You have just been told you need heart surgery. Somewhere in the conversation — maybe during a hurried clinic visit, maybe while you were still processing the word "surgery" — your cardiologist or surgeon mentioned a number. Your "STS score." Maybe they said it was 1.2%, or 3.8%, or 7%. Maybe they moved on quickly. Maybe you nodded but had no idea what that number meant or whether you should be worried.
You are not alone. Most patients I meet have heard of their STS score but do not truly understand what it represents, how it was calculated, or — critically — what it cannot tell them. This article is my attempt to close that gap. Because when you are making one of the most important medical decisions of your life, you deserve to understand the tools being used to guide it.
What Is the STS Risk Calculator and How Does It Work?
The STS risk calculator is a statistical tool developed by the Society of Thoracic Surgeons. It uses data from hundreds of thousands of real cardiac surgeries performed across the United States to estimate the likelihood of certain outcomes — most commonly, the risk of death (mortality) and the risk of major complications after a specific heart operation.
The calculator takes into account a range of patient-specific variables:
- Age and sex
- Body mass index (BMI)
- Diabetes status and severity
- Kidney function (creatinine level)
- Lung disease (COPD)
- Previous cardiac surgery
- Ejection fraction (how well your heart pumps)
- Urgency of the operation (elective vs. emergent)
- The specific procedure being performed (coronary bypass, valve replacement, combined operations, etc.)
The algorithm processes these inputs and returns a predicted risk — typically expressed as a percentage. For example, an STS predicted risk of mortality (PROM) of 2% means that, among a large group of patients with your same profile, roughly 2 out of 100 would be expected to die during or shortly after the operation.
You can explore how these variables interact using our free cardiac surgery risk calculator, which is designed to give patients and families an accessible starting point for understanding their surgical risk profile.
Understanding STS Score Meaning: What the Numbers Tell You
So what does your specific number mean? Let me put it in context.
For isolated coronary artery bypass grafting (CABG), the national average STS predicted mortality is roughly 1-2%. For isolated aortic valve replacement, it is in a similar range. Combined procedures — say, bypass surgery plus a valve replacement — carry higher predicted risk, often in the 3-5% range or above depending on patient factors.
The STS database groups risk into broad categories that most cardiac surgery programs use internally:
- Low risk: STS PROM less than 3-4%
- Intermediate risk: STS PROM approximately 4-8%
- High risk: STS PROM greater than 8%
- Extreme or prohibitive risk: STS PROM greater than 15%, or the presence of conditions that make surgery technically not feasible
These categories matter enormously. According to ACC/AHA guidelines, your risk category directly influences which treatment options are recommended. For example, in aortic stenosis, a patient at low surgical risk may be offered either surgical aortic valve replacement (SAVR) or transcatheter aortic valve replacement (TAVR), while a patient at prohibitive surgical risk would typically be directed toward TAVR or medical management alone. The STS score is one of the primary tools used to make that determination.
In other words, your STS score meaning is not just a number on a page. It is a gatekeeper — it shapes which doors are opened to you and which are closed.
Where the STS Score Falls Short: The Limitations Patients Need to Know
Here is where I need to be direct with you, because this is the part most patients never hear.
The STS risk calculator is an excellent population-level tool. It is one of the most rigorously validated risk models in all of surgery. But it has real limitations that can meaningfully affect your care — and you should be aware of them.
1. It predicts averages, not your individual outcome
An STS score of 4% does not mean you, personally, have a 4% chance of dying. It means that among a large cohort of patients with similar characteristics, approximately 4% experienced that outcome. You might be healthier than most people in that group in ways the model does not capture. Or you might be sicker. The score cannot distinguish between the two.
2. It does not account for surgeon or hospital quality
This is a significant blind spot. A patient with an STS PROM of 5% will have very different real-world outcomes depending on whether the operation is performed at a high-volume center with experienced surgeons or at a program that does the procedure infrequently. Studies consistently show that hospital volume and surgeon experience independently affect mortality, sometimes dramatically. The STS calculator does not factor in where or by whom your surgery will be performed.
3. Frailty is poorly captured
The STS model includes traditional medical comorbidities, but it does a poor job of measuring frailty — the overall physiologic reserve and resilience of a patient. Two 78-year-old patients can have identical STS scores, yet one walks two miles a day and lives independently while the other has difficulty standing from a chair. Their real surgical risks are not the same. Frailty indices like gait speed, grip strength, and the ability to perform daily activities are increasingly recognized as critical risk factors, but they are not part of the STS algorithm.
4. It was not designed for every procedure
The STS calculator has well-validated models for CABG, aortic valve replacement, mitral valve surgery, and certain combined procedures. But it does not cover every cardiac operation. Procedures like tricuspid valve surgery, thoracic aortic surgery, and certain reoperations may not have dedicated STS models or may use models that are less robust. If you are being quoted an STS score for one of these operations, ask your surgeon how reliable that estimate truly is.
5. It does not capture quality of life outcomes
The STS score predicts mortality and a set of defined complications (stroke, renal failure, prolonged ventilation, deep sternal wound infection, reoperation). What it does not predict is how you will feel six months after surgery. Will you be back to your normal activities? Will you have persistent pain or fatigue? Will your cognitive function be the same? These are outcomes that matter deeply to patients and families, and the STS model is silent on them.
How a Thorough Cardiac Surgery Risk Assessment Goes Beyond the Score
A good cardiac surgeon does not hand you an STS score and call it a day. The number is a starting point — one data input in a much more comprehensive cardiac surgery risk assessment that should include:
- A detailed review of your imaging: echocardiograms, cardiac catheterization, CT scans. The anatomy of your disease — how many vessels are blocked, how calcified your aorta is, whether your mitral valve is repairable — affects risk in ways the STS calculator does not fully model.
- A frailty assessment: Even an informal one. Can you walk a block without stopping? Do you live independently? Have you lost weight unintentionally in the past year?
- Consideration of alternative approaches: Is there a less invasive option — percutaneous intervention, a transcatheter valve procedure, a minimally invasive surgical approach — that might carry lower risk for your specific situation?
- An honest conversation about goals: Are you hoping for another 15 years of active life, or are you primarily trying to relieve symptoms? The right decision depends on what matters to you, and no algorithm can answer that question.
This is precisely the kind of comprehensive evaluation we provide at WhiteGloveMD. When you submit your records through our process, we do not just regenerate your STS score. We review your full clinical picture — imaging, lab work, operative notes if applicable, and your personal health goals — and give you an independent, expert assessment of your options and risks.
Practical Advice: What to Do With Your STS Score
If you have been given an STS score — or if you are about to be — here is what I recommend:
1. Ask for the exact number and what it includes. "What is my STS predicted risk of mortality? What about predicted morbidity or mortality?" Write it down. You have every right to this information.
2. Ask how the score influenced the recommendation. If you are being told you are "too high risk" for a surgical option, ask specifically: "What is the STS score that led to that conclusion? Are there factors not captured by the score that I should know about?"
3. Understand that the score is a tool, not a verdict. I have operated on patients with STS scores of 10% or higher who did beautifully, and I have seen patients with scores under 2% develop unexpected complications. The score informs the conversation. It does not end it.
4. Consider whether the score was calculated correctly. The STS calculator requires accurate input data. If your creatinine level was entered incorrectly, or your ejection fraction was estimated rather than precisely measured, or a comorbidity was overlooked, the resulting score may not reflect your true risk. Errors in data entry are more common than most patients realize.
5. Get a second set of eyes. If a surgical recommendation is being driven heavily by your risk score — especially if you are being told you are either "too high risk" for an operation you want or "low enough risk" for an approach you are not sure about — an independent review can be invaluable. A cardiac surgery second opinion can verify the score, contextualize it within your full clinical picture, and sometimes identify options that were not initially presented.
The Bottom Line
The STS risk calculator is one of the most important tools in modern cardiac surgery. It gives patients and surgeons a shared, evidence-based framework for talking about risk. Understanding your STS score meaning is a genuinely empowering step in taking ownership of your surgical decision.
But it is one tool. It has gaps. It was designed to serve populations, not to capture everything that makes you a unique patient. The best decisions happen when the score is combined with expert clinical judgment, thorough imaging review, honest goal-setting, and — when there is any doubt — an independent second perspective.
If you are facing a cardiac surgery recommendation and want to understand what your risk score truly means for you — not just for the average patient — a WhiteGloveMD second opinion can help. Our team provides a thorough, surgeon-led review of your complete medical records, including an independent risk assessment and a clear explanation of your surgical and non-surgical options. Start your review today and make your next decision with confidence.