Outcome measures in the lung.

The indolent progression of lung disease in SSc has caused great difficulty in therapeutic studies as outcome measures need to be sensitive. Idiopathic pulmonary fibrosis (IPF) has been more widely studied and can usefully be extrapolated to SSc, but is more rapidly progressive. In IPF, forced vital capacity (FVC) trends are the most accurate serial surrogate for mortality. Serial gas transfer trends have a lower prognostic value in IPF and may be confounded by pulmonary vascular disease in SSc. Unresolved issues include the optimal time interval between pulmonary function tests and the mode of expression of change (percentage change from baseline vs absolute change). It has yet to be determined whether changes in pulmonary function variables are best analysed continuously or categorically (i.e. according to whether a threshold for 'significant change' has been reached). The 6-min walk distance has proved disappointing as an outcome variable due to major inter-test variability over the course of therapeutic studies, ascribable to extra-pulmonary factors. Serial CT is promising in principle but an optimal scoring system has proved elusive. Dyspnoea and quality of life scales provide useful ancillary information as to the likelihood that pulmonary function trends are clinically significant. For the time being, serial change in FVC appears to be the best primary end-point.