Construct validity of the Functional Status Score for the Intensive Care Unit: a prospective observational study using actigraphy in mechanically-ventilated patients

Abstract

Background The Functional Status Score for the Intensive Care Unit (FSS-ICU) is a widely used instrument, designed to measure the mobility of critically ill patients that is available in English, Portuguese, Turkish and Spanish. The lack of a gold standard for measuring mobility limits the validation of outcome measures. However, actigraphy, from which measures movement intensity can be derived, is a potential alternative in the intensive care unit (ICU). Thus, this study aimed to assess the construct validity (hypotheses testing) of the FSS-ICU using continuous actigraphy from ICU admission to ICU discharge. Methods This prospective observational study included mechanically-ventilated patients selected from a 12-bed academic medical-surgical ICU. The Chilean-Spanish version of the FSS-ICU was used to evaluate its correlation with activity counts, activity time (>99 counts per minute) and inactivity time (0-99 counts per minute) measured by actigraphy ( GT9X Link ActiGraph). The FSS-ICU was assessed on awakening and at ICU discharge, while actigraphy variables were recorded from ICU admission to ICU discharge. Results Of 92 patients screened, 30 were analysed. The median FSS-ICU was 19 (IQR 10–26) points on awakening and 28.5 (IQR 22–32) at ICU discharge. There was no floor/ceiling effect of the FSS-ICU at awakening (0%/0%) and only a ceiling effect at ICU discharge that was acceptable (0%/10%). Less activity time was associated with better mobility on the FSS-ICU at both awakening (rho = -0.62, P<0.001) and ICU discharge (rho = -0.79, P<0.001). Activity counts and activity time were not correlated as expected with the FSS-ICU. Conclusions The FSS-ICU had a strong correlation with inactivity time during the ICU stay. These findings enhance the available clinimetric properties of the FSS-ICU.