We aimed to fine-tuning the Timed Water Swallow Test (TWST) screening procedure to provide the most reliable prediction of the Flexible Endoscopic Evaluation of Swallowing (FEES) assessment outcomes, with age, sex, and the presence of clinical signs of dysphagia being considered in the assessment. Participants were healthy people and patients with suspected dysphagia. TWST performance and participants' reported dysphagia symptoms were assessed in terms of their utility in predicting the outcome of a FEES assessment the same day. The FEES assessors were blinded to the nature of the TWST performance. The water swallowing capacity levels and clinical observations during a screening performance that were indicative of dysphagia/no symptoms in FEES were determined. Convergent validity was assessed as the agreement with the Functional Oral Intake Scale (FOIS) in the FEES assessment. TWST predicted FEES findings (aspiration and dysphagia) with a sensitivity of 72 and 45% and a specificity of 75% and 80%, respectively. Extended analysis of the TWST procedure (eTWST) identified aspiration (sensitivity = 92%, specificity = 62%) and dysphagia (sensitivity = 70%, and specificity = 72%) more accurately and showed a high correlation with FOIS (ɸ = 0.37). Excellent inter-rater reliability was further observed (Kw = 0.83). The extended evaluation of TWST performance has superior criterion validity to that of TWST. eTWST displayed high convergent validity and excellent interrater reliability. We therefore believe that eTWST can be highly relevant for clinical dysphagia screening.