Since the early 1970s, both nations have maintained cystic fibrosis patient data registries. Annually, the US Cystic Fibrosis Foundation, which helped fund the new study, and Cystic Fibrosis Canada have published separate reports based on registry data.
"We noticed that the median age of survival within these reports differed between the two countries," said Dr. Anne Stephenson, lead author of the new study and a respirologist at the Adult Cystic Fibrosis Center of St. Michael's Hospital in Toronto. "However, as they were calculated independently, we were uncertain whether the differences were real or due to technical issues."
The goal of the study was to see whether this survival gap between nations might have been due to differences in calculating statistics, or possibly disease severity.
But the differences aren't just about math -- they're real, the researchers soon discovered.
Factors behind the 'survival gap'
Stephenson and her colleagues examined national cystic fibrosis registry data between 1990 and 2013 for 5,941 patients in Canada and 45,456 patients in the United States. While survival increased in both countries over time, Canada began to see greater improvements than the US beginning in 1995, with a dramatic increase in Canadian survival rates in 2005.
For the most recent years, 2009 through 2013, the median age of survival has been 50.9 years in Canada, compared with just 40.6 years in the US. In particular, rates of Canadian males dying at older ages grew significantly faster than those of US males, while no differences in rates were seen among females.
"The reasons for the survival gap is definitely multifactorial and not based on one factor alone," Stephenson wrote in an email, adding that the study was not designed to determine any direct causes.
She has received personal fees from Cystic Fibrosis Canada for work unrelated to this study.
"We hypothesize that three factors may be playing a role in the survival gap: lung transplantation; differences in the two health care systems; the differential approach to nutrition in the 1970s that started first in Canada," Stephenson wrote. The impact of each of these factors remains unknown, however.
Transplants, insurance, diet
A higher proportion of cystic fibrosis patients receive a lung transplant in Canada than in the US. And a greater proportion of deaths in the US occurred in patients who had not received a transplant.
In 2005, when the United States began using a lung allocation score to prioritize people on the transplant waiting list, the survival gap also increased dramatically, the authors noted. The score may make access to a lung more difficult for cystic fibrosis patients, according to the authors.
This score, designed by the Organ Procurement and Transplantation Network, which operates under contract with the US government, is not used in Canada.
Since 2005, an individualized lung allocation score
, which estimates disease severity and chance of success after a transplant, has been assigned to every transplant candidate 12 and older, explained Anne Paschke of the United Network for Organ Sharing, which runs the network. This score, blood type and distances between the lung donor and patient all factor into who gets a lung first.
The study authors also found that differences in survival varied according to the insurance status of US patients. Those with private insurance had similar survival rates to those of Canadians, who have universal, publicly funded health care coverage.
However, Canadians had a 44% lower risk of early death than US patients receiving continuous Medicaid or Medicare coverage and a 36% lower risk than those receiving intermittent Medicaid or Medicare coverage. Compared with US patients with no health insurance (or unknown health insurance status), Canadians had a 77% lower risk of early death.
"Insurance status in the US is a complex construct and may reflect a combination of other unmeasured factors such as socioeconomic status," Stephenson said. "The impact of US health insurance/health care policy and survival in US (cystic fibrosis) patients needs further targeted study in order to make definitive conclusions."
Finally, Canadian doctors began recommending a high-fat diet with supplementation for patients with cystic fibrosis in the 1970s.
"Cystic fibrosis affects the ability of the digestive system to absorb fat," Stephenson explained. The high-fat diet, which increases nutritional absorption, prevents malnutrition and so probably helps explain improved survival rates north of the border starting in 1995, Stephenson said.
"Children born with CF in Canada in the 1970s would have been exposed to good nutrition from birth (or diagnosis) which likely reduced their risk of death later in life when they reached their 20s and 30s," Stephenson wrote in the email. "This approach to nutrition was adopted by the US in the 1980s and is considered standard of care for CF patients worldwide."
Health care disparities
Dr. Pamela L. Zeitlin, a professor of pediatrics at National Jewish Health in Denver, considers the study "very well-done" and important.
"Importantly, there was no difference in median lung function" between the two nations, noted Zeitlin, who was not involved in the study.
She speculated that unequal survival rates could result from differences in infections between the US and Canada, and these differences "in turn may reflect environmental exposures such as temperature, humidity, climate, air pollution and/or difference in medications used to treat these organisms."
Zeitlin underscored health care delivery disparities between the two nations.
"When CF patients in the USA registry with Medicaid or Medicare were excluded from the analysis, leaving the patients with private insurance or 'other', the difference between Canada and the US disappeared," Zeitlin wrote in an email.
Dr. Bruce C. Marshall, a co-author of the study and senior vice president of clinical affairs at the Cystic Fibrosis Foundation, noted that the new research "reinforces how critically important it is to strengthen Medicaid and Medicare, programs that provide thousands of people with CF with access to care and treatments.
Half of children and a third of adults with cystic fibrosis use Medicaid as a source of health care coverage, often as a secondary insurance to fill in gaps left by employer-sponsored and other plans, explained Marshall.
"Health status can be impacted by a complex interplay between social and environmental factors, such as where a person lives, socioeconomic status and social supports," he said.
Individuals covered by Medicaid may be disproportionately impacted by these factors. With regard to Medicare, cystic fibrosis patients receiving coverage under this government program are primarily eligible "because of age or due to disability status, both of which are associated with worsened health status in people with cystic fibrosis," Marshall said.
A 'revolutionized' diet from Canada
Joanna Valsamis, chief health care, research and advocacy officer at Cystic Fibrosis Canada, highlighted the importance of Canadian scientist, Dr. Douglas Crozier, who "revolutionized" the field when he implemented the high-fat diet.
In 1958, when standard practice included a low-fat diet, the mortality rate for patients at the clinic where Crozier practiced was 75% in the first three years of life. He wondered whether normal growth and better nutrition might positively influence the course of pulmonary disease.
So in 1972, he began a study where patients ate a "new diet high in saturated fat supplemented with high doses of pancreatic enzymes," explained Valsamis, who did not participate in the new study.
"From 1970 to 1973, the mortality rate at the clinic had fallen to less than 0.5%," Valsamis said. She also noted that "Canada has a rich history in lung transplantation," and transplants for cystic fibrosis patients have been supported in part by Transplant Centre Incentive Grants launched in 1995 by the Cystic Fibrosis Canada.
In the end, the exact cause of a survival gap between nations may be of secondary importance.
"Young Canadians and Americans continue to die prematurely each year from CF," Stephenson said. "We must continue research in CF in order to ultimately find a cure."