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The PHQ-15: Validity of a New Measure for Evaluating the Severity of Somatic Symptoms

From the Regenstrief Institute for Health Care and Department of Medicine (K.K.), Indiana University, Indianapolis, IN; and the New York State Psychiatric Institute and Department of Psychiatry (R.L.S., J.B.W.), Columbia University, New York, NY.

Address reprint requests to: Kurt Kroenke, MD, Regenstrief Institute for Health Care, RG-6 1050 Wishard Blvd., Indianapolis, IN 46202. Email: Kkroenke{at}regenstrief.org For a complimentary copy of reproducible PHQ materials, contact: Robert L. Spitzer, MD. Email: rls8@columbia.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION APPENDIX ACKNOWLEDGMENTS REFERENCES

 
OBJECTIVE: Somatization is prevalent in primary care and is associated with substantial functional impairment and healthcare utilization. However, instruments for identifying and monitoring somatic symptoms are few in number and not widely used. Therefore, we examined the validity of a brief measure of the severity of somatic symptoms.

METHODS: The Patient Health Questionnaire (PHQ) is a self-administered version of the PRIME-MD diagnostic instrument for common mental disorders. The PHQ-15 comprises 15 somatic symptoms from the PHQ, each symptom scored from 0 ("not bothered at all") to 2 ("bothered a lot"). The PHQ-15 was administered to 6000 patients in eight general internal medicine and family practice clinics and seven obstetrics-gynecology clinics. Outcomes included functional status as assessed by the 20-item Short-Form General Health Survey (SF-20), self-reported sick days and clinic visits, and symptom-related difficulty.

RESULTS: As PHQ-15 somatic symptom severity increased, there was a substantial stepwise decrement in functional status on all six SF-20 subscales. Also, symptom-related difficulty, sick days, and healthcare utilization increased. PHQ-15 scores of 5, 10, 15, represented cutoff points for low, medium, and high somatic symptom severity, respectively. Somatic and depressive symptom severity had differential effects on outcomes. Results were similar in the primary care and obstetrics-gynecology samples.

CONCLUSIONS: The PHQ-15 is a brief, self-administered questionnaire that may be useful in screening for somatization and in monitoring somatic symptom severity in clinical practice and research.

Key Words: somatization, • somatization disorder, • depression, • screening, • quality of life, • utilization.

Abbreviations: DSM-IV = Diagnostic and Statistical Manual of Mental Disorders, fourth edition;; PHQ-9 = Patient Health Questionnaire depressive symptom severity scale;; PHQ-15 = Patient Health Questionnaire somatic symptom severity scale;; SF-20 = 20-item Short-Form General Health Survey.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION APPENDIX ACKNOWLEDGMENTS REFERENCES

 
Somatic symptoms account for more than half of all outpatient encounters (1), and at least one third of these somatic symptoms are medically unexplained (2–6). Somatization is the association of medically unexplained somatic symptoms with psychological distress and health-seeking behavior and is present in at least 10% to 15% of primary care patients (7–11). Indeed, somatization together with depression and anxiety constitute the three most common psychiatric problems seen in primary care (8, 12, 13). The detrimental impact of somatization on multiple domains of health-related quality of life remains considerable even after controlling for comorbid depression and anxiety (7–9, 11). Moreover, compared with depression and anxiety, somatization results in more healthcare utilization and greater clinician frustration (6, 14–17). Although somatization is often comorbid with depression and/or anxiety (just as depression and anxiety frequently coexist), a third or more of patients have somatization alone (8, 10–12, 18). The importance of recognizing and evaluating somatization has been heightened by recent evidence of the effectiveness of specific treatment strategies (19–21).

Measures to identify and monitor somatic symptoms are important if researchers are to study somatization and clinicians are to evaluate and manage it. Unlike depressive symptom measures, measures to assess somatic symptoms are less well established. Limitations of existing measures (11, 22–25) include one or more of the following: their length, the need to inquire about lifetime as well as current symptoms, a predominant focus on identifying DSM-IV somatization disorder (which accounts for only a small proportion of clinically significant somatization in primary care), validation in psychiatric rather than general medical patient populations, and an assessment of symptom counts alone rather than both the severity and number of somatic symptoms. The few studies comparing multiple somatic symptom measures in the same sample have not demonstrated the superiority of any one particular measure (26, 27). Consensus is further complicated by the ongoing debate about the optimal classification of somatoform disorders (17, 28–32).

PRIME-MD (Pfizer Inc, New York, NY) is a brief instrument for making criteria-based diagnoses of mental disorders commonly encountered in primary care (8, 33). The Patient Health Questionnaire (PHQ) is an entirely self-administered version of the PRIME-MD that was recently validated in two studies involving 6000 patients in eight primary care clinics and seven obstetrics-gynecology clinics, respectively (34, 35). The PHQ assesses eight diagnoses, divided into threshold disorders (disorders that correspond to specific DSM-IV diagnoses: major depressive disorder, panic disorder, and bulimia nervosa) and subthreshold disorders (disorders whose criteria encompass fewer symptoms than are required for any specific DSM-IV diagnoses: other depressive disorder, other anxiety disorder, probable alcohol abuse/dependence, binge-eating disorder, and probable somatoform disorder).

In this article we analyze data for the 15-item somatic symptom scale, which we call the PHQ-15, to address two major questions. First, is the PHQ-15 a valid measure of somatic symptom severity as determined by its association with multiple domains of functional status as well as disability days and utilization? Second, how do somatic and depressive symptoms differ in their effects on these outcomes?

	METHODS

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Description of the PHQ-15
The PHQ-15 is a somatic symptom subscale (Appendix) derived from the full PHQ. It inquires about 15 somatic symptoms or symptom clusters that account for more than 90% of the physical complaints (excluding upper respiratory tract symptoms) reported in the outpatient setting (1, 36). Also, the symptoms inquired about in the PHQ-15 include 14 of the 15 most prevalent DSM-IV somatization disorder somatic symptoms (ie, those with a prevalence of 3% or greater in the general population) (32). Thirteen of the PHQ-15 somatic symptoms are included in the PHQ somatic symptom module, in which subjects are asked to rate the severity of each symptom as 0 ("not bothered at all"), 1 ("bothered a little"), or 2 ("bothered a lot"). Two additional physical symptoms—feeling tired or having little energy, and trouble sleeping—are contained in the PHQ depression module, in which subjects are asked: "Over the last 2 weeks, how often have you been bothered by any of the following problems?" For scoring, response options for these two symptoms are coded as 0 ("not at all"), 1 ("several days"), or 2 ("more than half the days" or "nearly every day"). Thus, in determining the PHQ-15 score, each individual symptom is coded as 0, 1, or 2, and the total score ranges from 0 to 30. Appendix 1 displays the recommended format for the PHQ-15 if used as a somatic symptom severity scale separate from the full PHQ.

PHQ Study Samples and Procedures
From May 1997 to November 1998, 3890 patients, aged 18 years or older, were invited to participate in the PHQ Primary Care Study (34). One hundred ninety declined to participate, and 266 started but did not complete the questionnaire (often because there was inadequate time before seeing their physician). Data for 434 patients were not entered into the data set because the equivalent of approximately one page (20 items) of the PHQ was incomplete. This resulted in 3000 primary care patients (1422 from five general internal medicine clinics and 1578 from three family practice clinics).

From May 1997 to March 1999, 3636 patients, aged 18 years or older, were approached to participate in the PHQ Obstetrics-Gynecology Study (35). Two hundred forty-five patients declined to participate, and 127 started but did not complete the questionnaire. Data (equivalent of approximately one page) were missing for 264 of these patients, resulting in 3000 subjects from seven obstetrics-gynecology sites. All sites used one of two subject selection methods to minimize sampling bias: either consecutive patients for a given clinic session or every nth patient until the intended quota for that session was achieved. Patient characteristics are summarized in Table 1. Besides being entirely women, the obstetrics-gynecology sample had a younger average age, more Hispanic subjects, lower average education, and less medical comorbidity. Medical comorbidity was assessed by asking physicians to indicate the presence or absence of nine types of physical disorders: hypertension, heart disease, diabetes, liver disease, renal disease, arthritis, pulmonary disease, cancer, or other disorders.

Before seeing the physician, all patients completed the PHQ as well as the Medical Outcomes Study Short-Form General Health Survey (SF-20) (37). The SF-20 measures functional status in six domains (all scores from 0 to 100, where 100 = best health). Patients also reported the number of physician visits and disability days during the past 3 months and provided a global rating of symptom-related difficulty by responding to the following question: "How difficult have these problems made it for you to do your work, take care of things at home, or get along with other people?" Response categories for this global rating are "not difficult at all," "somewhat difficult," "very difficult," and "extremely difficult."

Analysis
The PHQ-15 is intended to function as a continuous measure of somatic symptom severity. For this article the PHQ-15 score was divided into several categories to illustrate more clearly the relationship between graded increases in somatic symptom severity and various health outcomes. The categories were minimal (PHQ-15 score = 0–4), low (score = 5–9), medium (score = 10–14), and high (score = 15–30) levels of somatic symptom severity. These categories were chosen for several reasons. The first was pragmatic: the cutoff points of 5, 10, and 15 are simple for clinicians to remember and apply. The second reason was empiric: using different cutoff points did not noticeably change the associations between increasing PHQ-15 severity and measures of construct validity. The third reason is that patients with the most severe symptoms (score of 15 or higher) constituted approximately 10% of the sample, a prevalence comparable with the lower boundary of prevalence estimates for clinically significant somatization in primary care (7, 8, 11).

The internal reliability of the PHQ-15 was assessed using Cronbach’s . Construct validity of the PHQ-15 as a measure of somatization severity was assessed by examining functional status (the six SF-20 scales), disability days, symptom-related difficulty, and healthcare utilization (clinic visits) over the four PHQ-15 intervals.

The independent effects of somatic symptoms, depressive symptoms, and medical comorbidity on functional status and other outcomes were assessed using stepwise linear regression models. The PHQ has a nine-item depressive symptom severity scale (the PHQ-9) that ranges from 0 to 27. The PHQ-15 score, PHQ-9 score, and number of physical disorders were entered as independent variables in each model, adjusting for age, gender, minority status, education, and study site.

	RESULTS

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Distribution and Reliability of PHQ-15 Scores
Table 2 shows the distribution of PHQ-15 scores. Each sample was roughly divided into thirds, with scores of 0 to 4 representing the lower tertile of somatic symptom severity; 5 to 9, the middle tertile; and 10 or greater, the upper tertile. Within the upper tertile, one-third (or approximately 10% of both samples) had scores of 15 or greater. The internal reliability of the PHQ-15 was excellent, with a Cronbach’s of 0.80 in both the primary care and obstetrics-gynecology samples. The 15 individual symptoms showed moderate associations with one other: the majority of item-item correlations in both samples were in the 0.20-to-0.29 (45%) or the 0.10-to-0.19 range (33%). Only 6% of the item-item correlations were less than 0.10, and 9% exceeded 0.40, with the highest being the correlation between trouble sleeping and fatigue (0.55).

View larger version (52K): [in this window] [in a new window]   Fig. 1. Relationship between somatic symptom severity as measured by the PHQ-15 and decline in functional status as measured by the six subscales of the SF-20. The decrement in SF-20 scores are shown as the difference between each PHQ-15 severity group and the reference group (ie, those with PHQ-15 scores of 0–4). SF-20 scores are adjusted for age, sex, education, and number of physical disorders. Effect size is the difference in adjusted means divided by the pooled standard deviation for that scale.

	DISCUSSION

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Our data establish several desirable psychometric characteristics of the PHQ-15 as a measure of somatic symptom severity. First, the internal reliability of the PHQ-15 was high in both samples. Second, convergent validity was established by the strong association between PHQ-15 scores and functional status, disability days, and symptom-related difficulty. Third, discriminant validity was shown by the differential effects of somatic and depressive symptoms on various outcomes. Fourth, findings from the 3000 primary care patients were replicated in a second sample of 3000 obstetrics-gynecology patients, although the amount of change in various health outcomes with increasing PHQ-15 scores was somewhat less in the obstetrics-gynecology sample. The differences in magnitude of effect are probably due to demographic and comorbidity differences between the two samples, although the possibility of some other unmeasured construct cannot be excluded.

One limitation of the PHQ-15 as a self-administered measure is that it cannot distinguish between medically explained and unexplained symptoms, a distinction that typically requires a directed interview and clinical judgment. The PHQ-15 is therefore best characterized as a measure of somatic symptom severity rather than a diagnostic instrument for somatoform disorders. Patients who have high screening scores on the PHQ-15 should be further questioned to determine which symptoms might be medically unexplained. Unexplained symptom counts are more specific for somatoform disorders and predict adverse health consequences at lower thresholds. Still, total symptom counts (including unexplained and explained) are predictive of somatoform disorders (39, 40) and correlate strongly with psychological distress, functional impairment, and healthcare utilization (5, 14, 17, 41).

Two other study limitations should be noted. Because our sample was disproportionately female, studies involving more male patients are needed to determine the degree to which our findings can be extrapolated to men (42). Also, validity coefficients are based almost exclusively on self-report measures. Although patients are typically the criterion standard for evaluating somatic and depressive symptoms as well as functional status and health-related quality of life, independent measures of healthcare utilization would be desirable in subsequent studies.

The PHQ-15 score functions as a continuous measure. At the same time, scores of 5, 10, and 15 do represent valid and easy-to-remember thresholds demarcating low, medium, and high levels of somatic symptom severity. Moving from a lower to the next higher level of severity typically represented a moderate effect size for all functional status domains. In particular, scores of 15 or higher were associated with considerable impairment and high utilization. The fact that 8% to 10% of patients in the two samples have scores of 15 or greater is notable in that this is also the lower boundary of prevalence estimates for clinically significant somatization in primary care (7–9, 11).

Table 6 shows how the symptom coverage provided by the PHQ-15 compares favorably with other screeners for somatization: the World Health Organization Screener for Somatoform Disorders (22), the somatization scale from the Hopkins Symptom Checklist (23), and two screeners for somatization disorder developed by Swartz et al. (24) and Othmer and DeSouza (25). The PHQ-15 assesses 9 of the 12 WHO items, 7 of the 12 Hopkins items, 8 of the 11 Swartz et al. screen items, and 4 of the 7 Othmer and DeSouza screen items. This concordance rate of the PHQ-15 with the other instruments is superior to that of any two other instruments with one another. Of note, other measures designed to assess particular domains of somatization (eg, hypochondriasis and somatosensory amplification) or to screen for the somatic manifestations of depressive and anxiety disorders also correlate highly with one another (43). Although core diagnostic symptoms for depression, fatigue and sleep complaints are included in the PHQ-15 for several reasons. First, they are also included in one or more other somatization screening measures (Table 6). Second, 40% to 50% of primary care patients with fatigue or sleep complaints do not have a depressive or anxiety disorder diagnosis (44–46).

The association between number of physical disorders and various health outcomes is surprisingly weak. In part this may be because we used a simple count of physical disorders rather than a more sophisticated medical comorbidity measure that can capture the severity as well as number of disorders. Also, it may be only those disorders that are symptomatic (and the severity of those symptoms) that determines impairment. Many physical disorders (eg, hypertension, well-controlled diabetes, and stable coronary artery disease) are minimally symptomatic and thus produce less impairment than mental disorders or symptomatic physical disorders (47, 48). In both our samples, there was only a weak correlation between the number of physical disorders and somatic symptoms. Also, the conventional wisdom that somatic symptoms in medical patients commonly are secondary to the side effects of prescribed medications is challenged by studies showing that symptom prevalence may be equally high in patients receiving placebo or no medication (49–51).

Valid measures for assessing somatization severity are important given the emerging evidence for effective treatments. Two recent critical reviews of the literature have shown that somatizing disorders can respond to antidepressants (96 controlled trials) as well as cognitive-behavioral therapy (31 controlled trials) (20, 21). Although depression also responds to these types of treatment, there may still be reasons for differentiating somatization and depression. First, the benefits of these two treatment modalities in reducing somatic symptoms do not appear to be mediated entirely through amelioration of depressive or other psychological symptoms (20, 21). Second, the majority of antidepressant trials conducted in patients with somatic symptom disorders have focused on specific symptom syndromes rather than patients with multiple unexplained complaints. Third, the cognitions and behaviors targeted in depressed patients receiving cognitive-behavioral therapy may be differ from those emphasized in somatizing patients.

Nonpharmacologic treatments other than cognitive-behavioral therapy, including operant behavioral therapy, relaxation therapy, biofeedback, and problem-solving therapy, may also be beneficial for patients with chronic symptoms, especially pain (52–56). There may be more to offer the somatizing patient than the rather noninterventionist approach shown by Smith et al. (19, 57) to reduce costs with modest to no impact on the patients’ quality of life. In patients with milder or less chronic versions of somatization, even simple measures, such as attention to symptom-specific concerns and expectations, reassurance, and follow-up, may be useful (58–60).

Treatment trials of somatizing patients using the PHQ-15 as an outcome measure are necessary to establish its sensitivity to change. Also, test-retest reliability should be evaluated because it is possible that somatization, like depression, may peak the day of the primary care visit and diminish shortly afterward before treatment can potentially have an effect. Additional somatization measures may be warranted in some trials because patients with multiple somatic symptoms frequently have one or several symptoms that cause greater distress or impairment than the others. Thus, instruments that focus on the predominant symptom(s), such as 1-to-10 severity scales (61) or other symptom-specific measures (62), may complement generic somatization scales such as the PHQ-15 in monitoring treatment outcomes. Meanwhile, our validation data from two studies involving a total of 6000 patients establish the PHQ-15 as a promising measure for identifying patients with potential somatization in clinical practice as well as assessing somatic symptom counts and severity in clinical research.

	APPENDIX

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION APPENDIX ACKNOWLEDGMENTS REFERENCES

 

	ACKNOWLEDGMENTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION APPENDIX ACKNOWLEDGMENTS REFERENCES

Received for publication February 27, 2001.

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TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION APPENDIX ACKNOWLEDGMENTS REFERENCES

 

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