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STUDY FINDS NEW TRENDS IN INCIDENCE RATE OF OSTEOARTHRITIS

The number of people with arthritis is expected to increase as the Baby Boomer population ages.  The most frequent form of arthritis is osteoarthritis (OA), a disease commonly afflicting older people.  Based on demographic trends, experts have projected a 47 percent increase in the prevalence of arthritis in Canada between 1991 and 2031, and a 60 percent increase in the prevalence of arthritis in the United States between 2005 and 2050. The prevalence of OA has also been projected to increase among younger people, due to the rising prevalence of obesity. Prevalence is the focus of most population studies of arthritis. However, the better measure of disease dynamics in the population—because it responds more quickly to changes in the risk factors and is not influenced by disease duration—is incidence rate.

Surprisingly, considering this disease’s huge health and economic burden, information on trends in the incidence of OA is very limited. Struck by this observation, researchers in Canada decided to examine the changes in physician-diagnosed OA incidence rates in British Columbia (BC). Published in the July 2008 issue of Arthritis Care & Research (www.interscience.wiley.com/journal/arthritis), their findings include an increase in the incidence of OA in women over the past decade not resulting from population aging.

Led by Jacek A. Kopec, PhD, a Michael Smith Foundation for Health Research Senior Scholar with the University of British Columbia, this groundbreaking study used data on all visits to health professionals and hospital admissions covered by the Medical Services Plan (MSP) of BC—population: roughly 4 million—covering the span of years between 1991 and 2004. Researchers also obtained information about hospital admission and discharge dates, including diagnostic codes. Since the database offered no consensus on how to define OA, the team agreed to use 2 definitions: #1) at least 1 visit to a health professional or 1 hospitalization with a diagnostic code for OA and #2) at least 2 visits to a health professional within 2 years or 1 hospitalization with a diagnostic code for OA. Incidence rate was defined as the number of new cases of OA during a given period divided by person-time at risk. Age and sex specific incidence rates for OA were calculated for the fiscal years 1996-1997 through 2003-2004, with a 5-year run-in period to exclude prevalent cases.

Between 1996-1997 and 2003-2004 in British Columbia, the total number of new cases of OA based on definition #1 increased from 43,546 to 55, 911. Or, in terms of crude incidence rates, OA increased from 10.5 to 12.2 per 1,000 in men and from 13.9 to 17.4 per 1,000 in women. In men, the age-standardized rates fluctuated from year to year, but the differences were consistently small. In women, however, the age-standardized rates increased from 14.7 to 16.7 per 1,000. OA incidence rates increased substantially for women between the ages of 40 and 79, with the greatest increase—22 percent—in those ages 60 to 69 years. Approximately half of the increase in crude incidence rates of OA among women could be accounted for by an increase in the ranks of older people in BC. Incidence rates based on OA definition #2 were almost 50 percent lower across the board; the trends, however, were similar.

“Whatever the cause of the observed trends, OA is diagnosed more and more often by physicians, especially in women,” Kopec notes.  “More studies are needed to assess plausible scenarios for future OA prevalence based on demographic trends and changes in the major risk factors,” he adds. “Administrative databases can play an important role in OA surveillance, and more validation studies of administrative diagnosis, as well as efforts to standardize the definitions across studies and databases, are urgently needed.”

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Article: “Trends in Physician-Diagnosed Osteoarthritis Incidence in an Administrative Database in British Columbia, Canada, 1996-1997 Through 2003-2004,” Arthritis & Rheumatism (Arthritis Care & Research), July 15, 2008; 59:7, pp. 929-934.