The study was approved by an institutional human ethics committee and research committee, IEC NO 01/69/2014/MCTand informed consent was obtained from each patient. Assuming 70% prediction rate as per reference6 precision 10% and 95% confidence level, sample size needed was calculated as 300.

Consecutive male and female, 300 patients, aged 15-60 yrs, scheduled to undergo surgery under general anesthesia and endotracheal intubation, between March 2014 and February 2015, were considered for enrollment. Edentulous patients, those unable to open the mouth, with big oral tumours, or cervical spine pathology were excluded from the study.

Preoperative assessment of the airway is done using Mallampati criteria, Upper Lip Bite Test (ULBT) and Hyomental distance ratio (Hyomental distance at extreme extension devided by Hyomental distance at neutral position), taken with the patient in supine position. General anaesthesia induced as per institutional protocol. Laryngeoscopy is done by an experienced person who does not know Upper Lip Bite Test class and Hyomental distance ratio (HMDR). Statistical analysis was done using SPSS version 11. Catagorical variables are analysed using chi squire test and continuous variables are analysed using student’s t test. Cormack Lehane grading of laryngosopy was taken as the gold standard. Difficult laryngoscopy was defined as a Grade 3 or 4 laryngoscopic view. ULBT class 3, and HMDR with optimal cutoff point 1.2 are considered as difficult intubation predictors 8,10 . Stastistical parameters of ULBT are Upper lip bite test class3 is considered as a predictor of difficult intubation. Upper lip bite test class 1&2 are considered as predictors of easy intubation. True positive - Cormack lehane grade 3&4 + upper lip bite test class 3. True negative - Cormack lehane 1&2 + upper lip bite test class 1&2. False negative - Cormack lehane 1&2 + upper lip bite test class 3. False positive - Cormack lehane 3&4 + upper lip bite test class 1&2. Statistical parameters of HMDR are true positive - Cormack lehane 3&4+HMDR less than 1.2. True negatives- Cormack lehane 1&2 +HMDR more than 1.2. False negatives- Cormack lehane 1&2+HMDR less than 1.2.False positives Cormack lehane 1&2 +HMDR greater than 1.2.

Sensitivity, Specificity, Positive Predictive Vale (PPV), Negative Predictive Value (NPV) and Accuracy of both ULBT and HMDR separately and then both in combination are calculated and compared with those of MP.

Out of 300 patients recruited in this study, 84.7% were females. The mean age was 44 ± 12 years (mean ± SD). Difficult laryngoscopy (C–L grades III) was seen in 34 (11.33%) patients. Out of 300 patients recruited in this study, 254 (84.7%) were females.

Table 1 shows the demographic data of the enrolled cases. Table 2 reveals the statistical indices of the different diagnostic tests.

This study was designed to evaluate the efficacy of ULBT, MP, HMDR and HMDR+ULBT in forecasting a difficult intubation, and to draw a possible correlation between the tests and C–L grades.

We have taken more number of females in this study so the results are more applicable to females. 84.7% of study population are females. The incidence of difficult airway in this study is 11.33% which is consistent with a meta-analysis of nine studies that included 14,438 patients and a DVL incidence of 6%–27% .

The wide variations in the incidence of DVL may be related to factors such as age 11 [21] and ethnic differences among patients12,13 or types of laryngoscope blade used10. The incidence of difficult intubation is 1.3%, 15%, 1.8%, 3.5%, 4%, 45%, 4.9%, 7%, 8%, and 13% 7,11,14,15,16 depending on the criteria used to define it.

In this study 4% of patients under the age group 15-20 yrs. There are 12.7 % of patients under the age group 20-30 yrs. There are 23.3% and 27.3% under the age group 30-40 and 40-50 respectively. There are 32.7% under the age group 50-60yrs. There are 84.3% patients are under the age group 30-60yrs. Hence this study is more applicable in the age group 30 -60 yrs.

Total thyroidectomy patients constitutes 54% of the total number enrolled in this study. Chances of difficult airway in thyroid patients are higher, therefore in consideration to this we have included more number of thyroidectomy cases. Voyagis et al 17 had described that goiter, when accompanied by airway deformity, constitutes an aggravating factor for difficult intubation. Many articles have been published suggesting that goiter, when accompanied by tracheal compression, constitutes an aggravating factor for difficult airway 18,19,20.

Upper lip bite test is relatively new and easy test to predict the airway pre operatively. The new ULBT, was performed according to the following criteria: class I - lower incisors can bite the upper lip above the vermilion line; class II - lower incisors can bite the upper lip below the vermilion line, and class III - lower incisors cannot bite the upper lip8. ULBT class1 and 2 are considered as easy intubation and class 3 is difficult intubation. Hence there are 10% incidence of difficult intubations are predicted by this test. There are 76.7% patients having ULBT (Upper Lip Bite Test) class1 and 13.3% of patients are coming under class 2 Upper lip bite test in this study.

Classification of patients based on the ULBT is of a qualitative nature, making differentiation of classes easy and precise. In brief, the differences between the ULBT and the other tests are those between continuous and discrete variables. Thus, the ULBT is associated with the least inter observer variability, which adds to its advantage as an airway assessment test 21.

A test to predict difficult intubation should have high sensitivity, so that it will identify most patients in whom intubation will truly be difficult 22. It should also have a high PPV, so that only few patients with airways actually easy to intubate are subjected to the protocol for management of a difficult airway 17,23.

Oates et al predicted MMC as bedside test with higher predictive value but had multiple problems like inter-observer variation and phonation 21. In this study MP has only 17.65% sensitivity; 88.72% specificity, 16.67% positive predictive value, 89.39% NPV and 80.66% accuracy. These results are almost in consistent with a study conducted in a tertiary hospital of Pakistan which showed sensitivity 19.6%, (95% CI 10.9%-33.4%), specificity 91.8% (95%CI 87.6%-94.6%), PPV3 3.3%(95% CI 18.6%-51.6%), NPV 84.3% (95% CI 80%-88.6%)24. Our results are against the finding of Oates et al maybe due to inter observer variability and ethnic differences 12,16 25 . In a study accuracy of ULBT 93% is much higher than that of Modified Mallampati alone 81.6% 26 which are 98%, 33% and 80.66% in our study. The specificity of ULBT was 99.1% which is similar to this study 99.25%. But they got low sensitivity for ULBT because of low incidence of class 3 group in their study 26.

Upper lip bite test class of 1 coupled with good neck extension and flexion of 90° were sufficient criteria for us to predict with confidence that the intubation would be easy and thus proceeded with the rapid sequence protocol8 .

These findings are consistent with a previous report by Eberhart et al. 27 which showed that the inter observer reliability of the ULBT is higher than the Mallampati classification. The discriminating power of ULBT was high 0.90 (95% confidence interval, 0.84-0.95) than MP 0.55 (95% confidence interval, 0.47-0.64) indicating that ULBT is a good predictor of difficult intubation 27.

Sensitivity, specificity and NPV of ULBT in our study is comparable to an earlier study 28 (88.24%, 99, 62% and 98.51% versus 88%, 88.7% and 98.4% respectively), and some other studies 27,29 . Sensitivity (87.5% versus 88.72%.) and PPV (96.77% versus 28.9%) of ULBT was higher in our study compared to earlier studies 8,29,30,31 .

The probable reasons are lack of inter observer variance in our study as well as ethnic difference in a study. Comparison of six methods for predicting difficult intubation in obstetric patients, they got sensitivity, specificity, PPV, NPV of ULBT are 94.6%, 97.6%, 89.7%, 98.8% these results are almost similar to this study [38]. In this study, it is 88.24% sensitivity, 99.62% specificity, 96.77% positive predictive value, 98.51% negative predictive value and accuracy 98.33% for ULBT.

The hyomental distance (HMD) has been used to estimate the mandibular space, but the HMD alone was shown to have only a modest degree of diagnostic value 14. Takenaka et al defined the ratio of HMD in extreme of head extension to neutral position and as the hyomental distance ratio (HMDR) and demonstrated that it was a good predictor of a reduced occipito-atlantoaxial (OAA) complex extension capacity 32.

The angle required to expose the glottis during direct laryngoscopy was previously reported to be at least 12°, and the corresponding HMDR was calculated as 1.25 13 . HMDR showed 88.24% sensitivity, 99.25% specificity, 93.75% positive predictive value, 98.5% negative predictive value and 98% accuracy in this study. The sensitivity of HMDR in this study is comparable to Takanaka et al (88.2% versus 88%) 33 Limitations of the study are ULBT cannot be performed in edentulous patients, patients with low intellectual coefficient small children .The risk score requires further validation in larger study population.34,35,36,37,38,39,40

By combining ULBT and HMDR, the results showed sensitivity 82.35%, specificity100%, positive predictive value 100%, negative predictive value 97.79% and accuracy 98%. Observed improvement in positive predictive value, specificity and sensitivity, while there is slight decrease in negative predictive value in combination with ULBT and HMDR. Results show this is an excellent combination of airway assessment.

I would like to thank the patients who enrolled in the study. I sincerely thank the other contributor, Dr Harikrishnan S. and Dr Rejani Gandha for their support. There is no conflict of interest present and no external funding for this study.

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