Standard error of measurement and smallest detectable change of the Sarcopenia Quality of Life (SarQoL) questionnaire: An analysis of subjects from 9 validation studies

Objectives
The Sarcopenia Quality of Life (SarQoL) questionnaire, a sarcopenia-specific patient-
reported outcome measure, evaluates quality of life with 55 items. It produces 7 domain
scores and 1 overall quality of life score, all between 0 and 100 points. This study aims to
contribute to the interpretation of the SarQoL scores by calculating the standard error of
measurement (SEM) and smallest detectable change (SDC) in a sample of subjects from 9
studies.
Methods
Subjects from 9 studies (conducted in Belgium, Brazil, Czech Republic, England, Greece,
Lithuania, Poland and Spain) were included. The SEM, a measure of the error in the scores
that is not due to true changes, was calculated by dividing the standard deviation of the dif-
ference between test and retest scores (SDdiff) by p2. The SDC, defined as change beyond
measurement error, was calculated by multiplying SDdiff by 1.96. Bland-Altman plots were
assessed for the presence of systematic errors.
Results
A total of 278 sarcopenic subjects, aged 77.67 ± 7.64 years and 61.5% women, were
included. The SEM for the overall SarQoL score ranged from 0.18 to 4.20 points for the indi-
vidual studies, and was 2.65 points when all subjects were analyzed together. The SDC forhe overall score ranged from 0.49 to 11.65 points for the individual studies, and was 7.35
points for all subjects. The Bland-Altman plots revealed no systematic errors in the
questionnaire.
Conclusion
This study shows that, for individual subjects, a change in overall quality of life of at least
7.35 points (on a scale from 0 to 100) would have to be observed to confirm that a true
change, beyond measurement error, has occurred. It also demonstrated that the SarQoL
questionnaire is a precise instrument, with the observed scores within less than 3 points of
the theoretical “true score”.