You have just been told you need heart surgery. Somewhere in the conversation — maybe in passing, maybe on a printed sheet — someone mentioned your "STS score." Perhaps it was 1.2%. Perhaps it was 8%. You may have nodded, but you probably left the office wondering: What does that number actually mean for me? And should I be worried?
These are exactly the right questions. The STS risk score is one of the most important tools we use in cardiac surgery to assess risk, plan operations, and have honest conversations with patients. But it is also one of the most commonly misunderstood. In my years of practice as a cardiovascular and thoracic surgeon, I have seen patients terrified by a number that was actually reassuring — and, occasionally, patients who were not worried enough.
This article will explain what the STS risk calculator is, what your score means, where it falls short, and how you can use this information to make a more confident decision about your care.
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 (STS), the largest professional organization of cardiothoracic surgeons in the world. It uses data from millions of cardiac surgery cases performed across North America to estimate the likelihood of specific outcomes after heart surgery.
When your surgical team enters your information into the STS risk calculator, they input variables such as:
- Your age, sex, and body mass index
- The specific operation being considered (e.g., coronary artery bypass grafting, aortic valve replacement, mitral valve repair)
- Your existing medical conditions — diabetes, kidney disease, lung disease, prior stroke, peripheral vascular disease
- Whether the surgery is elective, urgent, or emergent
- Prior cardiac surgeries (reoperations carry higher risk)
- Lab values such as creatinine level and ejection fraction (a measure of heart function)
The calculator then generates several predicted risk percentages, the most commonly discussed being the predicted risk of operative mortality — the statistical likelihood of dying during or within 30 days of the procedure. It also estimates the risk of complications such as stroke, prolonged ventilation, kidney failure, deep sternal wound infection, and reoperation for bleeding.
The STS database contains outcomes from over 7.5 million cardiac surgery records and is updated regularly. It is, by a wide margin, the most rigorously validated cardiac surgery risk assessment tool in use today.
STS Score Meaning: What Your Number Actually Tells You
Let me be direct: your STS score is a population-level statistical estimate. It tells you what has happened, on average, to patients who share your combination of risk factors. It does not tell you what will happen to you.
Here is a general framework for interpreting the STS predicted risk of mortality for common cardiac operations:
- Less than 1%: Low risk. The vast majority of patients in this category do very well.
- 1% to 3%: Low to moderate risk. This is where a large proportion of cardiac surgery patients fall. Outcomes are generally excellent.
- 3% to 8%: Moderate to elevated risk. Surgery is still very reasonable, but the team will take extra precautions, and the informed consent conversation becomes more detailed.
- 8% to 15%: High risk. Surgery may still be the best option, but alternative approaches (such as transcatheter interventions) should be carefully considered if available.
- Greater than 15%: Very high or extreme risk. At this level, any intervention requires serious deliberation, and in some cases, medical management or comfort-focused care may be more appropriate.
To put this in perspective: the national average STS predicted mortality for isolated coronary artery bypass grafting (CABG) is approximately 2.2%. For isolated aortic valve replacement, it is roughly 2.5% to 3%. These are the benchmarks against which your individual score should be measured.
If your surgeon tells you that your STS predicted risk of mortality is 1.5% for a CABG, that means you are statistically below the national average risk — a favorable position. If it is 6%, you are above average, and that warrants a more careful conversation about what is driving the risk and whether there are ways to mitigate it.
You can estimate your own risk using our free cardiac surgery risk calculator, which provides a starting point for understanding where you stand.
What the STS Risk Calculator Does Not Tell You
This is where I want to be especially candid, because the limitations of the STS score matter just as much as the score itself.
It Does Not Account for Surgeon or Hospital Quality
Your STS score is the same whether your operation is performed by a high-volume surgeon at a major academic center or by a surgeon who performs the procedure a handful of times per year. But outcomes are not the same. Studies consistently show that higher surgical volume is associated with lower mortality for complex cardiac operations. According to data published in The Annals of Thoracic Surgery and JAMA Surgery, the difference in outcomes between low-volume and high-volume centers can be substantial — in some cases, doubling or tripling the actual risk compared to what the STS calculator predicts.
This means a patient with an STS predicted mortality of 3% could face a real-world risk that is meaningfully higher or lower depending on where and by whom the operation is performed.
It Does Not Capture Frailty Well
The STS calculator uses variables like age and comorbidities, but it does not directly measure frailty — a patient's overall physiological reserve, nutritional status, grip strength, walking speed, or cognitive function. Two 78-year-old patients with identical STS scores can have dramatically different outcomes if one is active and independent while the other is deconditioned and dependent. The ACC/AHA guidelines now recommend formal frailty assessment as part of any cardiac surgery risk assessment, particularly in older patients.
It Does Not Predict Your Quality of Life After Surgery
The STS score focuses on mortality and major complications within the perioperative period. It does not tell you how you will feel six months after surgery, whether you will return to your usual activities, or how long your recovery will take. For many patients, these quality-of-life questions matter just as much as — or more than — the mortality number.
It Is Based on Averages, Not Your Specific Anatomy
The calculator does not know the details of your imaging — the size and location of your aneurysm, the extent of your coronary artery disease, or the specific anatomy of your mitral valve. These details can significantly influence the actual difficulty and risk of your operation in ways the score cannot capture.
How to Use Your Cardiac Surgery Risk Assessment in Decision-Making
Knowing your STS score is useful. Knowing how to use it is essential. Here is my practical advice for patients and families:
1. Ask your surgeon to explain your score in context. Do not just accept a number. Ask: "How does my score compare to the average for this operation? What is driving my risk higher or lower? What is your personal or institutional outcome for patients like me?" A confident, experienced surgeon will welcome these questions.
2. Understand that risk is not a single number. Your STS mortality risk might be 2%, but your risk of stroke might be 1.5%, your risk of prolonged ventilation 8%, and your risk of new kidney problems 3%. Ask about all of the predicted outcomes, not just mortality. Surviving surgery but experiencing a disabling stroke is an outcome every patient wants to avoid.
3. Consider the risk of doing nothing. Every conversation about surgical risk should also include the risk of not having the operation. Severe aortic stenosis, for example, carries a mortality rate exceeding 50% within two years once symptoms develop if the valve is not replaced. A 4% surgical mortality risk looks very different when the alternative is a near-certain decline without intervention.
4. Get a second opinion if you have questions. If your STS score is moderate or high, if you have been told you are "too high risk" for surgery, or if you are uncertain whether the recommended approach is the right one, an independent review of your case by another cardiac surgeon can provide clarity. A cardiac surgery second opinion is not a sign of distrust — it is sound medical practice, and it is recommended by the American Heart Association for complex cardiac decisions.
5. Bring your family into the conversation. Risk assessment is not just a medical exercise — it involves values, priorities, and goals of care. The people who know you best can help you weigh the numbers against what matters most to you.
When a Second Look at Your Risk Makes a Real Difference
In my experience, the patients who benefit most from an independent cardiac surgery risk assessment are those in the gray zones — where the decision is not straightforward. These include:
- Patients told they are "borderline" or "high risk" who want to know if a safer approach exists
- Patients with multiple conditions who have been offered surgery but are unsure if the benefit outweighs the risk
- Patients who received conflicting recommendations from different physicians
- Patients considering aortic valve intervention and weighing TAVR versus surgical replacement
- Patients with complex coronary artery disease deciding between bypass surgery and stenting
- Older patients or those with frailty concerns who need a realistic assessment of expected recovery and quality of life
A thorough second opinion review includes not just recalculating your STS score, but examining your imaging, reviewing your complete medical history, assessing factors the calculator misses, and providing a clear, written recommendation you can take back to your care team.
You can see exactly how our review process works on our how it works page.
The Bottom Line on STS Scores and Cardiac Surgery Risk
Your STS score is a valuable starting point — not the final word. It is a statistical tool built from real outcomes data, and when interpreted correctly and in context, it helps guide some of the most important decisions you will ever make about your health. But it is only one piece of the puzzle. The skill of your surgeon, the quality of your hospital, your individual anatomy, your overall fitness, and your personal goals all matter enormously.
Do not let a single number make your decision for you. Understand it. Question it. And make sure the people advising you have looked at the full picture.
If you are facing a cardiac surgery decision and want to understand what your risk score really means for your situation, a WhiteGloveMD second opinion can help. Our reviews are conducted by a board-certified cardiovascular surgeon who will analyze your complete case — including your STS risk profile, your imaging, and the clinical details that no calculator can capture — and provide a clear, independent recommendation so you can move forward with confidence.