Formula predicts blood use during cardiac surgery

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US researchers have developed a simple formula that can predict how much blood is likely to be required during bypass surgery, according to study findings presented last week at the annual meeting of the Society of Cardiovascular Anesthesiologists in San Diego.

The formula, which takes into account various risk factors for transfusion, could help reduce the amount of unused blood that goes to the OR and is unavailable for immediate use by patients in other parts of the hospital. At the same time, the use of the formula could result in cost savings compared with simply typing and crossmatching all patients for the same amount of blood.

"For many years, it has been standard practice to type and cross bypass surgery patients for four units of blood," lead author Dr. George Lappas, from Duke University Medical Center in Durham, North Carolina, told Reuters Health. "There really is no good evidence to support this amount and, in fact, up to 40% of patients never require transfusion."

This led Dr. Lappas and his team to devise a method of estimating blood needs so that patients would only be typed and crossmatched for the amount required.

The researchers analyzed data from 5402 consecutive patients who underwent surgery at their center between 1993 and 2002. Data from two-thirds of the subjects were used to identify risk factors for transfusion and then create the formula itself, while data from one third was used to validate the predictive ability of the formula.

Age older than 75 years, body weight less than 121 lbs., and creatinine level above 1.4 mg/dL were identified as risk factors for transfusion. For each one of these factors that are present, the patient should be typed and crossmatched for 1.5 units of blood.

Age older than 65 years, female gender, and hematocrit less than 36% all indicated the need for 1 unit, the report indicates. A half unit of blood was reserved for each of the following: a creatinine level greater than 1.2 mg/dL, body weight less than 187 lbs, and moderately impaired heart function.

Thus, using the formula, the amount of blood for type and crossmatch could range from 0 to 7 units depending on the patient's risk factors.

Dr. Lappas said the formula is highly accurate in gauging blood needs, but emphasized that it only applies to bypass surgery, not to other types of cardiac surgery. The use of the formula did not impair patient outcomes and saved about $220 for each patient who avoiding being typed and crossmatched, he added.