A comparison of the rapid thermonuclease test and the lysostaphin susceptibility test in the presumptive identification of Staphylococcus aureus from positive bactec blood cultures

Abstract

We compared the ability of a rapid 2-hour thermonuclease test and a Gram-stained lysostaphin susceptibility test to presumptively identify Staphylococcus aureus from 72 blood cultures. The thermonuclease test identified 25 of 27 S. aureus; there were no false positives. The predictive values of positive and negative thermonuclease tests were 100% and 95.7%, respectively. The lysostaphin test correctly identified 24 of 27 S. aureus; however, there were eight false positives. The predictive values of positive and negative lysostaphin tests were 75% and 95.2%, respectively. Lysostaphin MICs were ⩾1.6 μg/ml for only 28 of 41 coagulase negative staphylococci strains. We now routinely apply the thermonuclease test in the clinical laboratory. Of the first 304 blood cultures tested in this setting, the thermonuclease test correctly identified 63 of 68 S. aureus. There have been no false positives. The thermonuclease test is superior to the lysostaphin test and accurately identifies S. aureus in blood cultures within 2 hours of the first positive reading.