2020B14 Mallampatti score is a diagnostic test for difficult intubation with a sensitivity of 30% and a specificity of 90%. Describe how this information and other statistics related to this test can be used for predicting difficult intubation. How does the prevalence of difficult intubation affect the performance of this test?





Difficult ETT

Not difficult ETT

Predictive values:

High MP score

a)True +ve

b)False +ve

PPV: a/(a+b)

Low MP score

c)False -ve

d)True -ve

NPV: d/(c+d)


Sensitivity: a/(a+c)


Specificity: d/(b+d)








Likelihood ratios:


Positive LR = sensitivity / (1-specificity) = 3


Negative LR = (1-sensitivity) / specificity = 0.78



Sensitivity & specificity

·     Will pick up 30% of those with difficult ETT

·     Will rule out 90% of those without difficult ETT


   ->> unhelpful at identifying difficult intubation

Predictive values

·     PPV and NPV prevalence

·     Prevalence of difficult intubation is low


   ->> low PPV for difficult intubation in all-comers

Likelihood ratios

·     +LR 3: slight ↑ post-test probability of difficult intubation if high grade MP score

·     -LR is <1: slight ↓ post-test probability of difficult intubation if low grade MP score

·     % Change in probability is unaffected by prevalence of difficult intubation


   ->> low utility in assisting prediction of difficult intubation

Receiver-operator characteristic

·        Relationship between sensitivity, specificity, test quality

·        ↑AUC associated with high utility



This is my weakest topic.

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