Comparison of the upper lip bite test with measurement of thyromental distance for prediction of difficult intubations.

BACKGROUND: The upper lip bite test (ULBT) introduced in 2003 is a simple method for predicting difficult intubations. According to this test, the higher a patient can bite the upper lip with the lower incisors, the lower the risk of difficult intubation. Given the necessity for evaluation and re-evaluation of a newer predictive method, this study was conducted to compare the ULBT with another method for difficult airway prediction, the measurement of thyromental distance (TMD).

METHODS: In this prospective, observational, single-blind study, 350 patients who required inhaled general anesthesia with endotracheal intubation for elective surgery were enrolled for evaluation of the accuracy in predicting difficult intubation by TMD measurement and ULBT. In the ULBT, the capacity of biting the upper lip is categorized into three classes: class I, a patient is able to raise the lower incisors above the vermilion line; class II, a patient is able to bite the upper lip below the vermilion line; and class III, a patient is unable to bite the upper lip. After induction of anesthesia with a standard protocol, the patient's grade of laryngeal view by the Cormack-Lehane classification was documented by an anesthesiologist with at least 3 years' experience who was unaware of prior evaluations. A TMD equal to or less than 4 cm and a class III ULBT were considered to be predictive of difficult intubation. A Cormack-Lehane class III or IV was considered a difficult intubation.

RESULTS: Only 20 (5.7%) of the patients were considered to have difficult intubations. Sensitivity, specificity, positive and negative predictive values, and accuracy were 70%, 93.3%, 39%, 98.1%, and 92.6%, respectively, for the ULBT, and 55%, 88%, 22%, 97%, and 86.3%, respectively, for TMD. Specificity and positive predictive value were found to be significantly higher for the ULBT than for TMD (p < 0.05). The sensitivity, negative predictive value, and accuracy were not significantly different between the two methods.

CONCLUSION: The findings suggest that the sensitivity of the ULBT and TMD for predicting difficult intubations may not be significantly different, but the specificity of the ULBT is significantly higher. The positive predictive value of the ULBT was significantly higher than that of TMD, signifying that a positive ULBT is more predictive of a difficult airway than is a positive TMD.