Abstract
In the midst of frequent reports about "the asthma epidemic," results from a number of studies by the Manitoba Centre for Health Policy have shown stable or decreasing prevalence of an overall indicator of respiratory diseases which includes asthma. To resolve these apparently contrary findings, we conducted a time trend analysis using administrative data. Results revealed significant potential for diagnostic exchange: asthma prevalence increased, but that of bronchitis decreased.
In Manitoba, as in Canada and elsewhere, many studies have reported increasing rates of asthma prevalence over time, especially among children (Akinbami and Schoendorf 2002; Burney et al. 1990; Erzen et al. 1995; Garner and Kohen 2008; Kozyrskyj and Hildes-Ripstein 2002; Kozyrskyj et al. 2004; Lawson and Senthilselvan 2005; Lundback et al. 2001; Manfreda et al. 1993; Mannino et al. 2002; Millar and Hill 1998; Moorman et al. 2007; Senthilselvan et al. 2003; Senthilselvan 1998). This "asthma epidemic" has raised concern among clinicians, researchers, policy makers and the public, sparking new studies to understand the causes of this increase. There are also significant potential implications for population health and the management of health services.
Contrary to this increasing trend for asthma, the results in a series of "atlas-style" population health reports by the Manitoba Centre for Health Policy (MCHP) since the mid-1990s have shown stable, then slightly decreasing rates of a larger grouping of respiratory conditions, including asthma and other respiratory diseases (Martens et al. 2003; Fransoo et al. 2005, 2009). These two sets of findings – increasing rates of asthma amid stable or decreasing rates of the larger grouping – seemed contrary, prompting further investigation.
Key messages:
The prevalence of asthma increased over time, especially among children, but seems to have leveled off by 1997.
The noted increase may not accurately reflect underlying population health status, but rather, temporal shifts in diagnostic categories used.
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The MCHP reports used a grouping called "total respiratory morbidity" (TRM), which combines several related diseases: asthma, bronchitis, emphysema and chronic obstructive pulmonary disease (COPD). The idea was developed by a team of respirologists doing research using clinical and administrative data (Erzen et al. 1995, 1997; Huzel et al. 2002; Manfreda et al. 1993). Their work revealed significant inconsistency in the diagnostic coding of respiratory diseases between generalists and specialists, and among individual physicians. That is, what one physician called "asthma" might be diagnosed as "chronic bronchitis" by another, depending on patient age and clinical characteristics, and the physician's background and training (Dodge et al. 1986; Erzen et al. 1997; Manfreda et al. 2004; Tinkelman et al. 2006). Similar observations have been reported by others, attributed to "diagnostic exchange" (Fletcher 1978; Dodge et al. 1986; Burney et al. 1990; Lundback et al. 2001; Tinkelman et al. 2006). The TRM grouping was created to overcome the limitations associated with inconsistent coding across these related diagnoses, and reflect the overall level of respiratory disease in a population served by a mix of providers.
The objective of this analysis was to describe long-term trends in the diagnosed prevalence of total respiratory morbidity, along with that of its constituent diagnoses. Rudimentary case definitions were used in order to provide a "big picture" view of changes in diagnostic codes assigned, knowing that diagnostic accuracy for each condition would be affected. The larger goal was to examine the possibility of diagnostic exchange among related disorders, versus true changes in the prevalence of disease in the population.
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