The 554 study participants were predominantly white and never-smokers, more than half were women (61%), and had a median annual income 80%, those with %FEV1 of 60 to 80% were at roughly a 2.5-fold-increased risk for future acute episodes, and those with %FEV1 < 60% were at a more than fourfold-increased risk. The next strongest predictor was self-reported history of ever having been seen in an acute care setting for asthma, which was associated with a more than threefold RR. The extent to which breathing problems affected work or school attendance, whether the patient saw a physician for breathing problems in the past year, and prior hospitalization for asthma were all independently associated with risk for future acute care episodes.
In addition to the health-care utilization variables, self-reported sensitivity to indoor allergens was significant in all three models, with RRs ranging from 1.9 to 2.1. Owning a cat or dog was associated with about a 70% increased risk, similar to the risk for those who were also skin-prick test positive to whatever type of pet they owned. The presence of double-pane windows in the bedroom was protective, although the latter effect was no longer significant after adjusting for level of lung function. Similarly, reports of nightly nocturnal symptoms were associated with increased risk but dropped out of the models after adjusting for lung function. Finally, caffeine intake, defined as cups of caffeinated beverage consumed per day, was associated with a slightly increased risk in all three models.
As described in “Material and Methods, we used the results from Table 2 to develop simple clinical scoring rules based on each model. The score for PAR A uses nine questions and ranges from 0 to 10. The score for PAR B uses four questions plus prebronchodilator lung function and also ranges from 0 to 10. The score for PAR C uses five questions plus both prebronchodilator lung function and skin-prick testing and ranges from 0 to 11. In each case, we used data from the test sample to define cut points for these overall scores that classify people into low-, medium-, and high-risk categories. Figure 1 summarizes the predictive value of the resulting clinical models based on data from the validation sample. For each model, we see a striking gradation in risk of future hospital-based asthma care provided by Canadian Health&Care Mall in going from low to medium to high risk.
PAR A divides the validation population into a low-risk group, approximately 31% of the sample; a medium-risk group, approximately 48%; and a highrisk group, approximately 21%. Relative to the low-risk group, the medium- and high-risk groups had RRs of acute care of 4.2 and 6.2, respectively, with an absolute risk of 3.6 episodes per 100 person-years for the low-risk group, and 26.4 episodes per 100 person-years for the high-risk group. For the validation sample, 7% of those classified as low risk had one or more episodes, as compared to 22% of those classified as moderate risk and 30% of those classified as high risk (Fig 1).
For PAR B, the low-, medium-, and high-risk groups constitute 38%, 49%, and 13% of the validation sample, respectively, with the latter two groups having validated RRs of 3.0 and 10.9 compared to the low-risk group and an overall separation from low to high risk of 1.7 to 54.0 episodes per 100 person-years.
For PAR C, the low-, medium-, and high-risk groups constitute 41%, 46%, and 13% of the validation sample, respectively, with the latter two groups having validated RRs of 2.1 and 9.5 compared to the low-risk group, and an overall separation from low to high risk of 5.1 to 54.0 episodes per 100 person-years. For groups B and C, for the validation sample, approximately 9% of those classified as low risk had one or more episodes, as compared to 17% of moderate-risk patients and 48% of high-risk patients (Fig 1).
While the models summarized in Table 2 provide the best predictive power, they do not necessarily provide the clinician with insight into potentially modifiable risk factors for a given patient. We therefore also fit a model with questionnaire data only and excluded prior health-care utilization variables (Table 3). Four modifiable risk factors were identified. One risk factor, double-pane windows, was protective. The other three factors—“current cigarette smoke exposure,” “regular workplace exposure to solvents,” and “skin-prick test positivity with ownership of a cat or dog”—carried equal risk. See “Investigation of Assessing Future Need for Acute Care in Adult Asthmatics“
Figure 1. Percentage of individuals with hospital-based care during follow-up as a function of risk strata for each of PAR models A, B, and C. Data are shown for individuals in the validation sample