When a patient undergoes a videofluoroscopic swallowing study (VFSS) or fiberoptic endoscopic evaluation of swallowing (FEES), one of the most important outcomes recorded is the Penetration-Aspiration Scale (PAS) score. This article explains what the PAS is, how each score is assigned, what it means clinically, and how it informs diet and management decisions.
The Penetration-Aspiration Scale is an 8-point ordinal scale developed by Rosenbek, Robbins, Roecker, Coyle, and Wood in 1996 at the William S. Middleton Memorial Veterans Hospital. It was designed to provide a standardised, reliable way to describe and record what happens when food or liquid enters the airway during swallowing.
Before the PAS existed, clinicians used inconsistent language — “mild aspiration,” “trace penetration,” “small entry” — that made comparison across clinicians and studies impossible. The PAS replaced that inconsistency with a single number from 1 to 8 that carries a precise meaning about airway invasion depth and patient response.
The scale is divided into three zones:
Material does not enter the laryngeal vestibule. This is the expected outcome for a fully functional swallow. The bolus is directed through the pharynx into the oesophagus without any entry into the airway above or below the vocal folds.
Material enters the airway space above the vocal folds (the laryngeal vestibule) but does not contact the vocal folds themselves. The material is expelled from the airway — cleared out by cough, swallow, or other mechanism — before or during the next swallow. This is considered a minor event and is sometimes seen in healthy adults, particularly with thin liquids swallowed rapidly.
Material enters the laryngeal vestibule, contacts the vocal folds, and is then expelled or cleared. The patient responds — typically with a cough or throat clear — and the material leaves the airway. The response is present and effective.
Material contacts the vocal folds but is NOT expelled from the airway despite the patient’s efforts (or lack thereof). This represents a more significant penetration event because material remains in contact with the vocal folds. Repeated events at this level carry increased aspiration risk as material may eventually pass through.
Material contacts the vocal folds and remains there without any observable patient response — no cough, no throat clear, no behavioural indication that the patient is aware of the penetration. This is sometimes referred to as “silent penetration” and is particularly concerning because it suggests reduced laryngeal sensation or impaired protective reflexes.
Material passes below the vocal folds into the trachea or subglottic space but is expelled by cough — the patient coughs the material back up and out of the airway. This is overt aspiration with a functional protective cough response. The material exits the lower airway, reducing immediate pneumonia risk.
Material enters below the vocal folds. The patient attempts to cough or clear but is unable to fully expel the material — some remains in the lower airway. This is clinically significant: material that cannot be cleared may accumulate and increase risk of aspiration pneumonia over repeated meals.
Material passes below the vocal folds with no observable patient response. The patient does not cough, does not change vocal quality, and shows no sign of awareness that aspiration has occurred. This is the most clinically dangerous PAS score. Silent aspiration is the primary reason that the Clinical Swallowing Examination (bedside assessment) cannot be relied upon alone — it cannot detect PAS 8 events, which can only be identified on VFSS or FEES.
During a VFSS or FEES, the patient is given multiple boluses of different textures and volumes. A PAS score is assigned for each bolus trial. By convention:
The SLP and reporting clinician review the pattern: Does aspiration only occur with thin liquids at high volumes? Does it occur across all textures? Is there a consistent response (cough) or is aspiration universally silent? These patterns determine the management plan.
PAS scores directly inform IDDSI (International Dysphagia Diet Standardisation Initiative) recommendations:
| PAS Range | Clinical Interpretation | Typical Management |
|---|---|---|
| 1–2 | Normal or minor laryngeal penetration | Regular diet; monitor only |
| 3–5 | Laryngeal penetration with or without response | Modified fluids or diet; texture modification; compensatory strategies |
| 6–7 | Aspiration with cough response | Texture/fluid modification; positioning strategies; may indicate instrumental follow-up |
| 8 | Silent aspiration | Strict texture/fluid restriction; often nil by mouth or alternative nutrition pending further assessment |
Importantly, a PAS 8 score on thin liquids does not automatically mean nil by mouth. The SLP considers:
For some patients — particularly those in palliative or long-term care — comfort feeding with accepted aspiration risk is an ethically appropriate and documented clinical decision, developed with the patient, family, and MDT.
It does not quantify volume aspirated. A PAS 8 event could involve a few drops or a large bolus — the scale does not distinguish these. Volume aspirated is a separate clinical judgment.
It does not assess oesophageal function. The PAS only captures events visible at the level of the larynx and trachea during the pharyngeal swallow phase.
It is observer-dependent. VFSS and FEES are real-time assessments; frame-by-frame review on VFSS video improves accuracy. Inter-rater reliability for PAS scores is moderate to good (Cohen’s kappa 0.50–0.80 across studies) but is best when trained raters use standardised slow-motion review protocols.
It does not predict pneumonia directly. Aspiration pneumonia risk depends on: volume and frequency of aspiration, bacterial load of aspirated material (oral hygiene), host immune status, and pulmonary clearance capacity. A patient with PAS 8 on thin liquids but excellent oral hygiene and good respiratory reserve may have lower actual pneumonia risk than a patient with PAS 6 but poor oral hygiene and chronic lung disease.
The PAS is the most widely used outcome measure in dysphagia research. It is used to:
Researchers have proposed a modified binary classification: PAS 1–2 = “safe,” PAS 3–8 = “unsafe” for clinical decision-making, though this simplification loses the clinically meaningful distinction between penetration (3–5) and aspiration (6–8).
PAS scoring is a standard component of VFSS and FEES reports at Hospital Authority facilities across Hong Kong, including Queen Mary Hospital, Prince of Wales Hospital, Queen Elizabeth Hospital, and Pamela Youde Nethersole Eastern Hospital. The score is documented in the SLP report and shared with the referring physician and dietitian as part of the dysphagia management plan.
Private VFSS is available at select private hospitals and radiology centres, with FEES more commonly performed in ENT or gastroenterology settings. Ask your reporting SLP to explain your PAS score at the feedback session following the assessment.