Upper Lip Bite Test (ULBT)

Definition and Background

The Upper Lip Bite Test (ULBT) is a simple bedside preoperative airway assessment tool introduced by Khan et al. (2003). It evaluates anterior mandibular mobility (jaw subluxation) by assessing the ability of the lower incisors to cover the upper lip. It is considered a more practical and arguably more accurate alternative to the modified Mallampati classification (MMC) for predicting difficult laryngoscopy and intubation.

How to Perform

The patient is asked to protrude the lower jaw and bite the upper lip with the lower incisors as far as possible. The patient sits upright; no special positioning is required. The result is graded based on how far the lower incisors can reach.

Classification

Upper Lip Bite Test - Class 1 (lower incisors cover upper lip mucosa), Class 2 (partial coverage), Class 3 (unable to bite upper lip)

Figure: Upper Lip Bite Test classes showing increasing difficulty from Class 1 (bottom) to Class 3 (top). - Fishman's Pulmonary Diseases and Disorders, Fig. 145-4

Class	Finding	Implication
Class I	Lower incisors can bite above the vermilion border of the upper lip (mucosa covered)	Normal jaw protrusion - easy intubation likely
Class II	Lower incisors can bite only at the vermilion border (partial coverage)	Intermediate - mild concern
Class III	Lower incisors cannot bite the upper lip at all	Poor jaw protrusion - difficult intubation predicted

Classes II and III are considered predictors of a difficult airway.

Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 571
Fishman's Pulmonary Diseases and Disorders, p. 2576-2577

What It Assesses

The ULBT is fundamentally a test of:

Temporomandibular joint mobility
Anterior mandibular protrusion (jaw subluxation ability)
Adequacy of the lower jaw to facilitate laryngoscopy

It correlates with the ability to align the oral and pharyngeal axes during laryngoscopy. Poor protrusion means the lower jaw cannot move forward to create the space needed for laryngoscope blade insertion.

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 114

Diagnostic Performance

From pooled data (Barash Clinical Anesthesia, 9e, Table 28-6):

Test	Sensitivity (%)	Specificity (%)
Mouth opening	22	94
Mallampati classification	53	80
Thyromental distance	37	89
Sternomental distance	33	92
Upper Lip Bite Test	67	92

Key points:

The ULBT has the highest sensitivity (67%) among common airway tests.
Specificity is ~92-96% (relatively high - a positive test strongly suggests difficulty).
In one prospective study, 60% of Class III patients had a difficult airway.
Positive predictive value (PPV) remains low in isolation - most patients predicted difficult are in fact normal.
The combination of abnormal ULBT + other abnormal tests greatly increases the probability of a truly difficult airway.

Advantages Over Mallampati

Feature	ULBT	Mallampati
Patient cooperation	Minimal	Requires mouth opening + tongue protrusion
Observer variability	Less	More (lighting, phonation, etc.)
Sensitivity	Higher (67%)	Lower (53%)
Specificity	Higher (92%)	Lower (80%)
Assesses	Jaw mobility	Pharyngeal space
Edentulous patients	Not applicable	May be assessed

Limitations

Cannot be used in edentulous patients (no lower incisors)
Cannot be assessed in patients with limited mouth opening from the start
Poor predictor as a single screening test - should always be combined with other assessments (Mallampati, thyromental distance, neck mobility, inter-incisor distance, sternomental distance)
Like all airway tests, no single test is 100% sensitive or specific - most difficult airways remain unanticipated
Limited utility in emergency settings where full cooperation may not be possible
Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1, p. prediction of difficulty section

Clinical Relevance for Anaesthesia

Part of the standard preoperative airway assessment battery
A Class III result should prompt:
- Senior anaesthetist involvement
- Awake fibreoptic intubation (AFOI) consideration
- Video laryngoscopy preparation
- Difficult airway trolley readiness
- Plan A/B/C airway strategy documentation
Useful in patients where the Mallampati is difficult to elicit (reduced cooperation, limited neck extension)
Often combined with Mallampati in a multivariate scoring system for better predictive accuracy

Summary Box (High-Yield for MCQ)

Introduced by: Khan et al., 2003
Tests: Anterior mandibular protrusion / TMJ mobility
Class III = difficult intubation (cannot bite upper lip)
Best sensitivity among common airway tests: 67%
High specificity: ~92-96%
Cannot use in edentulous patients
Better than Mallampati in sensitivity and specificity
Grades II and III = difficult airway predictors

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