Background and Objectives: Haemoglobin threshold for transfusion has been significantly decreased, but haemoglobin plasma concentration may not be sufficient to assess the need of red‐blood‐cell (RBC) transfusion. Central venous oxygen saturation (ScvO2) is a clue of metabolic matching between O2 transport and consumption, which could help to assess when transfusion is appropriate once anaemia has been diagnosed in ICU patients.Materials and Methods: Adult patients admitted consecutively to a cardiothoracic and vascular ICU were included in a prospective, observational and single‐centre study over a 6‐month period (September 2014 to February 2015), provided they were transfused with RBC. Patients with active bleeding or in unstable condition were excluded. Haemoglobin and ScvO2 were collected through a central venous catheter before and after transfusion. In order to identify a ScvO2 threshold, analysis of ScvO2 changes after transfusion was performed.Results: Fifty‐three patients received 100 RBC transfusions. Haemoglobin at the time of transfusion was 7·2 g/dl [6·8–7·7], while ScvO2 was 66·9% [60–73]. A 5% increase in ScvO2 after transfusion has the best specificity and positive predictive values, with a ScvO2 threshold of 65%. After transfusion (RBC units, 2 [1‐2]), ScvO2 increased only in patients with ScvO2 ≤65%, from 58% [53–62] to 69% [64–73] (P < 0·001).Conclusion: In anaemic patients, RBC transfusion induced a significant increase in ScvO2, provided it was low before transfusion. A 65% cut‐off value of ScvO2 before transfusion showed good specificity and good positive predictive value for a 5% increase after transfusion.