Clinical Utility and Patient-Level Analysis of IDDSI Level 1 (Slightly Thick) in Oropharyngeal Dysphagia: A Strategy to Mitigate Over-Thickening and Support Hydration.
IDDSI Level 1 (slightly thick) 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.
Key Findings
Results
IDDSI Level 1 reduced aspiration risk compared to Level 0 (thin liquid) by 12.0 percentage points in the full cohort.
Full cohort included 163 consecutive patients undergoing videofluoroscopic swallowing study (VFSS) in a single rehabilitation department.
Absolute risk reduction (ARR) of 12.0 percentage points for aspiration (PAS ≥ 6) when comparing Level 1 to Level 0.
Relative risk reduction (RRR) was 31.5% in the full cohort.
Number needed to treat (NNT) was 8.4 in the full cohort.
A de-escalation protocol was used (IDDSI Level 3 → 2 → 1 → 0) with stepwise bolus volumes of 2–3 mL followed by 5 mL at each consistency.
Results
IDDSI Level 1 reduced aspiration risk compared to Level 0 by 13.7 percentage points in the stroke subgroup.
The stroke subgroup comprised 87 patients, a pre-specified subset of the full cohort of 163.
ARR was 13.7 percentage points in the stroke subgroup.
RRR was 38.5% in the stroke subgroup.
NNT was 7.3 in the stroke subgroup, indicating stronger benefit than the full cohort.
Results
Incremental escalation from IDDSI Level 1 to Level 2 yielded only modest additional aspiration protection.
Incremental NNT for escalating from Level 1 to Level 2 was 18.4 in the full cohort.
Incremental NNT for escalating from Level 1 to Level 2 was 38.5 in the stroke subgroup.
These values contrast with the NNT of 8.4 (full) and 7.3 (stroke) for Level 1 versus Level 0, indicating diminishing returns from further thickening.
Results suggest a pattern of diminishing marginal benefit with each successive escalation step beyond Level 1.
Results
A clinically meaningful proportion of dysphagia patients were classified as 'Level 1-sufficient,' meaning Level 1 was their minimum safe consistency.
'Level 1-sufficient' was defined as unsafe on thin liquid (Level 0) but safe on Level 1.
13.7% of the full cohort (163 patients) met the Level 1-sufficient classification.
16.5% of the stroke subgroup (87 patients) met the Level 1-sufficient classification.
This classification used patient-level analysis to identify individuals who would not require escalation to Level 2 or higher.
Methods
The study used a retrospective de-escalation VFSS protocol with imputation of PAS scores at untested thinner consistency levels.
163 consecutive patients were included from a single rehabilitation department; stroke subgroup n = 87.
Testing was discontinued at thinner consistencies once aspiration (PAS ≥ 6) was observed.
PAS values at untested thinner levels following aspiration were imputed as 8 (the maximum score) in the primary analysis, reflecting 'the expected clinical outcome.'
Bolus volumes were 2–3 mL followed by 5 mL at each consistency level.
The de-escalation sequence was IDDSI Level 3 → 2 → 1 → 0.
Background
Over-thickening is associated with reduced hydration and impaired medication absorption, providing the clinical rationale for investigating Level 1 as a first-line option.
The authors note that over-thickening is a recognized clinical concern linked to reduced hydration.
Impaired medication absorption is also cited as a drawback of unnecessary thickening.
IDDSI Level 1 was introduced as a new intermediate consistency between thin liquid (Level 0) and mildly thick (Level 2) but its clinical utility had not been previously quantified.
The study frames Level 1 as a potential strategy to mitigate over-thickening while still providing aspiration protection.
Conclusions
The authors conclude that IDDSI Level 1 is a clinically useful and underutilized first-line thickened liquid option with substantial aspiration protection.
NNT of approximately 7–8 for Level 1 versus thin liquid was characterized as representing 'substantial aspiration protection.'
The authors explicitly describe Level 1 as 'underutilized' in current clinical practice.
The findings support Level 1 as a first-line option that may help avoid nutritional and hydration drawbacks of escalating directly to Level 2 or higher.
Diminishing returns from escalation beyond Level 1 (incremental NNT 18.4–38.5) support this recommendation.
What This Means
This research suggests that a slightly thickened liquid consistency — classified as IDDSI Level 1 — offers meaningful protection against unsafe swallowing (aspiration) in people with dysphagia (swallowing difficulties), particularly those who have had a stroke. The study analyzed 163 patients who underwent a specialized X-ray swallowing test and found that moving from plain thin liquid to Level 1 reduced the chance of dangerous aspiration by about 12–14 percentage points, meaning roughly 1 in 7–8 patients directly benefited from this step. Importantly, thickening liquids further — from Level 1 to the next level up (Level 2, mildly thick) — provided much smaller additional benefit, requiring treatment of 18 to 38 patients to prevent one additional aspiration event.
The study also found that approximately 14–17% of dysphagia patients were 'Level 1-sufficient,' meaning Level 1 was the thinnest consistency they could safely swallow without aspiration, and they did not need thicker fluids. This is clinically relevant because thicker liquids are harder to drink, less palatable, and are associated with dehydration and problems with absorbing medications. Currently, many patients are prescribed thicker consistencies (Level 2 or above) when Level 1 might be sufficient, a practice the authors describe as 'over-thickening.'
This research suggests that IDDSI Level 1 is an underused option that could improve quality of life and hydration for a meaningful subset of dysphagia patients by avoiding unnecessarily thick liquids. Clinicians conducting swallowing assessments may benefit from routinely testing Level 1 as a distinct consistency step rather than skipping directly to higher thickening levels. The findings are based on a retrospective single-center study, and some assumptions were required when patients could not complete all testing steps, so further research in larger, prospective settings would help confirm these results.
Kim J, Choi S, Lee J, Jang S, Han S, Park J, et al.. (2026). Clinical Utility and Patient-Level Analysis of IDDSI Level 1 (Slightly Thick) in Oropharyngeal Dysphagia: A Strategy to Mitigate Over-Thickening and Support Hydration.. Nutrients. https://doi.org/10.3390/nu18162703