A 79-year-old is significantly more likely than an 80-year-old to get a lifesaving heart bypass surgery—and the reason can't readily be explained away by differences in health, according to a study published last week in the New England Journal of Medicine.

How health care is like a car lot (no, really)

Previous research has found that small, otherwise insignificant numerical differences can have an outsized effect on consumer decision-making when there's a difference in the "left digit" of the values the consumer is considering. For instance, a large 2012 study found that consumers are willing to pay more for a used car when the odometer reads 9,999 than when the odometer reads 10,001, STAT News reports. This tendency to give more weight to the first digit we see is called "left-digit bias," and can often be seen in retail stores selling products for $4.99, instead of $5.00.

For the new study, the researchers wanted to see how left-digit bias might influence which elderly patients get heart bypass surgery.

To do that, researchers looked at Medicare records from 2006 to 2012 on 70,000 heart attack patients. Specifically, they looked at a group of patients who were no more than two weeks shy of their 80th birthday when they were admitted to the hospital for a heart attack and another group of patients who had turned 80 within the two weeks leading up to their heart attack.

What happens when you turn 80

The researchers found patients who'd already turned 80 were 24% less likely to receive a bypass surgery than the 79-year-old patients who were two weeks shy of their 80th birthdays, even though the patients presented with similar conditions. The researchers did not observe similar discrepancies in bypass rates for patients ages 77, 78, 79, 81, 82, or 83.

In addition, the researchers observed a higher death rate among the 80-year-olds than the 79-year-olds at 30 and 60 days after their hospitalizations.

Cardiac surgeon: 'We need to understand ourselves better'

The researchers said the findings suggest left-digit bias might be influencing doctors' treatment plans, though the cautioned that the findings do not prove causality and the phenomena should be further studied.

Anupam Jena, an associate professor of health care policy and medicine at Harvard University Medicine and lead author on the study, said the findings suggest doctors "are arbitrarily classifying the two groups of patients as young versus old instead of treating them as two groups who are basically the same age."

The phenomenon is "something that affects people's decisions in a subconscious way," Jena said. He added that doctors need to look beyond a patient's chronological age "because their biological age may be very different."

Andrew Olenski, an economics Ph.D. student at Columbia University and the paper's first author, said, "Studies like this are really to show physicians, 'Here's a common mistake or error that people make.'" That said, he added, "This is not to say, 'You should now be giving a lot more bypass surgeries to 80-year-olds than you would have.'"

John Spertus, director of health outcomes research at Saint Luke's Mid America Heart Institute, said while the study does not prove that left-digit bias was the cause, he believes "these types of cognitive biases are rampant in medical decision-making and this is a very clear demonstration of the phenomenon."

Still, Ruth Benson, a vascular surgeon at the University of Birmingham in England, cautioned that the results of the new study are correlational and can't definitively tell us what is causing the disparity. The study is "a snapshot that raises more questions than answers," Benson said.

Donald Redelmeier, a professor of medicine at the University of Toronto, added that the study "does not answer the question about which rate [of bypass surgery] is right, i.e. whether there's too much surgery going on beforehand or too little afterward."

George Tolis, surgical director of coronary bypass surgery at Massachusetts General Hospital, also said that the analysis doesn't provide enough details to prove whether left-digit bias was truly at fault for the discrepancy. For example, he noted that the analysis doesn't specify whether patients who didn't get the bypass surgery ever saw a surgeon. "A surgeon, in order to turn down a patient, needs to know about the patient. Did these surgeons know about these patients and either unconsciously or consciously turn them down? That's a key missing element here," he said.

But Thoralf Sundt, chief of cardiac surgery at Massachusetts General Hospital, said he believes the study serves as a good reminder for providers. "We need to understand ourselves better and understand these subtle biases so we can control for them," he said. "It's not sinister. It's human. We're all built this way" (Emery, Reuters, 2/19; Marchione, Associated Press, 2/19; Boodman, STAT News, 2/19).