We used stepwise multiple linear regression to examine which covariates, in addition to the symptom severity groups, had an influence on charges within each period. Because membership in a symptom severity group was not an ordinal measure, this variable was transformed into 4 dichotomous variables, one to denote membership for each group. The 2 moderate-severity groups and the high-severity group were entered into the regression model as a single block, with the low-severity group serving as the reference group. Medical comorbidity was entered into the model first, a stepwise block containing all demographic variables was entered, the symptom severity group block was entered, and finally the DSM-III-R variable. The ordering of variables was chosen to examine the relative impact on charges of the severity groups after first entering the comorbidity and demographic variables, and finally whether the presence of a diagnosed mood or anxiety disorder added additional information. As a part of our evaluation of each regression model, we also examined collinearity diagnostics.
Using the information from the regression models, the significant covariates were included in an analysis of covariance for each period to test for significant differences in adjusted charges between severity groups. For each symptom severity group, means for covariate adjusted charges were also estimated across each period.
Results
Sample Description
Mean age of subjects in our sample was 43 years (standard deviation = 15.7); 70% of our subjects were women, 39% white, 35% African American, and 26% Hispanic. Among our Hispanic patients, the level of acculturation by birth status was relatively high: 51% were at least second-generation US residents, and an additional 34% were first-generation US residents. A total of 57% of our sample had taken some courses beyond the high school level, and 53% had an annual household income of less than $20,000. Additional sociodemographic information is available in the original paper describing this sample.23
From our sample of 1333 subjects, 83 were excluded from the cluster analysis procedure because of incomplete responses to the 15-item symptom severity instrument. Another 18 subjects were excluded because of an inability to access their billing records, leaving 1232 subjects for our utilization analyses. Loss of charge data over time was 14.3%, with available charge data for 1055 subjects for the period 9 to 12 months after each subject’s index visit.
Symptom Severisty Group Descriptions
We clustered study subjects into 4 groups: low severity (n = 686), moderate anxiety/minor mood (n = 335), moderate anxiety/severe mood (n = 148), and high severity (n = 81). Sociodemographic information on subjects in each cluster and mean symptom severity scores for each cluster were presented in our initial paper.22 Membership in a higher-severity cluster tended to be associated with being female, unemployed, and having an annual income of less than $10,000. Significant differences were not seen between groups with respect to age, education, or presence of chronic health problems.
With the exception of the 2 moderate-anxiety groups, individuals in each group were distinguished by the level of severity across all symptoms. Subjects in the 2 moderate groups shared similar severity of anxiety symptoms but differed in the severity of their mood symptoms.
The Figure 1 displays the distribution across the 4 severity groups for those patients who met criteria for individual disorders. (For comparison, we also show the distribution for subjects who failed to meet criteria for any disorder.) This figure illustrates the lack of relationship between the severity groups and the diagnostic entities. The distribution does not follow an expected distribution of the majority of subjects with any particular disorder in the high-severity group. For major depressive disorder, partial remission or recurrence of a depressive disorder, and bipolar disorder the majority of subjects meeting criteria are actually found in the 2 moderate-severity groups. Subjects who failed to meet diagnostic criteria for any major depressive disorder have a symptom severity distribution similar to those who do. This suggests that for these mood and anxiety disorders, meeting the diagnostic criteria is not necessarily associated with a high level of symptom severity.
Charge Differences Between Clusters
Table 2 presents the unadjusted mean charges for each cluster group for each 3-month period of charge data obtained. Charges decreased for each severity group over the period of study, but the general trend toward higher charges in the high-severity group persisted over time. Even 9 to 12 months after the index visit, patients in the high-severity group had an average of more than 3 times the health care charges of patients in the low-severity group.
The largest significant differences were seen between the low- and high-severity clusters, except for the 6- to 9-month period. No statistically significant differences in charges were seen between the moderate-severity clusters for any period studied.