Assessing non-specific symptoms in epidemiological studies: Development and validation of the Symptoms and Perceptions (SaP) questionnaire.

OBJECTIVES To describe the development of the Symptoms and Perceptions (SaP) questionnaire, a new instrument measuring diversity and severity features and related perceptions of non-specific (physical and psychological) symptoms (NSS) and to test its reliability and validity within an epidemiological setting. METHODS First, a Delphi study was performed to develop the items of the questionnaire. Its psychometric properties were then tested within the context of an epidemiological study. This study yielded questionnaire data and general practice registry data from a sample of 5933 Dutch adults. Principal component analyses (PCA) were performed to test the factorial structure of the symptom assessment subscales of the SaP. Indicators of reliability and convergent validity were examined separately for the questionnaire parts of symptoms and perceptions. Criterion validity of the symptom scores of the SaP was also explored. RESULTS The factorial structure of the symptom subscales reflected meaningful sets of clusters, indicative of the representation of all relevant organ systems. The SaP showed high reliability and good convergent validity for both the symptoms and perceptions part. It was also able to identify, to a certain degree, cases of diagnosed somatoform disorders and multiple NSS in primary care. Additionally, the scores on the "psychological" factor fairly predicted the diagnosis of common mental disorders. CONCLUSIONS The SaP questionnaire is a reliable and valid self-reported measure of diverse features of NSS and corresponding perceptions. It can also be a useful tool for the identification of multiple somatic symptoms and related disorders in general practice and assessment of psychological problems in epidemiological studies. Further validation of the questionnaire in different samples and settings will establish the promising psychometric properties demonstrated in the present study.