Session 3 - standardisation and diagnostic tests

Question 1

Specificity

Answer 1

TN/TN+FP

Question 2

Sensitivity

Answer 2

TP/TP+FN

Question 3

Positive predictive value

Answer 3

TP/TP+FP

Question 4

Negative predictive value

Answer 4

TN/TN+FN

Question 5

Effects of maximising sensitivity

Answer 5

Fewer people with disease are missed

Question 6

Effect of maximising specificity

Answer 6

Avoid cost (emotional / £ / health) of treating inappropriately

Question 7

2 tests - Maximising Sensitivity

Answer 7

Parallel testing - 2 tests - positive for A OR B OR A&B = disease positive

Question 8

Maximising specificity

Answer 8

Sequential testing - Take all ‘test positive’ group and test again, Must test positive for A THEN B to = disease positive

Question 9

Effect of changing disease prevalence on sensitivity and specificity

Answer 9

Does NOT change as these are fixed characteristics

Question 10

Effect of changing disease prevalence on PPV

Answer 10

Prevalence Decreases - Positive predictive value Decreases = reducing numerator!
Prevalence Increases - PPV increases - more cases

Question 11

Effect of changing prevalence on NPV

Answer 11

Prevalence decreases - NPV Increases (increasingly unlikely you’re a missed case as disease is rare)
Prevalence increases - NPV decreases - lots of cases around to miss