Evaluation measures
We used the Medical Outcomes Study SF-36 subscales and component summary scale11,12 to assess health-related quality of life (HRQOL) in all subjects. Medical comorbidity was assessed using electronic medical record review as described previously.8 We also examined health care utilization using charge data from the billing system of UTMB. As previously described,8 we obtained all inpatient and outpatient charge data for a 15-month period beginning 3 months before the visit at which each subject was surveyed. Outpatient pharmacy data were not included. The results of the charge data subanalysis are presented online in Figure W1, at www.jfponline.com.
Analytic strategy
All subjects were screened with the Clinician Evaluation Guide mood module from the Primary Care Evaluation of Mental Disorders (PRIME-MD).13 A “DSM criteria positive” screen included major depressive disorder (MDD), dysthymia, and partial remission of MDD. Symptom severity was assessed using a 6-item Brief Depression Rating (BDR) scale (Table 1) derived from a principal components analysis of 15 mood and anxiety symptom severity questions used in the original study and our subsequent investigations.8 Factor analysis of the 6 BDR items confirmed that they occupy a domain distinct from the somatic symptoms included as PRIME-MD depression criteria.
Cronbach’s alpha for the BDR in our sample was 0.8911. Because the distribution of subjects was skewed toward lower severity (median = 9, mean = 10.47, skewness = 1.415), we chose the 75th percentile score13 as our cut point for a “positive” BDR. This choice reflected a more conservative definition of severity than the use of a standard cut point of 1 standard deviation above the mean (in this case, a score of 15).
We “filtered out” low-severity patients by matching BDR scores and DSM criteria to create 4 groups for comparison: “low severity and DSM negative,” “high severity only,” “DSM positive only,” and “high severity and DSM positive.”
TABLE 1
Brief Depression Rating*
Over the LAST 2 WEEKS, how often have you experienced any of the following?* |
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*Responses to questions are on a 5-point Likert scale ranging from “none of the time” to “all of the time.” |
Data analysis
We used analysis of variance to compare the 4 groups on demographic and outcome measures of interest. We made adjustments where demographic variables or medical comorbidity contributed significantly to the differences between groups by using analysis of covariance (ANCOVA). We examined interaction effects between the covariates and the severity/DSM groups. Where possible and appropriate, we used Bonferroni or Games-Howell adjustments for multiple comparisons between groups.
Results
Size and demographic comparisons
The distribution of the 1317 subjects available for analysis is depicted in Table 2. Fully 75% of the total sample fell below the BDR severity threshold. The BDR filtered out 29% of those subjects meeting DSM criteria because of low symptom severity. Conversely, 17% of subjects who did not meet DSM criteria had high symptom severity based on the BDR. Although the groups had similar demographic characteristics, subjects in the “high severity and DSM positive” group were significantly younger than subjects in the “low severity and DSM negative” group. The distribution of women in all groups was significantly higher than expected except for the “low severity and DSM negative” group. We found even distributions of subjects by ethnicity.
TABLE 2
Group demographics
Characteristic | PRIME-MD criteria (–) | PRIME-MD criteria (+) | ||
---|---|---|---|---|
BDR severity (–) | BDR severity (+) | BDR severity (–) | BDR severity (+) | |
Subjects, n | 893 | 119 | 91 | 214 |
Female subjects, % | 66.2 | 72.3 | 74.4 | 84.1 |
Race, % | ||||
White | 38.3 | 35.3 | 41.7 | 40.2 |
African American | 34.5 | 41.2 | 28.6 | 36.9 |
Hispanic | 27.2 | 23.5 | 29.7 | 22.9 |
Mean age, y | 43.9* | 43 | 42.5 | 40.0* |
Chi-square is significant for sex (P < .001) but not for racial distributions (P = .500). | ||||
*Significant differences exist for mean age by analysis of variance using Bonferroni adjustment (P = .012). | ||||
BDR, Brief Depression Rating; Prime-MD, Primary Care Evaluation of Mental Diseases. |
Mean HRQOL score comparisons
Figure 1 shows mean Mental Health Component Summary (MCS) scores for subjects in the 4 groups, after ANCOVA adjustments for significant covariates (age and African-American ethnicity, P = .003 for both). The groups of subjects that scored either positively or negatively on both the BDR and PRIME-MD occupy opposite poles of very low and very high functional status, respectively. The groups of subjects that scored positively on only the BDR or only the PRIME-MD share the middle ground with no significant difference in MCS-related functional status.
A similar pattern was seen for the Physical Component Summary (PCS) scores from the SF-36. PCS score means ranged from 41.60 to 44.17 among the 4 groups after ANCOVA adjustment for significant covariates (income, medical comorbidity, and Hispanic ethnicity, P < .001 for each). Only the “low severity and DSM negative” and “high severity and DSM positive” groups differed significantly at either end of this range; however, the absolute difference of 2.57 points carries minimal, if any, clinical significance.