Measuring Viscosity and Consistency in Thickened Liquids for Dysphagia: Is There a Correlation Between Different Methods?

Dysphagia is a common clinical condition, especially among older adults, associated with an increased risk of malnutrition, aspiration, and respiratory complications. A key therapeutic approach involves modifying liquid consistency using thickening agents to achieve safer swallowing. Although rotational rheometry offers accurate viscosity characterization, its complexity and cost limit routine application in clinical or domestic settings. This study evaluates and correlates different methods for measuring the viscosity of thickened liquids, comparing rheological data with empirical techniques such as the Ford cup, Bostwick consistometer, and Line-Spread Test (LST). Several thickeners were tested-guar gum, xanthan gum, a guar/xanthan blend, maltodextrin-based mixtures, and a commercial thickener-across a range of concentrations, temperatures, and preparation times. The results demonstrate that simple methods, particularly the Bostwick consistometer and LST, show strong correlations with rheometer measurements within the International Dysphagia Diet Standardisation Initiative (IDDSI) Level 2 (mildly thick) and Level 3 (moderately thick) ranges. However, limitations were observed at extreme viscosities, where certain methods lacked sensitivity or operational feasibility. These findings support the potential of empirical tools for practical viscosity screening in dysphagia management, especially where rheometry is unavailable. This work provides evidence-based guidance for clinicians, caregivers, and food service professionals seeking safe, reproducible, and standardized approaches to fluid consistency assessment.