The optimal cut-off points for each predictive tests were identified by using receiver operating characteristic analysis. Sensitivity, specificity and positive predictive value and negative predictive value (NPV) were calculated for each test. Multivariate analysis with logistic regression, including all variables, was used to create a predictive model. Comparisons between genders were also performed to explore possible differences in diagnostic value and cut-off points. Finally, the years of experience of the clinician and the number of tries needed to intubate the patient were compared to assess risk of bias.