69 4 Discussion The aim of this study was to determine the reliability of the WST for patients with HNC and healthy subjects. The results showed moderate to excellent reliability for all measures (ICC>0.70). The SEM values for patients with HNC were 0.28 (number of swallows), 2.24 (duration (s)), 4.83 (swallowing volume (mL)), 1.59 (swallowing capacity (mL/s)), and 0.20 (swallowing speed (s)), which are small considering the range of outcome possibilities. The SDC values were 0.79 (number of swallows), 6.21 (duration), 13.38 (swallowing volume), 4.40 (swallowing capacity), and 0.55 (swallowing speed), indicating that the outcomes of the WST have to change with at least these values before the observed change over time can be considered a true change in swallowing function and not potentially the result of a measurement uncertainty. The Bland-Altman plots show that 95% of the measures lie between the upper and lower LoA with a consistent variability. In all measures except swallowing speed, the SEM% and SDC% values were lower in healthy subjects in comparison to patients with HNC, indicating the importance of calculating these values for a specific population. In previous research, no significant differences were found in swallowing speed between the first and fourth test over a 48-hour period.12 This is in correspondence to the results found in this research, where there is a high reliability between the first and second test, over a 2-hour period. Swallowing speed is correlated with age, as found in this research.12,14,19 However, previous research is inconclusive about the correlation between swallowing speed and sex: although most research found a correlation,12,14,19 this was not always the case (including this research).13 With increasing age,13 speed decreases while time per swallow increases, and speed is most often lower in women in comparison to men. In addition, volume per swallow and swallowing capacity are greater in men.19 This is in correspondence to the results found in this research, where swallowing volume is correlated to sex. In addition, in this research, a significant effect for age on number of swallows, duration, and swallowing capacity was found, and an effect for sex on number of swallows. Strengths and limitations This study followed the COSMIN checklist (Consensus-based Standards for the selection of health Measurement INstruments) to ensure methodological and statistical quality, and to reduce bias.28 A large research population was used to test the reliability, and data were collected by the same author (JAV). However, only 3 female patients with HNC were tested, making it possible to have missed sex effects in this population. Therefore, the results found on sex differences between men and women should be tested again in a larger population. Because there were significant differences in age and sex between patients with HNC and
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