Level 1: Likely reliable evidence
The decision as to when to give blood transfusions is complex, requiring a careful assessment of risks and benefits. The TRACS trial compared the effects of giving blood transfusions perioperatively using a target of maintaining hematocrit at ≥24% vs. ≥30% in 502 patients having an elective coronary artery bypass graft(CABG) (JAMA. 2010;304:1559-1567). Patients were evaluated for a composite outcome that included 30-day all-cause mortality and severe morbidity (cardiogenic shock, acute respiratory distress syndrome or acute renal injury requiring dialysis or hemofiltration) during hospital stay.
The trial was designed to determine whether limiting blood transfusions with a goal of maintaining the hematocrit ≥24% was not inferior to the strategy of using transfusions to keep the hematocrit ≥30%. The researchers found that using a target hematocrit of ≥24% reduced the rate of transfusion (47% vs. 78%, P <0.001, NNT 4). There were no statistically significant differences in the composite outcome (11% vs. 10%, which met the prespecified noninferiority margin) (Level 2: Mid-level evidence), although there were nonsignificant absolute increases in mortality of 1% and in cardiogenic shock of 4% in the lower-target group. There were no significant differences in complications, severe bleeding or hospital stay.