Cancer screening statistics that show benefit in improved survival rates and increased early detection are susceptible to lead-time and overdiagnosis biases, but most primary care physicians don’t understand this crucial point. Few can correctly recognize that reduced mortality in a randomized trial is the best evidence of the benefit of screening, according to a study published in the March 6 issue of Annals of Internal Medicine.

“The benefit of screening tests is often communicated to physicians and patients in the form of improved survival rates,” wrote Odette Wegwarth, PhD, of the Max Planck Institute for Human Development in Berlin, and colleagues. “These data typically show a numerically large advantage for screening (for example, survival is 90 percent in early-stage disease but only 20 percent in late-stage disease). Although such statistics are intuitively appealing, they do not provide evidence of the benefit of screening.”

Problems stem from the fact that cancer-related mortality can remain the same despite changes in survival or detection based on screening. Lead-time bias, for instance, can result in radically different survival rates despite unchanging cancer-related outcomes. The authors offered the example of a group of patients who all die at age 70; if the patients are all diagnosed because of symptoms at age 67, their 5-year survival would be zero percent, but if they were diagnosed as a result of a screening at age 60, yet still die at 70, their 5-year survival is 100 percent even though the screening provided no benefit. Overdiagnosis also can bias screening results through the detection of nonprogressive cancers.

“Because lead-time and overdiagnosis biases do not affect mortality statistics, an extramural committee of the National Cancer Institute concluded that reduced mortality in a randomized trial is the only statistic that reliably proves that a screening test saves lives,” wrote the authors.

To see if physicians understood these common statistics, Wegwarth et al conducted a survey of 412 primary care physicians from 2010 to 2011. They were presented with two screening test hypotheticals—one describing improved five-year survival and increased early detection and the other describing decreased cancer mortality and increased incidence—and surveyed the physicians’ recommendation of screening and perception of its benefit.

When presented with irrelevant evidence (five-year survival increased from 68 to 99 percent), 69 percent of physicians recommended the test, compared with 23 percent when presented with relevant evidence (cancer mortality reduced from two to 1.6 in 1,000 persons). Responses to general knowledge questions about screening statistics showed the physicians largely couldn’t distinguish between irrelevant and relevant evidence. Just under half of the surveyed physicians incorrectly said finding more cases of cancer in screened as opposed to unscreened populations “proves that screening saves lives.”

Even providing the physicians with crib notes, of sorts, had an inconsistent effect. When presented with an explanatory note showing that higher survival with screening does not prove it saves lives, 29 percent said it made them more likely to recommend the screening test and 21 percent said it made them less likely. Similar results were seen after physicians were presented with an explanatory note highlighting the possibility of overdiagnosis.

“It is easy to be misled by statistics, particularly in the context of screening, where a familiar word like ‘survival’ takes on an unfamiliar meaning,” wrote Wegwarth et al. “However, to better understand the true contribution of specific tests, physicians need to be made aware that in the context of screening, survival and early detection rates are biased metrics and that only decreased mortality in a randomized trial is proof that screening has a benefit.”

In an accompanying editorial, Virginia Moyer, MD, MPH, chair of the U.S. Preventive Services Task Force, said that patients have a largely skewed view of the benefit of screening, believing it to be more beneficial than it has been demonstrated to be. Physicians could help dispel these misunderstandings, but the work of Wegwarth et al shows they might not be up to the task.

“Physicians clearly do not understand how to interpret cancer screening statistics themselves—expecting them to communicate this information to patients is a stretch,” wrote Moyer.

Moyer agreed with Wegwarth and colleagues that medical journals should present screening results in a way that avoids misinterpretation and medical educators should improve the quality of teaching about screening. She also recommended reaching beyond the medical field and pushing for better education of the public through the media.