Our purpose in this study was to determine the applicability, safety, validity, and test-retest and inter-rater reliability of the FTSST when used with critical care survivors from a general ICU at the time of ICU discharge.
As seen, only 30% of ICU survivors could perform the FTSST and 7% of patients presented with adverse events during the FTSST. Both inter-and intra-rater (test-retest) reliability were excellent (ICC > 0.95, p = 0.001) and higher scores were associated with lower muscle strength, longer hospital stay, and greater functional impairment at hospital discharge in adult survivors of critical diseases. Furthermore, a cutoff of 23.5 seconds was predictive of ICU readmission. We advocate that patients with FTSST scores higher than 23.5 seconds may be exposed to considerable risk of ICU readmission and these values should be used to guide physical recovery in order to mitigate such risk. We suggest that this threshold should be tested in another sample of critical care survivors.
Regarding test applicability, it is important to underline that, despite the relatively low median age of our sample, the clinical use of FTSST had to be restricted only to high-functioning post-ICU patients as Melo et al. [9] already reported.
We found the test to be safe with the remaining participants, as demonstrated by hemodynamic and respiratory variables responses (pre- and post-test) and a low rate (7%) of adverse events. Previous studies [9, 10] that used the FTSST in a hospital setting have also reported an absence of important adverse events.
Regarding test reliability, results revealed excellent test-retest and inter-rater reliability, with low percentages of error measurement, as shown by reliability methods, including ICCs and visual inspection of the Altman-Bland plot. This finding is in accordance with other studies that have investigated the test-retest and inter-rater reliability of the FTSST [9, 31,32,33,34]. This high inter-rater and test-retest reliability was deemed possible because of the straightforward test instructions, the researchers’ experience, and the objective nature of the test assessment.
Our study is the first to investigate the concurrent and predictive validity of the FTSST in patients at ICU discharge. The study of its clinical applicability through the safety, validity, and reliability of measurements is essential to allow a more accurate and earlier analysis of the functional status of critical care survivors. Our results suggest that the FTSST performance at ICU discharge is a clinically valid measure for determining muscle performance and functional status at hospital discharge. It was clear that patients with higher FTSST scores tended to show less peripheral muscle strength as evidenced by lower handgrip strength and lower functional scores in many FIM domains (mobility and transfers, locomotion, cognition, self-care and global score) at hospital discharge. These findings are important as the early performance of functional status evaluation may identify patients who are at higher risk for functional decline after hospital discharge and possible increased post-discharge mortality [35].
A recent study developed by Mayer et al. [10] has shown that the FTSST is highly associated with muscle strength in critical survivors and therefore could be used to reflect muscle dysfunction and physical disability at ICU discharge. Since instruments to assess muscle function generally are not available at bedside, the FTSST seems to be a practical and reproducible option to record muscle strength in hospitalized patients. Moreover, it is known that the FTSST has already been associated with reduced strength in the lower limbs [11], balance control [13], risk of falls [12] in older subjects, and risk of fall and functional recovery management in a hospital setting.
Interestingly, the FTSST performance in our study showed a moderate correlation with hospital length of stay and a moderate correlation with ICU length of stay and duration of ventilatory support. In other words, survivors of critical illness with lower scores at ICU discharge apparently tended to stay longer in the hospital, possibly due to greater physical fragility. Ventilatory support and ICU length of stay are suggested as potential contributors of functional deterioration as stated in previous studies [36,37,38,39].
Even though previous authors have found a strong association between clinical severity scores at admission and physical impairment [40], this association was not strong in our study, probably due to the severity profile of the included patients.
The present study is the first investigation of the FTSST applicability in a sample of survivors of critical disease at ICU discharge other than older subjects. The study findings suggested that the FTSST appears to be a safe, valid, reliable, and straightforward resource with the potential to contribute to the evaluation of the functional status performance of critical care survivors. The application of FTSST in this critical setting may also be useful to identify patients at risk of low functional recovery and falls during the hospital stay and after hospital discharge so as to guide the rehabilitation treatment.
Further studies are required to investigate the relationship between FTSST scores and short- and long-term outcomes, including hospital readmission rate and mortality. The main limitations of this study are the use of a convenience sample and a single center recruitment experience, as well as the relatively low median age of the patients included.