['The aim of this study was to explore the effect of red blood cell (RBC) transfusion on microdialysis-assessed interstitial fluid metabolic parameters in septic patients.', 'We conducted a retrospective study of 37 patients with severe sepsis/septic shock requiring transfusion of one to two RBC units. Interstitial fluid metabolic alterations were monitored by a microdialysis catheter inserted in the subcutaneous adipose tissue. Samples were collected before (T0) and after transfusion at two time-points: T1a and T1b; median post-transfusion times of 120 [interquartile range (IQR); 45-180] and 360 (IQR; 285-320) min. Lactate, pyruvate, glycerol and glucose concentrations were measured with a bedside analyzer, and the lactate/pyruvate (LP) ratio was calculated automatically.', 'RBC transfusions decreased the LP ratio from (T0) 18.80 [interquartile range (IQR); 14.85-27.45] to (T1a) 17.80 (IQR; 14.35-25.20; P < 0.05) and (T1b) 17.90 (IQR; 14.45-22.75; P < 0.001), while there was also significant interindividual variation. Post-transfusion LP ratio changes at T1a [r = -0.42; 95 % confidence interval (CI), -0.66 to -0.098; P = 0.01] and T1b (r = -0.68; 95 % [CI], -0.82 to -0.44; P < 0.001) were significantly correlated with the pre-transfusion LP ratio, but not with baseline demographic characteristics, vital signs, severity scores, hemoglobin level and blood lactate. RBC storage time and leukocyte reduction had no influence on the tissue metabolic response to transfusion.'] Does red blood cell transfusion affect microdialysis-assessed interstitial lactate/pyruvate ratio in critically ill patients with late sepsis?