New Assay Shown to Dramatically Reduce Time for Staphylococcus aureus Results

   Conventional methods for identifying and determining the susceptibility of Staphylococcus aureus take between 48 and 72 hours. A new laboratory assay  described at the 50th Interscience Conference on Antimicrobial Agents and Chemotherapy in Boston delivers results in approximately five hours, according to researchers. Further, the test determines methicillin resistance or susceptibility.

   Tanaya Bhowmick, MD, and colleagues in the Deptartment of Medicine at the University of Medicine and Dentristy/Robert Wood Johnson Medical School in New Brunswick, NJ, presented a poster detailing their evaluation of the MicroPhage assay, described as being a novel phage-based assay that identifies S. aureus and differentiates between methicillin-susceptible (MSSA) and methicillin-resistant S. aureus (MRSA) isolates directly from blood cultures. The assay uses mixed lytic bacteriophage to differentiate S. aureus from other bacteria and predicts susceptibility to penicillinase resistant-lactams.

The self-performing immunoassay measures the increase in concentration of bacteriophage antigens. Results are interpreted visually using a lateral flow immunoassay.

   For their part of a multisite trial, the New Jersey researchers compared the assay with both conventional identification methods and other commercially available assays. They tested a total of 322 specimens from 211 patients. Compared with the standard method, the MicroPhage assay was found to have a sensitivity of 95.5% for S. aureus, 93.8% for MSSA, and 94.1% for MRSA. The specificity was 96.5% for S. aureus, 97.9% for MSSA, and 98.6% for MRSA. 

   In the larger, multisite study, a total of 1123 total specimens were tested of which 356 were S. aureus. Analysis revealed 91.8% sensitivity and 97.5% specificity for detecting S. aureus. The assay had a sensitivity of 92.7% and specificity of 99% for MRSA. For MSSA, the sensitivity and specificity were 88.4% and 98% respectively.