Patient Health Questionnaire 15 as a generic measure of severity in fibromyalgia syndrome: Surveys with patients of three different settings
Introduction
The definition and content of fibromyalgia syndrome (FMS) have changed repeatedly in the last 100 years [1]. The most important change was the requirement for multiple tender points and chronic widespread pain that arose from the fibromyalgia classification criteria of the American College of Rheumatology [2]. By 2010, a second shift occurred with the preliminary American College of Rheumatology (ACR) diagnostic criteria [3] and the research criteria of fibromyalgia [4] that excluded tender points and placed reliance on patient-reported symptoms with chronic widespread pain, fatigue and cognitive difficulties (‘fibro fog’) as main and abdominal pain, depression and headache as minor symptoms. The new diagnostic criteria [3], [4] indexed FMS into functional somatic syndromes (FSS), which are defined by a typical cluster of chronic somatic symptoms and the exclusion of somatic diseases sufficient to explain the symptoms [5]. FSS are frequently associated with each other (e.g. FMS and irritable bowel syndrome) and with anxiety and depressive disorders [6], [7].
Recent evidence-based guidelines on FSS [8] and on FMS [9], [10] recommended a graduated treatment approach based on severity. Clinical criteria for severity are based on the amount of somatic and psychological distress, disability and health care use [8], [9], [10]. However, the lack of an internationally accepted instrument for severity grading of FSS and FMS is one major obstacle in their definition and management [10], [11]. The Polysymptomatic Distress Scale (PSD) [4] and the Fibromyalgia Impact Questionnaire (FIQ) [12] have been proposed as disease-specific measures of FMS-severity. However, cut-off values of the PSD have not been determined and the FIQ is difficult to analyze in routine clinical care.
The Patient Health Questionnaire (PHQ 15) is an easy to use measure of somatic symptom intensity. It provides cut-off scores for mild, moderate and severe somatic intensity. In 6000 unselected primary care patients, higher PHQ-15 scores were strongly associated with worsening function on all six Short Form Health Survey-20 scales as well as increased disability days and health care utilization [13], [14]. Recently, population-based cross-sectional studies demonstrated that the total somatic symptom score measured by the PHQ 15 was a valid predictor of health status and healthcare use over and above the effects of anxiety, depression and general medical diseases [15]. The PHQ 15 has been proposed for the grading of severity of somatic symptom disorder (SSD) by the Diagnostic and Statistical Manual of Psychiatric Diseases DSM V [16]. Therefore the total score of the PHQ 15 might be suited as a generic measure of overall severity of patients diagnosed with FSS, including those with FMS.
We tested if the cut-off scores for mild, moderate and severe somatic intensity measured by PHQ 15 provide a valid grading of overall severity of FMS. We hypothesized that FMS-specific measures of severity such as number of pain sites and fatigue as well as psychological distress and disability would increase with PHQ 15-defined severity (discriminant validity) and that these findings could be demonstrated in patients of different settings and countries (transcultural validity).
Section snippets
Patients and settings
We analyzed the data of three different settings:
- a.
FMS-cases within a cross-sectional survey of the general German population conducted between May and June 2008 [17].
- b.
2012 survey of the US National Data Bank of Rheumatic Disease (NDB) longitudinal study of rheumatic diseases outcomes [18]. Participants were volunteers, recruited primarily from the practices of US rheumatologists, who complete mailed or Internet questionnaires at 6-month intervals. They were not compensated for their participation.
Sample composition
The initial sample from the German population consisted of 4064 subjects of whom 2524 (62.1%) fully participated. Reasons for dropout included the following: three unsuccessful attempts to contact the household or selected household member (7.7%); the household or selected household member disagreed to participate (15.8%); and the household member was on a holiday break (4.1%). Furthermore 1.2% of the participants were excluded because they were not able to follow the interview because of
Discussion
The cut-off scores of mild, moderate and severe somatic symptom intensity of PHQ 15 provided a valid generic measure of the overall severity in persons meeting FMS-criteria. Somatic and psychological distress and disability increased with PHQ 15-defined FMS severity (discriminant validity). The results were consistent in persons meeting FMS-criteria of different settings (general population, patient data bank, clinical institution) and countries (Germany, USA) (transcultural validity).
In a
Conflict of interest
The authors of this article report no conflict of interest.
Funding source
The study was conducted without external funding.
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