Construct validity evaluation of the European Scleroderma Study Group activity index, and investigation of possible new disease activity markers in systemic sclerosis

Objectives. To evaluate the construct validity of the European Scleroderma Study Group (EScSG) activity index and to propose modifications if necessary. Methods. One hundred and thirty-one consecutive patients were investigated and re-evaluated 1 year later. Modified Rodnan skin score (MRSS), skin ulcers and joint contracture numbers, hand anatomic index (HAI), BMI, spirometry, carbon monoxide diffusing capacity (DLCO), left ventricular ejection fraction, pulmonary arterial hypertension, HAQ Disability Index (HAQ-DI), patient skin self-assessment questionnaire and several biomarkers were recorded, in addition to the data required for the EScSG activity index. Statistical analysis was performed by categorical principal component analysis (CATPCA). Results. The EScSG activity index appeared in the same dimension as the HAQ-DI, ulcer score and joint contractures, MRSS, patient-reported skin score and HAI by CATPCA. Parameters of lung involvement appeared in another dimension. We constructed a 12-point activity index that was equally associated with both dimensions, by adding the forced vital capacity/DLCO, change in DLCO, change in the ulcer scores, HAQ-DI and patient-reported skin score. Biomarkers including vascular endothelial growth factor, soluble P-selectin glycoprotein ligand-1, CRP and albumin were related to both the EScSG and the 12-point index, though they did not improve the total variance of the model. Conclusion. The construct validity of the EScSG activity index is good, though the lung-related disease activity may not be sufficiently represented. Further validation steps may be required for both the EScSG and our 12-point activity index.