You have been approached by a company which has developed a new biomarker of sepsis. They would like it tested in a cohort of critically ill septic patients. You test this biomarker in a cohort of 100 patients with proven bacteremia. You also test this biomarker in a cohort of 100 patients with drug overdose whom you use as a control. In the bacteremic group 70 patients had abnormal biomarker results. In the control group 60 patients had an abnormal biomarker results.

Calculate

a) Sensitivity

b) specificity

c) Positive predictive value

d) Negative predictive value

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College Answer

Values below expressed as a percentage .


a)

70/100

b)

40/100

c)

70/130

d)

40/70

Discussion

This question is identical to Question 19.1 from the first paper of 2010. However, the college changed the numbers a little, and made the question about pancreatic necrosis.

Going though the motions,

true positives = 70

false positives = 60

true negatives = 40

false negatives = 30

a) Sensitivity = True positives / ( true positives + false negatives)

= 70 / (70 + 30) = 70%

b) Specificity = True negatives / (true negatives + false positives)

= 40 / (40 + 60) = 40%

c) Positive predictive value = True positives / (true positives + false positives)

= 70 / (70 + 60) = 53.8%

d) Negative predictive value = True negatives / (true negatives + false negatives)

= 40 / (40 + 30) = 57.1%