Doctors and health practitioners take blood before surgeries and operations to identify any possible conditions that could interfere with the procedure and to check on the general health of the patient. The blood test checks for a number of possible conditions.

White Blood Cells

Most pre-op blood tests check for white blood cells. The number of white blood cells can give clues to underlying conditions, such as cancer or infections. The types of white blood cells will help the doctor determine what infections for which the patient might be at risk.

Red Blood Cells

High or low numbers of red blood cells can also point to abnormal conditions and help doctors determine the amount of transfusion blood needed for the procedure. Low numbers for red blood cells could be a cause for concern.

Platelet Count

The platelet count helps doctors determine how well the body can clot blood and if there might be problems in the use of blood thinners. The number of platelets can also give the doctor an idea of how the patient's bone marrow produces new platelets.

Mean Corpuscular Volume

Mean corpuscular volume is the index for the size of a patient's red blood cells. Differences in size could mean vitamin deficiencies or a lack of iron, a condition known as anemia.

Results

While pre-op blood tests are primarily used as a safety precaution before invasive surgeries, the tests can sometimes reveal dangerous underlying conditions such as leukemia or auto-immune disorders. But pre-op blood tests are a common practice in most hospitals and health centers.

Source:

Complete blood count

Stanford University School of Medicine

Resource:

Complete blood count