Clinical Utility and Patient-Level Analysis of IDDSI Level 1 (Slightly Thick) in Oropharyngeal Dysphagia: A Strategy to Mitigate Over-Thickening and Support Hydration

Background/Objectives: The International Dysphagia Diet Standardisation Initiative (IDDSI) introduced Level 1 (slightly thick) as a new intermediate consistency between thin liquid (Level 0) and mildly thick (Level 2). Although over-thickening is associated with reduced hydration and impaired medication absorption, the specific clinical utility of Level 1 has not been quantified. We aimed to determine the clinical benefit of Level 1 and identify the proportion of dysphagia patients for whom Level 1 is the minimum sufficient consistency. Methods: We retrospectively analyzed 163 consecutive patients undergoing a videofluoroscopic swallowing study (VFSS) in a single rehabilitation department, including a pre-specified stroke subgroup (n = 87). A de-escalation protocol was used (IDDSI Level 3 → 2 → 1 → 0) with stepwise bolus volumes (2–3 mL followed by 5 mL at each consistency), and testing was discontinued at thinner consistencies once aspiration (Penetration–Aspiration Scale [PAS] ≥ 6) was observed. PAS values at untested thinner levels following aspiration were imputed (assigned an assumed value reflecting the expected clinical outcome) as 8 in the primary analysis. We quantified Level 1 effectiveness using absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT). Results: In paired analyses, Level 1 reduced aspiration versus Level 0 by 12.0 percentage points (RRR 31.5%, NNT 8.4) in the full cohort and 13.7 percentage points (RRR 38.5%, NNT 7.3) in the stroke subgroup. Incremental escalation from Level 1 to Level 2 yielded only modest additional benefit (incremental NNT 18.4 and 38.5). Patient-level classification identified 13.7% (full) and 16.5% (stroke) of patients as “Level 1-sufficient”, defined as unsafe on thin liquid but safe on Level 1. Conclusions: IDDSI Level 1 provides substantial aspiration protection compared to thin liquid (NNT ≈ 7–8) with diminishing returns from further escalation, supporting Level 1 as a clinically useful and underutilized first-line thickened liquid option that may help avoid the nutritional and hydration drawbacks of over-thickening.