Comparative Analysis of The Efficacy of Upper Lip Bite Test and Thyromental Distance for Prediction of Difficult Intubation

Abstract

Background: Unanticipated difficult intubation remains a major contributor to perioperative morbidity and mortality, underscoring the need for reliable bedside airway assessment tools. The Upper Lip Bite Test (ULBT) and Thyromental Distance (TMD) are commonly used preoperative predictors, yet their comparative diagnostic performance remains clinically debated. Objective: To compare the diagnostic accuracy of ULBT and TMD in predicting difficult intubation among adult patients undergoing elective surgery under general anesthesia. Methods: In this comparative cross-sectional observational study, 119 adult patients scheduled for elective surgery requiring endotracheal intubation were consecutively enrolled. Preoperative airway assessment included ULBT and TMD classification using standardized criteria. Difficult intubation was defined as Cormack–Lehane grade III/IV or intubation requiring more than three attempts or prolonged duration. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), likelihood ratios, and accuracy were calculated with 95% confidence intervals. Results: Difficult intubation occurred in 21 patients (17.7%). ULBT demonstrated higher sensitivity than TMD (66.7% vs 47.6%) with comparable specificity (87.8% vs 89.8%). ULBT showed superior NPV (92.5% vs 88.9%) and lower negative likelihood ratio (0.38 vs 0.58), indicating improved rule-out capability, while PPV was moderate for both tests (53.8% vs 50.0%). Conclusion: ULBT provides greater sensitivity and stronger exclusion performance compared with TMD, supporting its preferential use as a primary bedside screening tool within a multimodal airway assessment strategy.