Using Viruses to Detect MRSA and MSSA in Blood Cultures

A newly FDA-approved test uses bacteriophages to rapidly identify Staphylococcus aureus— and to distinguish between methicillin-resistant and methicillin-susceptible strains — in blood cultures with gram-positive cocci.

Culture-based methods for detecting Staphylococcus aureus (SA) in blood and then determining whether the pathogen is methicillin-resistant (MRSA) or methicillin-susceptible (MSSA) can take up to 3 days. More-rapid identification allows for earlier optimization of therapy, which could improve outcomes and reduce unnecessary antimicrobial use. Although several nucleic acid detection tests have been FDA approved for rapid detection of MRSA or SA directly from blood cultures, acceptance has been limited by cost and performance issues.

The newly FDA-approved KeyPath MRSA/MSSA Blood Culture Test uses bacteriophages that replicate only in the presence of SA to detect and distinguish between MRSA and MSSA in blood cultures with gram-positive cocci in approximately 5 hours. In a manufacturer-sponsored multicenter trial that involved 1116 such samples, the test had a sensitivity of 91.8% and a specificity of 98.3% for SA detection, compared with culture. Among the SA-positive samples, the test was 98.9% and 99.4% accurate for determining MRSA and MSSA, respectively. In a separate study, the test accurately classified as MSSA 28 “empty cassette” SA variants (organisms that cause false-positive MRSA results in some polymerase chain reaction assays).

Comment

This simple immunoassay requires no special equipment to perform and meets the FDA performance standards for susceptibility tests. Whether the test becomes widely accepted in clinical laboratories will depend on cost and real-world performance. Negative predictive value will vary according to the prevalence of SA bacteremia in each hospital, but the reported sensitivity suggests that up to 8% of SA bacteremias could be missed, limiting the utility of a negative test result.

Author

Daniel J. Diekema, MD, D(ABMM)

Citation(s):

FDA clears first test to quickly diagnose and distinguish MRSA and MSSA [press release]. Rockville, MD: Food and Drug Administration; May 6, 2011. (http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm254512.htm)

Bhowmick T et al. Controlled evaluation of a novel method to detect Staphylococcus aureus in blood cultures within 5 hours. 50th Annual ICAAC Meeting, Boston, September2010. Abstract D-155. (http://viajwat.ch/lfbWFA)