Background
The
utility-weighted modified Rankin Scale (UW-mRS) is an outcome measure
recently proposed to improve statistical efficiency and interpretability
of the mRS. Statistical properties of the UW-mRS have been well
investigated, but construct validity has yet to be established.
Aims
To
investigate the construct validity of the UW-mRS as a primary outcome
measure by assessing variability in utility values within and between
mRS categories, over time post-stroke, and by different derivation
methods.
Methods
UW-mRS was derived using assessment of quality of life (AQoL-4D) and mRS scores at 3 and 12 months (
n = 2030)
from a large randomized controlled trial, A Very Early Rehabilitation
Trial (AVERT). Receiver operator characteristic (ROC) analysis of
AQoL-4D was conducted to differentiate between sequential mRS
categories. Intraclass correlation was used to explore variability in
utility values over time post-stroke, UW-mRS values, and derivation
methods from multiple studies.
Results
UW-mRS
values for mRS categories 0–6 at three months were 0.80, 0.78, 0.63,
0.37, 0.11, 0.03, and 0. Based on AQoL-4D utility values, areas under
the ROC curve varied from 0.54 to 0.87. Time post-stroke explained
42%–56% of variability in AQoL-4D utility values in patients with no
change in mRS between 3 and 12 months. The choice of the derivation
method contributed to 25% of the variability in UW-mRS values.
(Whatever this gobbledegook means. Obviously not meant for layperson survivors. If you can't explain this to normal people you have failed in your research.)
Conclusions
The
high variability in utility values between and within mRS categories,
over time post-stroke, and using different derivation methods is not
adequately reflected in the UW-mRS. These threats to construct validity
warrant caution when using UW-mRS as a primary outcome measure.
Trial registration
Australian New Zealand Clinical Trials Registry (ACTRN12606000185561
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