Week 11: Diagnostic Validity

Question 1

What is 
True positive (A)
False positive (B)
False negative (C)
True negative (D)

Answer 1

True positive: test shows positive when person does have condition (a)
False positive: test shows positive when person doesn’t have condition. (B)
False negative: saying you don’t have a condition when you do (c)
True negative: sayiing you don’t have a condition when you don’t (d)

Question 2

Sensitivity =

Specificity =

Answer 2

Sensitivity =
A/ (A + C)
Specificity =
D/ (B + D)

Question 3

What are the 3 statistics of diagnostic validity

Answer 3

1. Sensitivity - true +’ve
2. Specificity -proportion with the -ve result
3. Likelihood ratios

Question 4

What is the ideal diagnostic test

Answer 4

Would always correctly discriminate between those with and those without the condition
Ie
-always positive for those with the condition
-always negative for those without it

Question 5

SnNout
Sensitivity (true positive rate)
What is SnNout

Answer 5

Sensitivity: true positive rate
-proportion if patients with condition who have a positive test result
-tests with high sensitivity have few false negatives, therefore a negative result rules out the condition
SnNout–> increased sensitivity and (-)result = rule out
Helpful in ruling out a diagnosis
-only helpful when you have negative result
-often used as screen tests

Question 6

SpPin
Specificity
What is SpPin?

Answer 6

Specificity: (true negative rate)

• proportion if patients without the condition who have negative test results
• tests with high specificity have few false positives, therefore a positive result RULES IN the condition

Question 7

Statistics of diagnosis

What is the Likelihood ratio?

Answer 7

• incorporates both sensitivity and specificity of the test and provides a direct estimate of how much a test result will change the odds I’d having a disease.
• the positive likelihood ratio tells you how much the odds if the disease increase when a test is positive (sensitivity/1-specificity)
• the negative likelihood ratio tells you how much the odds of the disease decrease when a test is negative (1-sensitivity/specificity)