Certain types of surgery are associated with occult blood loss, which is hard to detect intraoperatively by intermittent conventional, invasive Hb concentration measurements using the clinical standard of Hb monitoring by satellite laboratory analysis (Hbsatlab). The investigators want to see whether continuous non-invasive transcutaneous Hb measurement using a finger sensor (SpHb), (a) reduces the total time (area under the curve, AUC) a patient's Hb is below a predetermined transfusion threshold (HbAUC) for administration of red blood cell concentrate (RBC), and (b) prevents a decrease in total oxygen delivery (DO2) possibly associated with transfusion below a critical haemoglobin concentration. Furthermore, the investigators want to study if SpHb monitoring changes the timing of RBC administration and reduces the need for intra- and post-operative RBC transfusion.

Inclusion Criteria:

• Patients at the age of 18 years or older, planned for elective surgery at-risk for undetected blood loss.
• American Society of Anesthesiologists (ASA) classification I, II, III, IV

Exclusion Criteria:

• Patient refusal
• Emergency surgery

30 adult patients scheduled for surgery associated with undetected, i.e., difficult to reliably monitor intraoperative blood loss