Why Your Surgeon Talks About Risk Scores — And Why You Should Listen Carefully
If you or someone you love has been told they need heart surgery, chances are you have heard a number. Maybe it was 1.2%. Maybe it was 6%. Maybe it was a number that scared you.
That number almost certainly came from a cardiac surgery risk assessment tool — most commonly the STS risk calculator, developed and maintained by the Society of Thoracic Surgeons. It is the most widely used risk prediction model in cardiac surgery in the United States, and it plays a significant role in how surgical teams plan your care, counsel you about your options, and decide whether an operation is the right path forward.
But here is what rarely gets explained well in a 15-minute office visit: what that number actually means, how it was generated, and — just as importantly — what it cannot tell you.
As a board-certified cardiovascular and thoracic surgeon, I use risk scores every day. They are indispensable. But I also know their limitations, and I believe patients deserve to understand both. That understanding can be the difference between a well-informed decision and one driven by fear or false confidence.
What the STS Risk Calculator Measures — and How It Works
The STS risk calculator is a statistical model built from data on millions of cardiac surgery cases performed across the United States. When your surgeon enters your clinical information — age, sex, kidney function, ejection fraction, diabetes status, prior surgeries, and dozens of other variables — the calculator generates a predicted risk of mortality (death within 30 days of surgery) and a predicted risk of major complications such as stroke, prolonged ventilation, kidney failure, deep wound infection, and reoperation.
The STS score meaning is straightforward on the surface: it estimates the likelihood that a patient with your profile would experience these outcomes, based on how similar patients have done historically. For example, an STS predicted risk of mortality of 2% means that among 100 patients with a similar clinical profile, approximately 2 would be expected to die within 30 days of the procedure.
The model is procedure-specific. There are separate calculators for:
- Coronary artery bypass grafting (CABG)
- Aortic valve replacement
- Mitral valve repair or replacement
- Combined procedures (e.g., CABG plus valve surgery)
This matters because the risk profile for an isolated CABG is very different from a combined CABG and mitral valve replacement. The calculator accounts for that complexity.
If you want to explore how these calculations apply to your situation, our free cardiac surgery risk calculator can give you a starting point — though I always recommend reviewing the results with a qualified surgeon.
What Your STS Score Meaning Really Is — Risk Categories Explained
One of the most important ways the STS score is used clinically is to stratify patients into risk categories. This classification directly influences treatment recommendations, especially for conditions like aortic stenosis where both surgical (SAVR) and transcatheter (TAVR) options exist.
According to ACC/AHA guidelines, patients are generally categorized as follows:
- Low risk: STS predicted risk of mortality less than 3%
- Intermediate risk: STS predicted risk of mortality between 3% and 8%
- High risk: STS predicted risk of mortality greater than 8%
- Extreme or prohibitive risk: STS predicted risk of mortality greater than 15%, or other factors that make surgery inadvisable
These categories are not arbitrary. They are tied to landmark clinical trials — PARTNER, PARTNER 2, PARTNER 3, Evolut Low Risk — that compared surgical and transcatheter approaches in each risk group. Your risk category may determine which procedures are recommended, which are considered appropriate, and which are covered by your insurance.
But here is a critical nuance: the category boundaries are guidelines, not laws. A patient with an STS score of 2.9% and one with a score of 3.1% are clinically almost identical, yet they technically fall into different risk groups. Good surgical teams understand this and make individualized recommendations rather than treating the number as an absolute threshold.
When the Number Is Misleading
I want to be direct about something. The STS risk calculator is a population-level tool being applied to an individual. It tells you what happens on average to patients who look like you on paper. It does not know:
- Your frailty level. Two 78-year-old patients can have the same STS score, but if one walks three miles a day and the other cannot get out of a chair without assistance, their actual surgical risks are vastly different. Frailty is not fully captured in the model.
- Your surgeon's experience. Studies consistently show that surgeon and hospital volume correlate with outcomes. A high-volume surgeon at an experienced center may achieve mortality rates well below what the STS calculator predicts. The score does not adjust for where or by whom your surgery will be performed.
- Anatomic complexity. A severely calcified or porcelain aorta, hostile mediastinum from prior radiation, or unusual coronary anatomy can dramatically increase surgical difficulty. These factors are not inputs in the calculator.
- Your nutritional status, social support, mental health, and rehabilitation potential. These "soft" factors significantly affect recovery and long-term outcomes but are invisible to the algorithm.
This is precisely why a cardiac surgery risk assessment should never end with a number on a screen. It should begin there.
How Surgeons Actually Use Risk Scores in Practice
In my practice, the STS score serves several purposes:
1. As a communication tool. It gives me a framework to explain to patients and families what we are facing. When I say your predicted risk of mortality is 1.5%, that is a different conversation than when the number is 12%. Both patients deserve surgery if the benefit outweighs the risk, but the counseling, preparation, and decision-making process look different.
2. As a benchmarking tool. Hospitals and surgeons report their outcomes to the STS National Database. Their actual mortality rates are compared to their expected rates (based on the risk profiles of the patients they operate on). This is how STS star ratings are generated. A program that operates on sicker patients but achieves outcomes better than predicted earns high marks. This is a far more honest measure of quality than raw mortality numbers.
3. As one input into a Heart Team discussion. For conditions like aortic stenosis, guidelines recommend a multidisciplinary Heart Team — including cardiac surgeons, interventional cardiologists, imaging specialists, and anesthesiologists — to review each case. The STS score is one piece of evidence on the table, alongside imaging, functional status, patient preferences, and clinical judgment.
4. As a reality check. Sometimes, a risk score confirms what clinical intuition already suggests: that a patient is too frail for open surgery and would be better served by a less invasive approach, or by medical management alone. Other times, the score is reassuringly low, and it helps a nervous patient feel more confident about proceeding.
The Danger of Risk Score Tunnel Vision
I have seen cases where patients were told they were "too high risk" for surgery based solely on an STS score, without a thorough evaluation by an experienced surgical team. And I have seen the opposite — patients pushed toward surgery when a more careful assessment would have revealed that the score was underestimating their true risk because of factors the model does not capture.
Both scenarios are harmful. Both can be avoided with a thorough, individualized evaluation. This is one of the most common reasons patients seek a cardiac surgery second opinion — not because they distrust their doctor, but because they want someone to look beyond the number and explain what it means for them.
What You Should Do With Your Risk Score
If you have been given an STS score or another risk estimate, here is my practical advice:
- Ask your surgeon to explain what went into the calculation. Were all the variables entered correctly? Was your most recent creatinine used? Was your ejection fraction from an up-to-date echocardiogram? Small input errors can meaningfully change the output.
- Ask what the score does not capture. Does your surgeon think your actual risk is higher or lower than what the model predicts? Why? A good surgeon will have a thoughtful answer to this question.
- Understand risk in context. A 4% risk of mortality sounds frightening — but if the natural history of your untreated condition carries a 30-50% risk of death over the next two years, that 4% looks very different. Risk is only meaningful when compared to the alternative.
- Do not compare your score to someone else's. Your neighbor who had bypass surgery with a "low" score had a different body, different disease, and different circumstances. Your risk assessment is yours alone.
- Consider the full picture. Mortality risk is important, but so is the risk of stroke, kidney injury, prolonged recovery, and reduced quality of life. Ask about all of these, not just the headline number.
When a Second Look at Your Risk Score Can Change Everything
Research published in the Journal of the American College of Cardiology and other peer-reviewed sources has shown that second opinions in cardiac surgery change the recommended treatment plan in a meaningful percentage of cases — some studies suggest as many as 30-40% of the time. Sometimes the change is a different procedure. Sometimes it is a different timing. And sometimes it is a more accurate understanding of the risk itself.
At WhiteGloveMD, we built our review process around exactly this kind of deep, individualized analysis. We do not just re-run your STS score. We review your imaging, your clinical history, your functional status, and the nuances that no algorithm can capture. Then we provide a clear, evidence-based opinion about your options — written by a board-certified cardiac surgeon, enhanced by AI-powered analysis to ensure nothing is missed.
Your risk score is a starting point. It is an important one. But it is not the whole story, and you deserve to know the full picture before making one of the most consequential decisions of your life.
If you are facing a cardiac surgery recommendation and want to understand what your risk score truly means for your individual situation, a WhiteGloveMD second opinion can help. We provide surgeon-led, AI-enhanced reviews that go beyond the numbers — giving you the clarity and confidence you need to make the right decision for your heart and your life.