Objectives: Determining the minimal clinically important difference (MCID) of questionnaires on an interval scale, the trait level (TL) scale, using item response theory (IRT) models could overcome its association with baseline severity. The aim of this study was to compare the sensitivity (Se), specificity (Sp), and predictive values (PVs) of the MCID determined on the score scale (MCID-Sc) or the TL scale (MCID-TL).
Study design and setting: The MCID-Sc and MCID-TL of the MOS-SF36 general health subscale were determined for deterioration and improvement on a cohort of 1,170 patients using an anchor-based method and a partial credit model. The Se, Sp, and PV were calculated using the global rating of change (the anchor) as the gold standard test.
Results: The MCID-Sc magnitude was smaller for improvement (1.58 points) than for deterioration (-7.91 points). The Se, Sp, and PV were similar for MCID-Sc and MCID-TL in both cases. However, if the MCID was defined on the score scale as a function of a range of baseline scores, its Se, Sp, and PV were consistently higher.
Conclusion: This study reinforces the recommendations concerning the use of an MCID-Sc defined as a function of a range of baseline scores.
Keywords: Item response theory; Minimal clinically important difference; Patient-reported outcomes; Questionnaires; Rasch models; Sensitivity and specificity.
Copyright © 2014 Elsevier Inc. All rights reserved.