2017A09 A new test called the “intubation score” has a reported 90% sensitivity and 70% specificity
when used to predict difficult intubation. Describe how this information and other statistics related
to this test can be used in predicting difficult intubation. How will the incidence of difficult intubation
 affect the performance of this test?



·      Table

·      Meaning

·      Effect of prevalence

·      Receiver operator characteristic



Difficult ETT

Not difficult ETT


Predicts difficult ETT

a)True +ve

b)False +ve

PPV: a/(a+b)

Predicts not difficult ETT

c)False -ve

d)True -ve

NPV: d/(c+d)


Sensitivity: a/(a+c)


Specificity: d/(b+d)





·         90% sensitivity: will pick up 90% of those with difficult ETT

·         70% specificity: will rule out 70% of those without difficult ETT


Effect of prevalence

·         ↑Prevalence -> ↑PPV and ↓NPV

·         ↓Prevalence -> ↓PPV and ↑PPV


Receiver operator characteristic

·         Relationship between sensitivity, specificity, test quality



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