Our results are unlikely to have been affected by the inability to link more than 83.2% of participants in the survey to physician claims. Linked and nonlinked individuals did not differ with respect to sex, age or the frequency of symptoms reported. The incomplete linkage may be due to strict requirements for matching and to the fact that the study population was relatively young (20 to 44 years) and mobile; different addresses may well have been given in the survey and in the administrative data. Almost all subjects (94.7%) in the analysis were covered by the Manitoba Health Insurance Plan throughout the five-year period. Restricting the analysis to individuals covered over the full five years would probably have improved the agreement only slightly.
Several reasons may underlie the only moderate agreement found in the present study. First, the survey and the administrative data record different aspects of asthma. The physician claims note whether the subjects went to see a physician for asthma in the previous 12 months, while the survey asked if the subjects had experienced symptoms and/or were using asthma medications. People with asthma attacks or using medications may be expected to see a physician at least annually. However, in most instances, the asthma was not severe. A sample of survey respondents (n=518) were questioned in greater detail about their disease when examined in the laboratory. Only 55.0% of participants who had had an asthma attack or who were using medication in the previous 12 months reported visiting a doctor because of a breathing problem or shortness of breath in the preceding 12 months. Apparently, a substantial fraction of people who reported symptoms suggestive of asthma did not feel sick enough to seek medical care.