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Introduction. Several studies have used matrix-assisted laser desorption ionization-time of flight MS (MALDI-TOF) with a serum separator tube (SST) to perform rapid identification of microorganisms directly from positive blood cultures (BCs), with different performances and methodologies. Hypothesis / Gap Statement. The use of TSS could significantly reduce the time of identification of microorganisms that produce bacteremia. Aim. Our goals were to evaluate bacterial identification by MALDI-TOF using a method based on an SST and compare it with MALDI-TOF after subculture for 18–24 h. Methodology. BCs no more than 1 h after a positive growth signal were included in the study. Analysis of results was expressed as a score. Information about time to a positive signal and number of microorganisms was collected. Results. In total, 253 BCs were analysed; 45.5 % gave a reliable result, 23.3 % an unreliable result and 31.2 % an error in identification. In gram-negative and gram-positive bacteria, the percentages of reliable results were 83.5 and 21.8 %, respectively. According to time to positive signal, the percentages of correct identification and mean score were 81.1 % (99/122) and 1.89±0.30 in Group 1 (<15 h); and 57.2 % (75/131) and 1.70±0.32 in Group 2 (>15 h), respectively (P <0.001). According to the number of microorganisms, the corresponding percentages of correct identification and mean scores were: Group 1 [≤50 microorganisms observed per field (MOF)], 50/94 (53.19 %) and 1.72±0.32; Group 2 (51–100 MOF): 44/66 (66.67 %) and 1.85±0.34; Group 3 (>100 MOF): 79/93 (84.94 %) and 1.84±0.31. Conclusion. This method allowed us to obtain a high percentage of the aetiological agent of bacteraemia in less than 30 min after a positive BC.

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