Why Your Risk Score Is Not the Whole Story
If you or someone you love has been told they need heart surgery, chances are a number has come up in conversation — a percentage that is supposed to represent the risk of the operation. Maybe your surgeon said your risk of mortality is 1.2%, or 4%, or 8%. Maybe you saw it printed on a report or heard it referenced in passing.
That number almost certainly came from a cardiac surgery risk assessment tool, most likely the STS risk calculator developed by the Society of Thoracic Surgeons. It is one of the most validated and widely used scoring systems in all of medicine. I use it for every patient I evaluate. It matters.
But here is what I want you to understand as a patient: that number is a starting point for a conversation, not the final word on your outcome. And how that number gets interpreted — by whom, in what context, with what nuance — can genuinely change the course of your care.
How the STS Risk Calculator Works: A Plain-Language Explanation
The STS risk calculator is a statistical model built on data from millions of cardiac surgery cases performed across the United States. When your surgical team enters your clinical information — your age, kidney function, lung disease status, diabetes, prior surgeries, ejection fraction, the specific operation planned, and dozens of other variables — the calculator generates predicted risks for several outcomes:
- Operative mortality (risk of dying during or shortly after surgery)
- Stroke
- Renal failure
- Prolonged ventilation (needing a breathing machine longer than expected)
- Deep sternal wound infection
- Reoperation (needing to go back to the operating room)
- A composite morbidity/mortality score
The model was developed and is continuously updated using the STS Adult Cardiac Surgery Database, which captures outcomes from more than 90% of cardiac surgery programs in the country. That is a remarkable dataset, and it gives the calculator real predictive power at the population level.
You can explore how these risk factors apply to your own situation using our free cardiac surgery risk calculator, which walks you through the key inputs in a patient-friendly format.
What the STS Score Meaning Really Is — And Where It Falls Short
Here is what I tell my patients: the STS score meaning is essentially this — among a large group of patients who look like you on paper, this is the average rate of complications we have observed historically.
That is genuinely useful information. But it is not a prophecy. And there are specific ways it can mislead if taken at face value.
1. The Score Doesn't Know Your Surgeon or Your Hospital
This is perhaps the most important limitation. The STS risk calculator produces a prediction based on national average outcomes. It does not adjust for the experience, volume, or skill of the specific surgeon who will operate on you. It does not account for the quality of the ICU team, the anesthesiology group, or the perfusionists running the heart-lung machine.
We know from extensive research — including data published by the STS itself — that outcomes vary significantly between institutions and between individual surgeons. A patient with an STS-predicted mortality of 5% might face a real-world risk closer to 2% at a high-volume center with an experienced team, or closer to 8% at a program that rarely performs that particular operation.
This is one of the reasons I believe so strongly in the value of a cardiac surgery second opinion. The score gives you a baseline. An experienced surgeon reviewing your case gives you context.
2. Frailty Is Underrepresented
The STS calculator captures many clinical variables, but it does not fully account for frailty — that complex combination of muscle weakness, low energy reserves, slow walking speed, and diminished physiologic resilience that affects so many older patients. Studies published in the Annals of Thoracic Surgery and the Journal of the American College of Cardiology have shown that frailty independently predicts worse outcomes after cardiac surgery, even after adjusting for STS score.
In other words, two patients can have the same STS-predicted risk, but if one is frail and the other is robust, their actual outcomes may be very different. This is especially relevant for patients in their late 70s, 80s, and beyond who are considering procedures like aortic valve replacement or coronary artery bypass grafting.
3. The Score Cannot Capture Every Relevant Anatomy
A heavily calcified or "porcelain" aorta, prior chest radiation, unusual coronary anatomy, a very small or very large body habitus — these factors influence surgical complexity and risk, but they are not always fully captured by the standard risk model inputs. An experienced surgeon reviewing your imaging studies will identify these features. The calculator cannot.
4. It Predicts Short-Term Outcomes, Not Long-Term Benefit
The STS calculator focuses primarily on 30-day (operative) mortality and short-term morbidity. It does not tell you how much longer or better you might live because you had the surgery. A patient with a 6% operative mortality risk might still be an excellent candidate for surgery if the operation is expected to add years of functional life. Conversely, a patient with a 1% risk might not benefit much if the underlying condition is mild and the natural history is favorable without intervention.
This is where clinical judgment — the kind that comes from years of training and thousands of patient encounters — becomes irreplaceable. Risk is only half the equation. Benefit is the other half, and no calculator quantifies that for you.
How Cardiac Surgery Risk Assessment Should Inform Your Decision
So if the risk score isn't the final answer, how should you use it? Here is my practical advice for patients and families navigating this process.
Ask for the Number — Then Ask What It Means for You Specifically
You have every right to know your predicted STS score. Ask your surgeon to share it with you. But then ask the follow-up questions that matter:
- "How does your program's actual outcome rate compare to this predicted risk?"
- "Are there factors in my case that might make my real risk higher or lower than this number?"
- "What is the risk of not having surgery?"
- "What outcome can I realistically expect if everything goes well?"
These questions move the conversation from a sterile number to a real clinical assessment. According to ACC/AHA guidelines, treatment decisions for conditions like valvular heart disease and coronary artery disease should be made by a multidisciplinary heart team that weighs both procedural risk and expected benefit in the context of the individual patient. The score is one input among many.
Understand Where You Fall on the Risk Spectrum
As a general framework, here is how STS-predicted operative mortality is typically interpreted in clinical practice:
- Low risk: less than 3% predicted mortality. Most patients undergoing isolated coronary bypass or single-valve surgery fall into this category.
- Intermediate risk: 3% to 8%. These patients often have multiple comorbidities or are undergoing more complex procedures. Careful patient selection and optimization matter significantly here.
- High risk: greater than 8%. At this level, surgeons and heart teams weigh surgical options against less invasive alternatives (such as transcatheter procedures) or, in some cases, medical management alone.
These thresholds are not rigid cutoffs — they are guideposts. And they are heavily referenced in the landmark trials (such as PARTNER and COAPT) that have shaped modern guidelines for procedures like aortic valve replacement and mitral valve intervention.
Get a Second Set of Eyes on Your Case
If your risk score is anything other than very low — or if the recommended surgery is complex, the diagnosis uncertain, or the treatment options debatable — a second opinion is not a sign of distrust. It is a standard of good care.
I review cases every week where the risk score told only part of the story. Sometimes a patient was told they were "too high risk" for surgery when, in fact, a more experienced team could offer a reasonable operative approach. Other times, a patient was being steered toward an operation when a less invasive strategy or even watchful waiting would have been more appropriate.
At WhiteGloveMD, we use AI-assisted analysis combined with direct surgeon review to evaluate your specific case — your imaging, your lab work, your clinical history — not just your score. You can learn more about how our process works.
The Human Side of Risk Numbers
I want to end with something that no algorithm can measure.
When a patient sits across from me and asks, "What would you do if this were your father?" — I do not answer with a percentage. I answer by integrating everything I know: the data, the imaging, the patient's goals, their family situation, their vitality, their fear, their resilience. That synthesis is what good surgical judgment is.
Risk scores are essential tools. I rely on them daily. But they are instruments — not oracles. They inform decisions. They do not make them. And you, as the patient, deserve a physician who understands the difference.
If you have received a cardiac surgery risk assessment and are trying to make sense of what it means for your specific situation — or if you have been told your risk is too high (or too low) and something feels off — a WhiteGloveMD second opinion can help you understand your options with clarity and confidence. Our team provides detailed, surgeon-led case reviews that go far beyond a single number. Start your review today.