Reliability and minimal clinically important differences of forced vital capacity: Results from the Scleroderma Lung Studies (SLS-I and SLS-II).

OBJECTIVES

To assess the reliability and the minimal clinically important differences (MCID) for FVC% predicted in the Scleroderma Lung Study I and II.

METHODS

Using data from SLS I and II (N=300), we evaluated the test-retest reliability for FVC% predicted (FVC%; screening vs. baseline) using intra-class correlation (ICC). MCID estimates at 12 months were calculated in the pooled cohort (SLS-I and II) using 2 anchors: Transition Dyspnea Index (≥change of 1.5 units for improvement and worsening, respectively) and the SF-36 Health Transition question: "Compared to one year ago, how would you rate your health in general now?", where "somewhat better" or "somewhat worse" were defined as the MCID estimates. We next assessed the association of MCID estimates for improvement and worsening of FVC% with patient reported outcomes (PROs) and computer-assisted quantitation of extent of fibrosis (QLF) and of total ILD (QILD) on HRCT.

RESULTS

Reliability of FVC%, assessed at a mean of 34 days, was 0.93 for the pooled cohort. The MCID estimates for the pooled cohort at 12 months for FVC% improvement ranged from 3.0 % to 5.3% and for worsening from -3.0% to -3.3%. FVC% improvement by ≥MCID was associated with either statistically significant or numerical improvements in some PROs, QILD, and QLF, while FVC% worsening ≥MCID was associated with statistically significant or numerical worsening of PROs, QILD, and QLF.

CONCLUSION

FVC% has acceptable test-retest reliability, and we have provided the MCID estimates for FVC% in SSc-ILD based changes at 12 months from baseline in two clinical trials. Clinical trial registration available at www.clinicaltrials.gov, IDs NCT00004563 and NCT00883129.