Does upper lip bite test predict difficult intubation?

Sir,

We read the article by Khan and Arbabi describing the utility of upper lip bite test (ULBT) in predicting difficult intubation with great interest.[1] However, we would like to point out few issues, which need clarification.

There was no blinded comparison of ULBT (diagnostic test) with the laryngoscopic view (gold standard). In its absence, it will lead to bias which will result in over/under interpretation of the value of diagnostic test.[2] In this study it could have led to over interpretation of the post-test probability (from 5.88% to 37.5%). The authors have excluded patients with gross anatomical abnormality. Ideally patients of all the severity (less or very severe) should be included in a diagnostic study to get the correct value of the test of interest. Moreover, there is a mention of matching of the control subjects with the cases. This is not possible and is never done in any diagnostic study because both the diagnostic test and the gold standard are done independently. As a result, no one is able to differentiate the case from the control. If one knows the case and the control from the beginning then it defeats the basic purpose of the study.[2] It would have been good to mention the likelihood ratios so that the results can be directly applied to Indian population too. The advantage of the likelihood ratio is that, it is independent of the prevalence/pre-test probability which varies from one geographical area to another geographical area.[3] The authors have used the cut off points of various predictors as suggested by their respective originators. It will be great if the authors try to calculate the new cut-offs of various predictors (like thyromental distance, sternomental distance etc.) which will be given maximum. sensitivity and reasonable specificity. The new cut-offs should be compared with the old cut-offs in finding out the improvement in the post-test probability.

Despite these limitations, we congratulate the authors for generating new evidence in the field of ‘predicting difficult intubation’ which is not only a common problem but also an important area of interest both for intensivists and anaesthesiologists.