Validity and responsiveness of GHC-index in patients with amalgam-attributed health complaints

Objective Many patients have medically unexplained physical symptoms (MUPS); some of them attribute their health complaints to dental amalgam fillings. The aim of this study was to assess the validity and responsiveness of General Health Complaints index (GHC-index) for measuring the symptom load in MUPS patients compared to the widely used symptom outcome measure, Giessen Subjective Complaints List (GBB-24). Methods Three outcome measures - GHC-index, GBB-24, and Munich Amalgam Scale (MAS) - were administered at baseline and 12 months after removal of all dental amalgam restorations. The validity and responsiveness of these symptom measures were tested against external anchors: bodily distress syndrome (BDS), SF-36 vitality, and visual analogue scale (VAS). We tested both convergent and known group validities. We also examined the predictive validity and responsiveness to changes for each instrument. Results All the main outcome measures showed evidence of convergent and known group validities. The GHC-index, GBB-24 and MAS were all able to detect the anticipated differences in BDS and Energy. But the GBB-24 was more efficient in discriminating the BDS compared with the GHC-index (relative efficiency: RE = 0.69; 95% CI: 0.41-0.96) and MAS (RE = 0.59; 95% CI: 0.32-0.86). Each main outcome variable revealed good predictive validity for vitality (standardized coefficient: b approximate to 0.71 and R-2 approximate to 0.50). Moderate to high sensitivity to change over time was demonstrated, with GHC-index performing better. Conclusion The GHC-index is a valid and responsive instrument for assessing symptom load in MUPS patients attributing their health complaints to amalgam fillings and undergoing amalgam removal.