Why Your Cardiac Surgery Risk Score Matters More Than You Think
If you or a family member has been told you need heart surgery, there is a good chance someone has mentioned a "risk score." Maybe a surgeon quoted a number — "Your predicted mortality is 2.3 percent" — and moved on. Maybe no one mentioned a score at all. Either way, understanding how cardiac surgery risk assessment works is one of the most important things you can do before consenting to an operation.
As a board-certified cardiovascular and thoracic surgeon, I use risk scores on every single patient I evaluate. They are part of the foundation of surgical decision-making. But I have also seen them cause confusion, fear, and — in some cases — lead to decisions that were not in a patient's best interest. The problem is rarely the score itself. It is how the score is communicated and interpreted.
This article will walk you through what cardiac surgery risk scores are, how the most commonly used ones work, what your score actually means for you, and the critical limitations every patient should understand.
How the STS Risk Calculator Works in Practice
The most widely used tool in North America is the STS risk calculator, developed and maintained by the Society of Thoracic Surgeons. It is a statistical model built from data on millions of cardiac surgery cases performed across hundreds of hospitals.
Here is the basic concept: your surgeon or surgical team enters a series of clinical variables — your age, sex, ejection fraction, kidney function, diabetes status, whether you have had a prior cardiac surgery, the specific procedure being planned, and dozens of other data points. The calculator then generates a predicted risk of several outcomes:
- Operative mortality — the probability of dying during or within 30 days of surgery
- Major morbidity — the probability of a serious complication such as stroke, prolonged ventilation, deep sternal wound infection, renal failure, or reoperation
- A composite endpoint — mortality or major morbidity combined
The output is a percentage. For example, a predicted mortality of 1.5% means that among a large group of patients with similar clinical profiles, roughly 15 out of every 1,000 would be expected to die within 30 days of surgery. The current STS models are updated periodically and are generally well-calibrated across broad populations, meaning the predictions align reasonably well with observed outcomes at the national level.
If you want to get a preliminary sense of your own risk, you can use our free cardiac surgery risk calculator, which can help you frame the conversation with your surgical team.
What Your STS Score Meaning Really Is — And Common Misunderstandings
This is where things get nuanced, and where I spend a significant amount of time in my consultations.
A risk score is a population estimate, not an individual prophecy
Your STS score tells you what happens, on average, to patients who look like you on paper. It does not tell you what will happen to you. This is a critical distinction. Two patients can have identical STS-predicted mortality rates and have vastly different actual risk profiles based on factors the model does not fully capture — things like frailty, nutritional status, the specific anatomy of their coronary arteries or valve disease, their social support structure, and the skill and volume of the surgical team performing the operation.
Not all risk factors are weighted equally
Patients often fixate on one variable — "I'm 80 years old, so my risk must be very high" — without understanding that age is only one of many inputs. A healthy, active 80-year-old with preserved kidney function and a good ejection fraction may have a lower predicted risk than a sedentary 65-year-old with diabetes, chronic kidney disease, and reduced heart function. The STS risk calculator accounts for interactions between these variables, which is why the final number can sometimes surprise people.
The score does not capture surgeon or hospital quality
This is arguably the most significant limitation. The STS model predicts risk based on patient characteristics. It does not adjust for where or by whom the surgery is performed. A patient with a predicted mortality of 3% may face genuinely different actual risk depending on whether the operation is performed at a high-volume center with experienced surgeons or at a low-volume program. According to extensive data published in journals like The Annals of Thoracic Surgery and The New England Journal of Medicine, surgical volume and institutional experience are among the strongest predictors of outcomes, particularly for complex procedures like mitral valve repair or reoperative surgery.
The score does not decide your treatment — the Heart Team does
ACC/AHA guidelines emphasize a multidisciplinary Heart Team approach for complex cardiac surgical decisions, particularly for valve disease and high-risk coronary artery disease. The STS score is one input into that discussion. It helps classify patients into low, intermediate, or high-risk categories, which in turn may influence whether a surgical, transcatheter, or medical approach is recommended. For example, in aortic stenosis, the STS predicted risk of mortality plays a direct role in determining whether a patient is better suited for surgical aortic valve replacement (SAVR) or transcatheter aortic valve replacement (TAVR). But the score alone should never be the sole basis for a recommendation.
When Cardiac Surgery Risk Assessment Goes Wrong
In my practice reviewing cases for second opinions, I see recurring patterns where risk scores are either misapplied or incompletely communicated:
- Risk is quoted without context. Telling a patient "Your risk is 8 percent" without explaining what that means relative to the risk of not having surgery is incomplete. For many conditions — severe aortic stenosis, critical left main coronary disease, acute aortic pathology — the risk of declining surgery far exceeds the operative risk. A proper risk discussion must include both sides of the equation.
- Frailty is ignored. The standard STS model does not include formal frailty metrics, though frailty is one of the most powerful predictors of poor surgical outcomes in elderly patients. Studies from groups at Columbia, Duke, and other major centers have consistently demonstrated that frail patients have two to three times the mortality and complication rates compared to non-frail patients with similar STS scores. If your surgeon has not assessed your functional status, gait speed, or grip strength, the risk picture may be incomplete.
- The score is used to deny surgery rather than to plan it. I have reviewed cases where patients with elevated STS scores were told they were "too high risk" for surgery without a thorough evaluation of whether risk-modifying strategies — nutritional optimization, prehabilitation, staged procedures, or referral to a higher-volume center — could change the equation. A high score is a starting point for discussion, not necessarily an endpoint.
- Only one score is used. The STS calculator is excellent, but it is not the only tool available. The EuroSCORE II is widely used in Europe and offers a complementary risk perspective. In some cases, the two models produce meaningfully different risk estimates for the same patient, which can inform a more nuanced discussion.
If any of these patterns sound familiar — if you were quoted a risk number without full context, told you were too high-risk without further exploration, or if you simply want an independent interpretation of your risk profile — getting a second opinion is a reasonable and often valuable step.
How to Use Your Risk Score as a Patient
Here is practical advice I give to every patient and family I work with:
1. Ask for the actual numbers
Do not accept vague language like "high risk" or "low risk" without a specific number. Ask your surgeon: "What is my STS-predicted mortality? What is my predicted risk of major morbidity?" You are entitled to this information. It is calculated for every cardiac surgery patient at STS-participating centers.
2. Ask what the risk of NOT having surgery is
This is equally important. For severe symptomatic aortic stenosis, the one-year mortality without intervention can exceed 50%. For significant left main coronary artery disease, medical management alone carries meaningfully higher long-term mortality compared to revascularization. Risk is always relative.
3. Ask about the surgeon's and hospital's actual outcomes
STS-participating programs report their observed-to-expected mortality ratios — a measure of whether their actual outcomes are better or worse than what risk models would predict. A program with an O/E ratio below 1.0 is outperforming expectations. You can and should ask about this.
4. Ask whether frailty and functional status were assessed
If you are over 70, or if you have significant debility regardless of age, a formal frailty assessment should be part of your preoperative evaluation. If it was not performed, your risk assessment may be incomplete.
5. Get an independent review if something does not feel right
I say this not as a marketing statement but as a clinical one: the data consistently shows that second opinions in complex cardiac cases change the recommended treatment plan in a meaningful percentage of cases. A 2019 study in The American Journal of Medicine found that second opinions led to a change in diagnosis or treatment in up to 88% of cases across specialties. Cardiac surgery is no different. If you have questions about your risk score, your diagnosis, or the recommended approach, an independent review by a cardiac surgeon who has no stake in performing your operation can provide clarity.
A Score Is a Tool — Not the Final Word
The STS risk calculator and other risk models represent some of the best work in evidence-based medicine. They are invaluable for benchmarking, quality improvement, and framing surgical discussions. But they are tools, not oracles. They work best when combined with clinical judgment, honest communication, and a genuine commitment to shared decision-making between surgeon and patient.
Your risk score is a piece of the puzzle. Understanding it — truly understanding it — puts you in a stronger position to make the right decision for your life.
If you are facing a cardiac surgery recommendation and want an independent, expert interpretation of your risk profile and treatment options, a WhiteGloveMD second opinion can help. Our reviews are conducted by board-certified cardiac surgeons using the same risk models and clinical frameworks employed at leading academic medical centers — delivered directly to you, with clear explanations and actionable guidance. Learn how our process works and take the next step toward confidence in your care decisions.