistance other than or in addition to beta-lactamase are responsible for resistance of Nocardia to ampicillin and carbenicillin.