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Somatic Complaints in Primary Care: Further Examining the Validity of the Patient Health Questionnaire (PHQ-15)

Received May 4, 2005; revised September 28, 2005; accepted October 13, 2005. From the UMDNJ-Robert Wood Johnson Medical School, Dept. of Psychiatry. Address correspondence and reprint requests to Dr. Interian, Dept. of Psychiatry, UMDNJ-Robert Wood Johnson Medical School, 671 Hoes Lane D306, Piscataway, NJ 08854-5635. e-mail: interial{at}umdnj.edu

ABSTRACT

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The authors examined the reliability and validity of the PHQ-15, a measure of current somatic complaints. An index of medically unexplained symptoms was used as a key criterion. Data were utilized from medical outpatients enrolled in a treatment study for moderate-to-severe somatization (N=172). Approximately 68% of the sample was Hispanic. Results showed that the PHQ-15 was moderately related to a history of medically unexplained symptoms among non-Hispanic participants. Results indicated ethnic differences on the validity profile of the PHQ-15 showing that the criterion variables were less predictive of the PHQ-15 among Hispanics than among non-Hispanics. Also, among the Hispanic group, the PHQ-15 was less related to medically unexplained symptoms and more to psychiatric distress. General support was provided for using the PHQ-15 with clinical samples composed of non-Hispanics. Also, the PHQ-15 appears to measure medically unexplained symptoms, psychiatric distress, and physical functioning. Further study is recommended to better evaluate ethnic variations and other types of validity for the PHQ-15.

INTRODUCTION

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Somatic symptoms that are medically unexplained have been estimated to account for approximately one-third of medical visits.^1,2 Patients with high levels of these somatic symptoms have been shown to be disproportionately high utilizers of healthcare services^3,4 and to suffer from high rates of anxious and depressive disorders.^5–8 Given these and other clinical correlates, these patients constitute an important clinical group. Therefore, properly assessing the severity of their somatic symptoms is critical.

The DSM-IV provides a number of diagnoses that can be used to describe the clinical presentation of patients with somatic symptoms, including somatization disorder, somatoform pain disorder, undifferentiated somatization disorder, and somatoform disorder, not otherwise specified. Measures such as the Structured Clinical Interview for DSM-IV (SCID)⁹ and the Composite International Diagnostic Interview (CIDI)¹⁰ can be used to identify clinical presentations of somatoform disorders on the basis of a determination of meeting or not meeting specific diagnostic criteria. Both of these measures involve a structured interview that queries patients’ lifetime experiences with various somatic symptoms.

A number of instruments are also available to measure the severity of somatic symptoms. For example, the SCL–90¹¹ and the SUNYA Psychosomatic Checklist¹² are self-report questionnaires assessing current experience with a number of somatic complaints. Derived scores represent a range of severity, thereby measuring current somatic distress along a hypothesized continuum. These measures are of value for research concerning the treatment outcome of somatoform disorders. Potentially beneficial treatments for somatoform disorders have ranged from management recommendations for primary-care physicians¹³ to cognitive-behavior therapy^14–16 and pharmacotherapy.¹⁷

Another measure of severity of somatic complaints is the PHQ-15, which is the somatic symptom module of the Patient Health Questionnaire.¹⁸ The Patient Health Questionnaire is a self-report questionnaire designed for assessing Axis I disorders in primary-care patients. The PHQ-15 assesses current severity of 15 somatic complaints that have been demonstrated to be among the most common somatic symptoms.¹⁹ In addition to being a brief and simple measure to administer, the PHQ-15’s reliability and validity have received initial support in one study that utilized 6,000 patients from general internal medicine and family-practice clinics.²⁰ In that study, the PHQ-15 demonstrated good internal consistency (=0.80) and was related to criterion indices of physical dysfunction, self-reported disability days, clinic visits, and amount of difficulty that patients attributed to their symptoms. Furthermore, linear regressions examined the ability of the PHQ-15, along with other variables, such as depression and medical comorbidity, to independently predict medical outcomes (e.g., bodily pain, physical functioning). These analyses showed that the PHQ-15 accounted for the greatest proportion of variance related to medical outcomes.

Given that the Patient Health Questionnaire has been used in numerous studies,^21–23 the PHQ-15 holds the potential for being a commonly used measure to assess the severity of somatic complaints. Moreover, the PHQ has been translated into numerous languages and has been examined with samples in foreign countries (e.g., Saudi Arabia, Germany, and Belgium^23–25), thereby having potential for comparison among ethnic groups.

The purpose of the current study was to examine further the convergent validity of the PHQ-15 in a sample of participants with high levels of medically unexplained symptoms. Since Kroenke and his colleagues²⁰ had already studied samples comprising participants with and without somatoform disorders, we sought to examine the ability of the PHQ-15 to detect severity differences within a sample of participants with medically unexplained somatic symptoms. Such an examination was intended to provide information on the instrument’s potential performance in those clinical studies in which samples are composed solely of participants with high levels of medically unexplained somatic symptoms. Also, the current study included a large proportion of Hispanic participants, which provided an opportunity to examine whether the instrument performed differently among Hispanic and non-Hispanic participants. Our analyses also advance previous research by comparing the PHQ-15 with a criterion that indexed somatic symptoms that were likely to be medically unexplained. This measure of medically unexplained symptoms was our key variable of interest.

METHOD

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Participants
Participants were primary-care patients enrolled in a clinical trial for treating moderate-to-severe somatization with a cognitive-behavioral intervention.²⁶ Participants were included in the study if they met criteria for Abridged Somatization, which required a history of six medically unexplained symptoms for women and four medically unexplained symptoms for men.²⁷ The first 172 participants that were entered into the study were included in these analyses.

Measures
The Patient Health Questionnaire (PHQ)
Participants completed the PHQ,¹⁸ a self-report questionnaire with modules that assess various Axis I disorders. The 15 somatic symptoms of the PHQ (PHQ-15²⁰) have been utilized as a measure of somatic complaints experienced during the month before the evaluation. The PHQ-15 score ranges from 0 to 30.

The Composite International Diagnostic Interview
The participants’ lifetime history of medically unexplained symptoms was evaluated using the somatoform module of the Composite International Diagnostic Interview (CIDI)¹⁰. The somatoform module evaluates the presence of 41 possible somatic symptoms and contains a structured set of probes that directs the interviewer in determining whether those symptoms were significantly distressing and medically unexplained. Referring physicians and/or nurses were consulted to clarify the organic basis of symptoms whose medical explanation was unclear. The questions inquire about patients’ lifetime experience with symptoms. The CIDI was used to generate a variable (CIDI Symptom Count) that indexed participants’ history of medically unexplained symptoms. It was derived by summing the number of medically unexplained symptoms.

The CIDI was developed by the World Health Organization for international research on psychiatric epidemiology. The somatoform module is one of the most widely used instruments for assessing medically unexplained symptoms.^8,28–30 A review of the test–retest reliability of the somatoform module of the CIDI reported estimates that ranged from 0.66 to 0.74.²⁸ The diagnosis of somatization disorder based on the CIDI demonstrated substantial interrater agreement (=0.68) in an international sample.³¹

RAND MOS Short-Form Health Survey
For the assessment of functional status, participants completed the RAND MOS Short-Form Health Survey (MOS SF–12),³² a self-report questionnaire that assesses overall health and physical functioning. The analysis utilized the physical functioning subscale whose 10 items pertain to participants’ ability to perform certain physical tasks (e.g., climb stairs, walk more than one mile, etc.). Higher scores on this measure indicate better physical functioning. The variable derived from scoring these 10 items was labeled MOS SF–10. The MOS SF–10 demonstrated good internal consistency with the current sample (=0.89).

The Hamilton Rating Scale for Depression (Ham-D) and the Hamilton Rating Scale for Anxiety (Ham-A)
The 17-item Ham-D³³ and the Ham-A³⁴ were administered in interview format by trained clinicians to measure severity of depression and anxiety, respectively. To assess interrater reliability, six Ham-D/Ham-A interviews (three Spanish, three English) were randomly selected to be re-rated by all clinicians by use of audiotape. Each interview was re-rated by one clinician, who provided Ham-D/Ham-A ratings; pairs of raters were alternated randomly. Intraclass correlation coefficients were used to measure the interrater reliability of these pairs (Ham-D: r=0.98; Ham-A: r=0.97). This procedure was repeated 1 year later to assess rater drift (Ham-D: r=0.97; Ham-A: r=0.98).

Procedures
Participants were referred by primary-care physicians and nurses if they presented with a history of at least three medically unexplained symptoms. Research staff subsequently prescreened participants by telephone to ensure that they met criteria for the study. Research staff included four doctoral-level psychologists and four psychology doctoral candidates.

After prescreening, clinical interviews were performed by the research staff. These clinicians collected basic demographic data and administered the CIDI Somatoform module, Ham-D, and Ham-A. Participants subsequently completed the MOS SF–12 and PHQ. Of the 172 assessments conducted, 111 (65%) were conducted in Spanish with Spanish translations of the measures.

Of a total of 416 participants referred to the study, 172 were enrolled in the study, and 244 were excluded. Participants were excluded if they did not meet criteria for Escobar’s definition²⁷ of Abridged Somatization (N=75), as determined by the Somatoform section of the CIDI; were not between the ages of 18 and 65 (N=2); reported significant current psychiatric symptoms, such as psychotic symptoms or clinically significant active suicidal ideation (N=21), current substance abuse/dependence (N=1), or acute medical illness (N=18); concurrent legal issues (N=6); concurrent mental health treatment (N=22); or pregnancy (N=2). Data were collected as part of a treatment study, and exclusions were made to ensure internal validity. The remainder of potential participants were lost because of difficulty scheduling/reaching (N=40) or participant’s declining participation (N=56).

RESULTS

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Table 1 summarizes the demographic variables separately for Hispanic and non-Hispanic subsamples. Although the groups appeared similar in terms of age and gender, they were quite different in terms of cultural variables and level of education. Participants in the Hispanic group were more likely to be Spanish speakers, to be born outside the United States, and to have fewer years of education. To examine ethnic effects, the sample was divided into Hispanic and non-Hispanic subsamples. Separate subsamples were not created for African American or Asian participants because of insufficient sample sizes.

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TABLE 1. Summary of Demographic Variable

Table 2 compares the groups on the various measures. We examined means differences between the groups with t-tests. These analyses showed that the Hispanic group had significantly higher levels of depressive (Ham-D) and anxious (Ham-A) symptoms. Also, Hispanic participants reported lower physical functioning (MOS SF–10), but did not differ in their severity of somatic complaints (PHQ-15) or history of medically unexplained symptoms (CIDI Symptom Count).

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TABLE 2. Symptom Severity Differences Between Hispanic and Non-Hispanic Subgroups

The internal reliability of the PHQ-15 was assessed with Cronbach’s alpha test. Among Hispanics and non-Hispanics combined, internal consistency was acceptable (=0.79). Identical levels of internal reliability were produced with the Hispanic (=0.79) and non-Hispanic (=0.79) subgroups.

Analyses also evaluated the convergence between the PHQ-15 and the CIDI Symptom Count (history of somatic symptoms that were likely to be medically unexplained). A multiple regression was first created that determined the degree to which PHQ-15 was predicted by the Ham-A and Ham-D. This was done to create a residual variable for PHQ-15 (PHQ15-residual) in order to isolate the effects that were independent of psychiatric distress as measured by the Ham-A and Ham-D. The correlations of these PHQ-15 variables (PHQ-residual and PHQ-15) with the CIDI Symptom Count are summarized in Table 3. Overall, there appear to be greater magnitude correlations for the non-Hispanic group. The correlations in Table 3 were analyzed to determine whether those generated for the Hispanic group were significantly different from those generated for the non-Hispanic group. The correlation between CIDI Symptom Count and PHQ-15 (z = –2.13; p=0.016) was significantly different between the ethnic groups, as well as the one between CIDI Symptom Count and PHQ15-residual (z = –2.16; p=0.015).

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TABLE 3. Convergent Validity With CIDI Symptom Count

Linear-regression analysis was used to assess whether the criterion measures independently contributed to PHQ-15 variance. Criterion measures were selected that would not contribute to multicollinearity. Thus, we selected the CIDI Symptom Count, as well as a measure of psychiatric distress (Ham-A) and physical functioning (MOS SF–10). The Ham-D was not selected, given than it would likely have been collinear with the Ham-A, and it demonstrated a lower correlation with the PHQ-15 (Ham-A: r=0.63; Ham-D: r=0.51). To examine the effects of ethnicity, we also included ethnicity and an interaction term (Ethnicity x CIDI Symptom Count) to examine whether ethnic differences existed. We also included effects related to education to better delineate the contribution of culture versus that of socioeconomic status.

The results of the linear regression are summarized in Table 4. Model 1 utilized the combined sample to analyze the effects of the criterion variables and determine whether ethnic effects existed. Model 1 predicted nearly half of the PHQ-15 variance, but Ham-A and MOS SF–10 were the only significant main effects. The interaction term for ethnicity (ethnicity x CIDI Symptom Count) was also found to be significant, suggesting that the relationship between the PHQ-15 and CIDI symptom count differs according to ethnicity. The main and interaction effects related to education were not significant.

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TABLE 4. Regression Values Between the PHQ-15 and the Criterion Measures

Because the association with CIDI Symptom Count varied by ethnicity, separate regression models were generated for the Hispanic (Model 2) and non-Hispanic (Model 3) groups. For the Hispanic group, Model 2 predicted 39% of the PHQ-15 variance, and all criterion variables, including the CIDI Symptom Count, significantly contributed to the equation. Among non-Hispanics, Model 3 predicted a greater amount of the PHQ-15 variance (R²=0.65), and all criterion variables significantly contributed to the equation.

DISCUSSION

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The results of this study provide further support for the internal consistency of the PHQ-15. Analyses among the Hispanic, non-Hispanic, and combined samples all showed an acceptable level of internal consistency. This supported the reliability of the PHQ-15 and suggested that no ethnic differences existed on the domain of reliability.

A more complex picture emerged regarding the PHQ-15’s validity; it seemed to perform differently between the ethnic groups. Among non-Hispanic participants, the PHQ-15 demonstrated a moderate to high-moderate degree of convergence with a history of medically unexplained symptoms, even when effects related to psychiatric distress were controlled. Among the Hispanic participants, however, convergence with medically unexplained symptoms occurred at a lower magnitude, suggesting that the validity of the PHQ-15 for predicting medically unexplained symptoms is lower among this group. The ethnic variation of the relationship between the PHQ-15 and history of medically unexplained symptoms was also revealed in the linear-regression analyses. The regression model showed this ethnic difference to be significant, even while controlling for effects related to level of education.

Ethnic differences on the PHQ-15 also were suggested by the relative contribution of psychiatric distress versus medically unexplained symptoms. Among both groups, the PHQ-15 was related to a history of medically unexplained symptoms, psychiatric distress, and lower physical functioning. Among Hispanics, the PHQ-15 seemed to capture mostly psychiatric distress. In contrast, the PHQ-15 seemed to be equally related to psychiatric distress and a history of medically unexplained symptoms among non-Hispanics.

To the best of our knowledge, this study is the first to examine the validity of the PHQ-15 within a clinical sample and to compare its performance with an index that differentiated medically explained versus unexplained symptoms. Some of the current findings replicate previous research on the PHQ-15.²⁰ Specifically, the PHQ-15’s internal consistency and relationship to lower physical functioning was consistent with results reported by Kroenke and his colleagues. Given that the PHQ-15 demonstrated independent relationships with a history of medically unexplained symptoms, psychiatric distress, and lower physical functioning, it seems to capture phenomena consistent with the construct of somatization. The evidence linking somatization to measures of anxiety and physical functioning is abundant.^5,8,27,30 Therefore, the current results support the use of the PHQ-15 in clinical studies among non-Hispanic, primarily Caucasian, samples.

However, the ethnic differences noted in the current study indicate that further research on the PHQ-15’s cross-ethnic application is warranted. Not only did does this measure account for less variance on the criterion measures for Hispanics in our sample, it also appeared more closely associated with anxiety than with medically unexplained symptoms or physical functioning. If these findings are replicated in the future, additional research could examine whether the ethnic variation represents a limitation with the PHQ-15’s validity or whether this measure is accurately capturing ethnic variation in somatic complaints. The notion that somatic complaints and psychiatric distress represent a more unified construct has been raised by cross-ethnic factor-analytic studies of the Center for Epidemiological Study–Depression Scale (CES–D).

The current study examined a sample with high levels of medically unexplained symptoms, which permitted an evaluation of the PHQ-15’s performance within clinical samples. At the same time, our clinical sample also posed a methodological limitation because it did not allow a determination of the extent to which the PHQ-15 differentiates between patients with and without medically unexplained symptoms. This line of inquiry would best be pursued by studies that compare somatoform patients with medically ill patients and depressed and/or anxious patients.

Another limitation of our study pertains to the unbalanced ethnic groupings within the sample. In fact, the non-Hispanic group included primarily Caucasians, with few Asian American and African American participants. Future studies should therefore continue to assess for the presence of cross-cultural effects, particularly among ethnicities not well represented in the current sample.

Finally, although two studies have now examined the internal consistency of the PHQ-15, to our knowledge, none have examined its test–retest reliability. This type of analysis should be conducted by future studies to form a more complete picture of the PHQ-15’s reliability.

ACKNOWLEDGMENTS

 
This work was supported in part by NIMH grant R01 MH-60265.

REFERENCES

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Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Email this article to a Colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My Articles & Searches Download to citation manager Citing Articles via Google Scholar Articles by Interian, A. Articles by Díaz-Martínez, A. M. Search for Related Content PubMed Citation Articles by Interian, A. Articles by Díaz-Martínez, A. M. Primary Care Somatoform Disorders

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