Upper Lip Bite Test Grading System
The upper lip bite test uses a three-class grading system based on the patient's ability to bite their upper lip with their lower incisors, with Class I indicating the lower incisors can bite above the vermilion line, Class II below the vermilion line, and Class III indicating inability to bite the upper lip at all. 1
Classification System
The ULBT categorizes patients into three distinct classes based on mandibular protrusion capability:
- Class I: Lower incisors can bite the upper lip above the vermilion line (mucosa is visible) - indicates easy intubation 1, 2
- Class II: Lower incisors can bite the upper lip below the vermilion line (mucosa is not visible) - intermediate difficulty 1, 2
- Class III: Lower incisors cannot bite the upper lip at all - predicts difficult intubation 1, 2
Clinical Interpretation
Class III is considered a positive test result predictive of difficult laryngoscopy and intubation. 3, 2 This classification reflects the patient's temporomandibular joint mobility and mandibular space, which directly correlates with laryngoscopic view quality.
Performance Characteristics
The test demonstrates superior specificity compared to the modified Mallampati classification:
- Specificity: 93.3% for ULBT versus 88% for thyromental distance 2
- Positive predictive value: 39% for ULBT, significantly higher than alternative bedside tests 2
- Interobserver reliability: Excellent (kappa = 0.79), superior to Mallampati score (kappa = 0.59) 4
- Sensitivity: Approximately 70% for predicting difficult intubation 2
Important Clinical Caveats
The test cannot be applied in approximately 12% of patients (compared to <1% for Mallampati), typically those with dental abnormalities, missing lower incisors, or severe temporomandibular joint dysfunction 4. In such cases, alternative assessment methods must be used.
When used alone, the ULBT has limited discriminating power (area under curve 0.60), indicating it performs poorly as a single screening test 4. The combination of ULBT with modified Mallampati classification significantly improves predictive accuracy, particularly specificity and positive predictive value 3.
Special Population Considerations
In morbidly obese patients (BMI >40 kg/m²), the ULBT demonstrates particularly favorable performance with likelihood ratios of 6.35-9.47, substantially higher than modified Mallampati (3.21-3.16) 3. However, in acromegalic patients, sensitivity drops significantly, with ULBT failing to predict 73% of difficult laryngoscopies 5.
Practical Application Algorithm
- Perform ULBT during preoperative airway assessment alongside (not instead of) modified Mallampati classification 3
- Document the specific class (I, II, or III) in the anesthesia record 1
- Class III result warrants preparation for difficult airway management with alternative intubation equipment immediately available 2
- If test cannot be performed due to dental issues, rely on alternative predictive methods and document the reason 4