Dysphagia Knowledge Hub — 吞嚥困難知識庫

IDDSI Level 1 (Slightly Thick): Clinical Guide to Flow Testing, Allowed Foods, and Prescribing Criteria

What Is IDDSI Level 1?

IDDSI Level 1, designated Slightly Thick, is the first step of liquid modification above normal thin liquids. It flows more slowly than water but is barely detectable in texture by most patients. In the IDDSI framework, Slightly Thick sits between Level 0 (Thin) and Level 2 (Mildly Thick), and occupies a narrow but clinically meaningful viscosity range.

Level 1 is the smallest clinical intervention in liquid management. Many patients prescribed Level 1 report that the liquid “tastes normal” or “just feels slightly heavier.” This is both its advantage — improved patient acceptance — and its risk, because caregivers unfamiliar with IDDSI may underestimate how precisely this level must be prepared and tested.

Syringe Flow Test: Level 1 Values

The IDDSI 10-second syringe flow test is the primary objective verification method for liquid Levels 0–2.

Test procedure:

Use a clean, standard 10 mL catheter-tip syringe.
Draw exactly 10 mL of the liquid at serving temperature.
Hold the syringe vertically, tip pointing downward.
Release the plunger and allow the liquid to flow freely for exactly 10 seconds.
Read the volume remaining in the barrel.

Level 1 result: 1–4 mL remaining after 10 seconds.

If 0 mL remains, the liquid is Level 0 (Thin) — too fast. If more than 4 mL remains, it is Level 2 (Mildly Thick) — too slow. The window is narrow and temperature-sensitive. Always test at serving temperature, as cooling increases viscosity and may push a compliant Level 1 liquid into Level 2 range.

Clinical Rationale: Who Benefits from Level 1?

Level 1 is prescribed for patients with mild, selective swallowing impairment — specifically those who can manage most liquids safely but show evidence of thin-liquid aspiration at the margins of their swallowing capacity:

Post-neurological insult with partial recovery. Patients recovering from mild stroke, transient ischaemic attack, or peripheral nerve injury may retain near-normal pharyngeal function but show delayed swallow trigger or mild laryngeal closure impairment. The slight resistance of Level 1 provides a fraction of additional time for the swallow reflex to complete before the bolus clears the pharynx.

Fatigue-associated dysphagia. Patients with motor neuron disease (ALS/MND), myasthenia gravis, or Parkinson’s disease may swallow safely in the morning but aspirate on thin liquids by afternoon as neuromuscular fatigue accumulates. Level 1 during vulnerable periods can reduce this risk without restricting the patient to more viscous levels throughout the day.

Paediatric thin-liquid aspiration. In children with mild neurological dysphagia, Level 1 may be selected as the least restrictive safe modification when thin liquids are documented to be aspirated.

Transition level during rehabilitation. Patients being stepped down from Level 2 to Level 0 during dysphagia recovery may use Level 1 as an intermediate step confirmed by repeat instrumental assessment.

Level 1 should not be prescribed without a formal swallowing assessment. It is not a universal precautionary measure — it carries real risks of under-thickening and inconsistent preparation.

Allowed and Excluded Items at Level 1

Level 1 is exclusively a liquid level. The IDDSI framework does not assign food textures to this tier — all texture modification for solid and semi-solid food begins at Level 3 and above.

Liquids that typically test at Level 1 (by formulation or preparation):

Commercially thickened beverages formulated to Level 1
Water or juice with a small measured dose of powder or gel thickener
Certain nutritional supplement drinks when mildly warmed (verify by syringe test)

Liquids that do NOT naturally reach Level 1 without thickener:

Plain water, fruit juice, cordial, tea, coffee — all are Level 0
Standard milk (typically Level 0 or borderline 0/1 — test required)
Sports drinks and electrolyte solutions — Level 0

Items to exclude:

Unverified thickener-to-liquid ratios
Liquids prepared at one temperature and served at another without re-testing
“Naturally thick” claims for drinks such as smoothies or nectars — these vary widely and must be tested individually

Common Caregiver Errors at Level 1

Rounding up from Level 0 without testing. Caregivers sometimes add a small amount of thickener and assume it has reached Level 1 without performing the syringe test. Under-thickening is the most frequent error at this level.

Using the wrong syringe type. The IDDSI test requires a standard 10 mL syringe. Using a wider-barrel or Luer-lock design with different internal friction produces different results and is not a valid test.

Inconsistent thickener brand substitution. Different commercial thickener brands (starch-based vs. xanthan gum-based) reach viscosity targets at different doses. Switching brands without recalculating the dose is a common source of error. Xanthan gum-based thickeners are generally more temperature-stable than starch-based products.

Preparing liquids too far in advance. Starch-based thickeners continue to thicken over time and may push a freshly prepared Level 1 liquid into Level 2 range after 20–30 minutes. Prepare close to serving time or verify with a second syringe test.

Assuming hot drinks thin out uniformly. Hot tea or coffee prepared with thickener will thin slightly when very hot, then thicken as they cool. Both extremes may shift the liquid out of the Level 1 range. Test at the intended serving temperature.

HK and International Food and Drink Examples

Common HK drinks at or near Level 1 with thickener:

Thickened pu-erh or oolong tea — popular in HK elderly care settings; requires careful thickener dosing
Thickened soy milk — commonly used in community care; soy milk itself is Level 0 without thickener
Thickened warm water — frequently the first liquid prescribed post-stroke in HK hospitals

International examples:

Thickened apple juice or cranberry juice — commonly used in residential care internationally
Thickened oral nutritional supplements (e.g., Ensure, Fortisip at diluted concentrations) — check manufacturer’s IDDSI designation; some products require additional thickener to reach Level 1
Thickened sports rehydration drinks — used in some paediatric settings

Documentation and Communication

Level 1 prescriptions must use the full IDDSI designation: Level 1 / Slightly Thick. Do not use institutional shorthand such as “mildly thickened,” “nectar thick,” or “stage 1” — these terms belong to older pre-IDDSI classification systems and create dangerous ambiguity at care transitions.

All thickener-to-liquid ratios used to achieve Level 1 should be documented and verified by syringe test before the prescription is formalised. Where possible, caregivers and nursing staff should be trained to perform the syringe test themselves as a quality-check routine.

Summary

IDDSI Level 1 (Slightly Thick) is the minimum effective liquid modification for patients who aspirate thin liquids but can manage slightly slowed flow. The syringe flow test target is 1–4 mL remaining at 10 seconds. Level 1 benefits patients with mild pharyngeal impairment, fatigue-associated dysphagia, and those in the later stages of swallowing rehabilitation. The main clinical risks at this level are under-thickening, temperature-related viscosity drift, and inconsistent preparation. Precision in preparation and regular syringe testing are the foundation of safe Level 1 management.