The study included patients who underwent free flap surgery between January 2006 and December 2019 in the Hospital de San José and Hospital Infantil Universitario de San José in Bogotá, Colombia. Criteria for exclusion were history of deep vein thrombosis, pulmonary embolism, systemic lupus erythematosus, antiphospholipid syndrome, hypercoagulability syndrome, and previous medication with oral or parenteral anticoagulant within the past year of the surgery. A standardized Google App for Work form was created where participant's medical records were registered. The exposure criteria were RBCT, and the primary outcome was the occurrence of VPT, which was defined as the intraoperative visualization of arterial or venous thrombosis in the vascular pedicle, observed up until seven days after the procedure. As a secondary outcome, the presence of clinical signs of arterial flap suffering (pallor, decreased capillary filling time, decreased skin turgor, , venous congestion: blue dermal bleeding, and poikilothermia). As well as the absence of blood flow measured by Doppler ultrasound that was registered from the beginning until the outcome. Different clinical characteristics were recorded such as: age, gender, body mass index (BMI), diabetes (fasting blood glucose \>126 mg/dl or random glucose value \>200 mg/dl), arterial hypertension (persistently high value \>140/90 mmHg or use of antihypertensive medication for adequate control) anesthetic risk (ASA Classification), most recent pre-surgical hemoglobin (Hb mg/dl), most recent pre-surgical hematocrit (Ht %), anemia (Hb \<12mg/dl or Hto \<36% in children under 14 and women of childbearing age, Hb \<13 mg/dl or Hto \<39% in men over 14 or postmenopausal women), history of chemotherapy during the past year, history of radiation therapy in the flap donor or free flap recipient site In regard to the surgery performed, the following variables were analyzed: indication of procedure, free flap donor site, free flap recipient site, type of flap, whether or not other concomitant surgical procedures were being performed (oncological resections, osteosynthesis, tumor resections) and intra-operative bleeding according to the records from anesthesiologists' evaluations. The information was exported from the standardized formats to a database for statistical analysis through statistical packages R and Stata 15® licensed by the Fundación Universitaria de Ciencias de la Salud - FUCS.
The exposed group corresponded to patients who received red blood cell transfusion during the perioperative period of free flap surgery (defined as: 24 hours before, during and up to 24 hours after the procedure).
\- The unexposed group corresponded to patients who did not receive red blood cell transfusion during the perioperative period of free flap surgery (defined as: 24 hours before, during and up to 24 hours after the procedure).
