Thermonuclease test accuracy is preserved in methicillin-resistant Staphylococcus aureus isolates

Introduction. The nuc gene encodes a thermonuclease which is present in Staphylococcus aureus but not in coagulase-negative staphylococci (CoNS) and is the target of the rapid phenotypic thermonuclease test. The effect of nuc gene variation in methicillin-resistant S. aureus (MRSA) on the performance of PCR testing has been noted, although there are no reports about the effect of MRSA on the activity of the thermonuclease enzyme. Aim. Our goals were to examine the sensitivity and specificity of the thermonuclease test used to distinguish S. aureus from CoNS cultured from blood. In addition, we aimed to assess differences in the sensitivity, specificity and accuracy of the thermonuclease test between methicillin-sensitive S. aureus (MSSA) and MRSA isolates. Methodology. We performed a retrospective analysis of 1404 isolates. Each isolate from a positive blood culture was identified as a Gram-positive coccus by microscopy then analysed with the thermonuclease test (Southern Group Laboratory) prior to confirmatory identification using VITEK microbial identification platforms (bioMérieux) and cefoxitin disc diffusion testing. Results. Of 1331 samples included in the final analysis, 189 were thermonuclease-positive, of which 176 were identified as S. aureus . Of the 1142 thermonuclease-negative samples, 13 were finally identified as S. aureus , giving a sensitivity of 93.1 % (95 % confidence interval [CI] 88.5–96.3) and specificity of 98.9 % (95 % CI 98.1–99.4). Of the nine proven MRSA samples, eight were thermonuclease-positive, giving a sensitivity of 88.9 % (95 % CI 51.8–99.7). Thermonuclease test accuracy for MSSA and MRSA isolates was 98.1 % (95 % CI 97.2–98.8) versus 98.8 % (95 % CI 98.0–99.3), respectively. Conclusions. In the era of increasing use of molecular-based microbiology assays, the thermonuclease test remains a simple, inexpensive and robust test for the presumptive identification of S. aureus cultured from blood, irrespective of methicillin sensitivity.