60 significant inter-rater differences or differences between tests over a 48 hour period.12 The WST has proven to be an excellent test to help identify patients at risk for dysphagia and aspiration, and can be used to monitor swallowing performance over time.11,16 In order to detect changes that may occur in the WST outcomes after treatment, test-retest reliability is an important test criterion, most often measured with an Intraclass Correlation Coefficient.17 Besides, to interpret repeated measurement scores, it is important to use the Smallest Detectable Change (SDC) scores to determine whether a change in scores is significant and not a measurement uncertainty. The SDC is crucial for clinicians and researchers to determine the real change in repeated measurements for individual patients.18 The reliability of the WST has been tested in patients with motor neuron disease, in which a high inter-rater reliability was found.19 However, to our knowledge, test-retest reliability has not been performed in patients with HNC yet. The purpose of this study was therefore to assess the reliability of the WST in patients with HNC. In order to detect differences in reliability that may occur in a different population, the reliability was tested in healthy subjects as well. Methods Patients were included when they had been diagnosed with oral, oropharyngeal, hypopharyngeal, laryngeal, or unknown primary HNC. Patients were included at the University Medical Center Utrecht (UMCU), the Netherlands, and were referred for either RT, chemoradiotherapy, or surgery, with a curative intent, between September 2016 and June 2018. Patients with recurrent or residual disease, cognitive impairments and patients having trouble understanding and reading the Dutch language were excluded. Healthy subjects could respond to a flyer outside the hospital, and were included when they were 18 years or older. The study protocol for patients with HNC is part of the NET-QUBIC research,20 and was approved by the Medical Ethics Committee of the Netherlands (NL45051.029.13). A random selection of the total NET-QUBIC research (n=154) was taken, and patients were asked before the start of the measurements if they would want to perform the WST twice. The study protocol for healthy subjects was approved by the Medical Ethics Committee of the UMCU (18/701). General information about age, sex, tumor site, tumor stage, and treatment were collected for patients with HNC, and about age and sex for healthy subjects. Before participating, all subjects received oral and written information about the study, before signing written informed consent.
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