Why doctors predict life expectancy—and why they get it wrong

This probably goes without saying, but: Doctors don’t know when you’re going to die.

I’ve had patients with a prognosis of six months to live who continue to visit me 10 years later. And I’ve had patients die unexpectedly when I believed they had plenty of time remaining. Call it a miracle or bad luck—there’s always an unpredictable aspect to mortality.

So why do doctors continue to offer such prognoses? Where do we get our numbers? And can our predictions influence whether our patients live longer or die sooner?

Let’s start with that first question.

Why do doctors predict life expectancy?

Many doctors are reluctant to attach numbers to a prognosis because of the uncertainty surrounding such a prediction. That said, these are essential conversations for doctors to have with their patients.

I’ve found that most patients actually want to know how much time they might have left. This gives them some idea of how quickly they need to get their affairs, such as wills and advance directives, in order.

They may also be looking down the road at the milestones they don’t want to miss, such as seeing their youngest graduate from college or walking a daughter down the aisle. These patients often ask me to tell it to them straight—and to do everything I can to help them reach that next milestone.

The stakes are high here, which is why it’s key that doctors are using the most up-to-date information to determine a prognosis. This brings us to the next question.

How do doctors determine how long you have to live?

What we know about a prognosis for a patient with any medical disease or disorder is largely based on those who came before them. What you’re really looking at is the risk of the population—that is, how long anyone else with the same disease survived.

So we’d say patients with Class 3 or 4 heart failure, for example, may have a 20% risk of dying in the next year because that’s the risk for the entire population: We’d expect 20 out of 100 to die while the remaining 80 survived the year.

There are some emerging tools that are helping to individualize the risk of death by factoring in a variety of personal characteristics, such as a patient’s age, gender and condition. These formulas can generate risk prediction scores that better show a patient’s risk of dying in a given number of years or being hospitalized for their condition.

Regardless of what prediction tools are available, it’s crucial that doctors stay current on the prognoses for certain diseases, because they change frequently over time. In my field, for example, 20 years ago we’d predict that about half of all patients with heart failure would be dead in five years no matter how severe their disease. With advances in medications, devices and treatment, that prediction no longer holds—but some doctors still cite the old data to their patients.

So if we get it wrong, does that matter? Well, it could. We’ll talk about that in the next question.

What should you do after a doctor tells you how long you might have to live?

If a doctor told me with complete certainty that I had six months to live, I’d say, “Thank you for your honesty.” And then I’d get a second opinion. In fact, I always encourage such patients to seek second opinions.

There’s a risk that patients who hear such grim prognoses might experience overwhelming anxiety or depression, possibly to the point of giving up. That’s why it’s important that doctors not only have the most accurate information, but that they deliver the news with the appropriate context and measure of hope.

I often couch these discussion in, “Let’s talk about this, but please understand that you’re an individual and your odds of survival might be very different compared to what happens to another patient or patients on average.”

It’s also perfectly OK to ask. Your doctor may be reluctant to share a prognosis, but this is your care—you should feel encouraged to be an active participant in what will happen to you.

Overall, patients should know that no matter the prognosis, their doctors aren’t giving up on them. I always give patients hope that they can beat the odds, and I’m thrilled when they do.

William Abraham is a cardiologist at The Ohio State University Wexner Medical Center and a professor in The Ohio State University College of Medicine.