Hemoglobin oxygen saturation in superior vena cava (ScvO(2)) is used as a parameter to guide hemodynamic management in shock patients and it can be continuously read through a central venous catheter equipped with a fiberoptic spectrophotometric probe (Edwards PreSep catheter) connected to a specific monitor (Edwards Vigileo). We report of an episode of erroneous ScvO(2) reading by this technology in a patient with septic shock who was receiving an erythrocytes transfusion through the PreSep catheter main lumen. We think this artifact should be known by intensivists since it can lead to ScvO(2) misinterpretation and subsequent erroneous therapeutic decisions.