Highlight
– The PHQ-15 and SSS-8 are widely used tools to quantify somatic symptom burden in clinical and general populations.
– Both instruments demonstrate robust internal consistency and correlate moderately to strongly with other measures of somatic symptoms.
– Factor analysis reveals multidimensional constructs including specific symptom groups and a general burden factor.
– Evidence on longitudinal measurement properties such as test-retest reliability and responsiveness is less conclusive, requiring further research.
Study Background and Disease Burden
Somatic symptoms—defined as bodily complaints that may or may not be linked to identifiable medical conditions—frequently present in health care settings. They can profoundly affect quality of life, increase health care utilization, and challenge clinical management, especially when symptoms are medically unexplained or chronic. Accurately assessing somatic symptom burden through patient-reported outcomes is essential for diagnosis, monitoring, and intervention evaluation. Among several instruments, the Patient Health Questionnaire-15 (PHQ-15) and the Somatic Symptom Scale-8 (SSS-8) are prominent. Both scales are brief, patient-administered, and designed to quantify overall physical symptom load, facilitating screening and research across diverse populations.
Study Design
The systematic review and meta-analysis by Hybelius et al. (2024) synthesized evidence from 305 studies involving a cumulative sample of 361,243 participants. The search was comprehensive, sourcing data from Medline, PsycINFO, and Web of Science up to February 1, 2024. Included studies, published in English, reported on psychometric properties such as factor analysis, internal consistency, construct validity, cutoffs, area under the receiver operating characteristic curves (AUROCs), minimal clinically important difference (MCID), test-retest reliability, and sensitivity to change for the PHQ-15 and SSS-8. The populations studied spanned routine clinical care, general population cohorts, and specialized groups. Data extraction involved independent rating and pooled analyses using random-effects meta-analytic methods, with quality assessment employing three validated instruments. Reporting adhered to PRISMA 2020 guidelines, reinforcing methodological transparency and rigor.
Key Findings
The meta-analysis illuminated several psychometric dimensions of the PHQ-15 and SSS-8:
- Factor Structure: Both instruments demonstrated multifactorial constructs. Factor analysis confirmed domain-specific symptom clusters—cardiopulmonary symptoms, fatigue, gastrointestinal complaints, and pain—alongside a unifying general symptom burden factor. This underscores the complexity and heterogeneity in somatic symptom presentation.
- Internal Consistency: The pooled Cronbach’s alpha for the PHQ-15 was 0.81 (95% CI: 0.80–0.82), indicative of good reliability. However, certain items—menstrual problems, fainting spells, and sexual problems—exhibited low item-total correlations (<0.40), suggesting variability in item performance or relevance across populations. The SSS-8 had a comparable pooled alpha of 0.80 (95% CI: 0.77–0.83).
- Construct Validity: Correlations with other validated somatic symptom measures were robust—0.71 (95% CI: 0.64–0.78) for PHQ-15 and higher for SSS-8 at 0.82 (95% CI: 0.72–0.92)—supporting convergent validity.
- Discriminative Ability: AUROC values for identifying somatoform disorders ranged 0.63–0.79 for PHQ-15 and 0.71–0.73 for SSS-8, reflecting moderate diagnostic performance. This is clinically meaningful, although the moderate AUROC suggests that these scales should be part of a broader diagnostic assessment rather than stand-alone tools.
- Minimal Clinically Important Difference (MCID): Both scales shared an MCID of 3 points, providing practical thresholds for meaningful change in clinical or research contexts.
- Test-Retest Reliability: Data were heterogeneous and could not be meta-analyzed. PHQ-15 showed test-retest correlations ranging from 0.65 to 0.93 and an intraclass correlation coefficient (ICC) of 0.87. For SSS-8, very high reliability was reported (r = 0.996, ICC = 0.89). These results suggest acceptable stability; however, inconsistency in PHQ-15 calls for cautious interpretation.
- Sensitivity to Change: PHQ-15 demonstrated tentative sensitivity (effect size, g = 0.32), indicating capacity to detect clinically relevant symptom fluctuations. Evidence for the SSS-8 in this domain was insufficient, highlighting a key evidence gap.
Expert Commentary
The findings affirm the PHQ-15 and SSS-8 as reliable and valid measures for somatic symptom burden, with widespread applicability in routine and research settings. The multidimensional factor structures reflect clinical reality, wherein somatic symptoms span multiple physiological domains and challenge uni-dimensional conceptualizations. Low item-total correlations for select items highlight the need for contextual and cultural considerations in interpretation and possibly tailored modifications. The moderate AUROCs emphasize that these scales should complement, not replace, clinical judgment and additional diagnostic workup.
Limitations include sparse longitudinal evidence, particularly on sensitivity to change for the SSS-8 and inconsistent test-retest data for the PHQ-15, which must be addressed by future prospective studies. The review’s breadth is a strength, but heterogeneity across studies in populations and methodologies may limit generalizability. Clinicians and researchers should apply these tools within a broader biopsychosocial framework.
Conclusion
This extensive systematic review and meta-analysis consolidates the evidence base for the PHQ-15 and SSS-8 scales in measuring somatic symptom burden. Both instruments exhibit good internal consistency, construct validity, and clinical utility. Recognition of their complex, multifactorial measurement structures is essential for nuanced interpretation. Further research is warranted to strengthen evidence concerning longitudinal reliability and responsiveness, particularly for the SSS-8. Integration of these tools into clinical workflows may enhance symptom monitoring and patient-centered care, ultimately improving outcomes for individuals afflicted with somatic symptom disorders.
References
Hybelius J, Kosic A, Salomonsson S, et al. Measurement Properties of the Patient Health Questionnaire-15 and Somatic Symptom Scale-8: A Systematic Review and Meta-Analysis. JAMA Netw Open. 2024;7(11):e2446603. doi:10.1001/jamanetworkopen.2024.46603
Kroenke K, Spitzer RL, Williams JB. The PHQ-15: validity of a new measure for evaluating the severity of somatic symptoms. Psychosom Med. 2002;64(2):258-266.
Moss-Morris R, Petrie KJ, Horne R, et al. The Revised Illness Perception Questionnaire (IPQ-R). Psychol Health. 2002;17(1):1-16.
Löwe B, Hahn SR, Mundt C, et al. Responsiveness of the Somatic Symptom Scale-8 (SSS-8) and the Patient Health Questionnaire-15 (PHQ-15) in primary care patients. Psychosom Med. 2020;82(2):141-149.
