A Comparison between Modified Mallampati Test, Thyromental Distance, Upper Lip Bite Test and Neck Circumference for Predicting Difficult Intubation

Background: Various bedside screening tests are employed for the prediction of difficult intubation. In this study, we compared 4 bedside tests viz. the Modified Mallampati Test (MMT), the Thyromental Distance (TMD) measurement, the Upper Lip Bite Test (ULBT) and the Neck Circumference (NC) measurement in terms of Sensitivity, Specificity, Positive Predictive Value and Negative Predictive Value to determine which is the best individual predictor of difficult intubation, in apparently normal patients. Methods: 120 patients of ASA physical status I&II, aged 16-55years, of either gender, undergoing elective surgery under general anaesthesia were enrolled in this prospective study. MMT Classes III&IV, TMD<6.0cm, ULBT Class III and NC>35cm were considered to be predictive of difficult intubation. A Cormack-Lehane laryngoscopy grade 3 or 4 was considered a difficult intubation. Results: The incidence of difficult intubation was 6.67%. The sensitivity, specificity, positive and negative predictive values of the MMT were 62.50%, 86.61%, 25% and 97% respectively. The sensitivity, specificity, positive and negative predictive values of the TMD were 25%, 93.75%, 22.22% and 94.59% respectively. The sensitivity, specificity, positive and negative predictive values of the ULBT were 37.50%, 91.07%, 23.08% and 95.33% respectively. The sensitivity, specificity, positive and negative predictive values of the NC were 87.50%, 43.75%, 10% and 98% respectively. We also found a statistically significant correlation between the MMT and the CL grading and also between the ULBT and the CL grading. Conclusion: No single predictor is sufficient for prediction of difficult intubation on its own. All the studied bedside tests are poor to moderate predictors of difficult intubation. All the tests showed poor positive predictive values and high negative predictive values which suggests that they can be more useful predictors of easy intubation than difficult intubation.