The value of MALDI-TOF failure to provide an identification of Staphylococcal species direct from blood cultures and rule out Staphylococcus aureus bacteraemia: a post-hoc analysis of the RAPIDO trial.

Abstract

Introduction Rapid differentiation between Staphylococcus aureus (SA) and coagulase-negative staphylococci (CoNS) is critical in clinical infection. Direct matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) identification from blood culture is highly accurate, but is associated with a significant failure rate, delaying identification. However, MALDI-TOF failure may itself be indicative of CoNS infection.Aim We sought to examine whether failure of MALDI-TOF direct ID was indicative of CoNS infection and could be used as a diagnostic tool to promote antimicrobial stewardship.Methodology Results of Gram stains, MALDI-TOF identification and formal identification were extracted from the large, multi-centre RAPIDO trial. All blood cultures with presumed staphylococci were included. MALDI-TOF performance (correct identification, incorrect identification, failed identification) was calculated for each sample and across sites. Risk of SA disease was calculated for each group (correct, incorrect, failed) and across sites. Logistic regression was used to identify if clinical features are associated with MALDI-TOF performance.Results In the RAPIDO trial, 4312 patients were allocated to the MALDI-TOF arm. After exclusions, 880 patients were eligible and had a blood culture with a Gram stain consistent with presumed staphylococci. In total, 204 of these (23.2 %) were ultimately identified as SA. MALDI-ID was successful 83.9 % of the time, and was 100 % accurate when successful. Failure was more common in CoNS isolates (124/641, 19.3 %) than in SA (13/191, 6.4 %). When MALDI-TOF failed, the risk of SA disease was 9.2 % across the whole cohort, although failure rates and risk of SA disease varied significantly between centres. MALDI-TOF failure was independent of clinical characteristics.Conclusion Presumed staphylococci that fail direct MALDI-TOF identification from blood culture are significantly more likely to be CoNS isolates than SA. In low-risk or low-prevalence settings, SA therapy can be withheld if MALDI-TOF is unsuccessful.