ABSTRACT Background: Surgical procedures can cause significant blood loss, which can decompensate the patient's homeostasis and require modification of treatment during surgery. A quick way to quantify bleeding is to estimate it visually, which is widely used because of its speed, but is very inaccurate. The gold standard for bleeding is blood count, but in an emergency the result may take too long, so one of the alternatives is estimated bleeding by gasometry, since it provides the patient's acid-base status, in addition to a hematocrit and hemoglobin, with which the bleeding calculation can be performed quickly. Objective: To evaluate the correlation index, through Pearson's correlation as well as the Bland Altman diagram, between estimated bleeding by arterial blood gas analysis and estimated bleeding by blood count in patients undergoing surgical procedures in surgeries performed at the General Hospital of Zone 33. Methodology: Analytical prospective observational study, simple random sampling of patients 18-70 years of age, both sexes, undergoing surgical procedures in neurosurgery, general surgery, otorhinolaryngology and urology, with an expected transoperative bleeding greater than 100 ml, ASA I-IV, BMI 18.5- 39.9, with a blood count taken no more than 3 months prior to the surgical procedure, in which the transoperative bleeding has not altered the condition the patient's hemodynamics and had the need for transfusion of blood products, with consent and signed assent, to whom at the end of the surgical procedure, an arterial blood gas and a blood count were taken, and calculations of the estimated bleeding were made taking the hematocrit from the previous blood count and as final hematocrit that from the arterial blood gas and blood count respectively.