The STS CABG composite score is calculated using a combination of 11 quality measures divided into four categories or domains.
1. Absence of Operative Mortality
The first domain is measured by the percentage of patients who do not die during the hospitalization in which the surgery was performed and those who do not die within the 30 days of the surgery if they were discharged from the hospital. STS adjusts the results for each surgeon and hospital by accounting for the severity of their patient's illnesses, a process known as risk adjustment. To level the playing field, statistical techniques have been developed to account for the condition of patients before surgery.
2. Absence of Major Morbidity
A second domain is the risk-adjusted absence of major morbidity, the percentage of patients who leave the hospital with none of the five most serious complications (often referred to as morbidities)—reoperation, stroke, kidney failure, infection of the chest wound, or prolonged need to be supported by a breathing machine (ventilator).
3. Use of Internal Mammary Artery
The third domain measures the percentage of CABG procedures that include the use of at least one of the arteries from the underside of the chest wall—the internal mammary (or internal thoracic) artery—for bypass grafting. This artery has been shown to function much longer than vein grafts, which can become blocked over time.
4. Receipt of Required Perioperative Medications
The fourth domain measures how often all of the four medications believed to improve a patient's immediate and long-term outcomes were prescribed. Those drugs include beta-blocking drugs prescribed peroperatively, as well as aspirin (or similar drugs to prevent graft clotting), and additional beta blockers and cholesterol-lowering medicines prescribed postoperatively.
Each database participant is scored in all of the four categories. The scores are combined using a statistical formula and the result is the overall composite score. In addition to a composite score, each participant also is given a star rating of one to three stars. Those participants who perform worse than expected receive a star rating of one, those who perform as expected receive a two star rating, and participants performing better than expected receive a three star rating.