BackgroundBloodstream infections are a leading cause of mortality. Their detection relies on blood cultures (BCs) but time to positivity is often between tens of hours and days.d-lactate is a metabolite widely produced by bacteria but very few in human. We aimed to evaluated-lactate,d-lactate/l-lactate ratio andd-lactate/total lactate ratio in plasma as potential early biomarkers of bacteraemia on a strictly biological standpoint.MethodsA total of 228 plasma specimens were collected from patients who had confirmed bacteraemia (n= 131) and healthy outpatients (n= 97). Specificl-lactate andd-lactate analyses were performed using enzymatic assays and analytical performances ofd-lactate,d-lactate/total lactate andd-lactate/l-lactate ratios for the diagnosis of bacteraemia were assessed.ResultsA preliminaryin vitrostudy confirmed that all strains ofEscherichia coli,Klebsiella pneumoniaeandStaphylococcus aureuswere able to produced-lactate at significant levels. In patients, plasmad-lactate level was the most specific biomarker predicting a bacteraemia profile with a specificity and predictive positive value of 100% using a cut-off of 131 μmol.L−1. However, sensitivity and negative predictive value were rather low, estimated at 31% and 52%, respectively.d-lactate displayed an Area Under Receiver Operating Characteristic (AUROC) curve of 0.696 with a P value < 0.0001. There was no difference ofd-lactate levels between BCs bottles positive for Gram-positive or Gram-negative bacteria (p = 0.55).Conclusiond-lactate shows promise as a specific early biomarker of bacterial metabolism. The development of rapid automated assays could raise clinical applications for infectious diseases diagnosis including early bacteraemia prediction.