Chemical Pathology Math

Question 1

Suggest the formula for Positive Predictive Value

Suggest the formula for Negative Predictive Value

Answer 1

Suggest the formula for Positive Predictive Value
TP/(TP+FP)
Denominator: All ppl tested positive

Suggest the formula for Negative Predictive Value
TN/(TN+FN)
Denominator: All ppl tested negative

Question 2

Suggest the formula for Sensitivity

Suggest the formula for Specificity

Answer 2

Suggest the formula for Sensitivity
TP/(TP+FN)
Denominator: All ppl actually have the disease

Suggest the formula for Specificity
TN/(TN+FP)
Denominator: All ppl NOT have the disease

Question 3

Relationship between Threshold and Sensitivity and Specificity (REFER TO DIAGRAM on notes)

Fill in the blanks with TP, FP, TN, FN
The left curve is normal; the right curve is diseased.

At point P or rightwards, there is only __________, no more _______________.

At point K or leftwards, there is only ____________, no more _______________.

So when the threshold increases (moves from left to right), the ____________ increases.

So when the threshold decreases (moves from right to left), the _____________ increases.

Answer 3

Relationship between Threshold and Sensitivity and Specificity (REFER TO DIAGRAM on notes)

Fill in the blanks with TP, FP, TN, FN
The left curve is normal; the right curve is diseased.

At point P or rightwards, there is only TRUE POSITIVE, no more FALSE POSITIVE.

At point K or leftwards, there is only TRUE NEGATIVE, no more FALSE NEGATIVE.

So when the threshold increases (moves from left to right), the SPECIFICITY increases.

So when the threshold decreases (moves from right to left), the SENSITIVITY increases.

Question 4

Linking Sensitivity with NPV

When Sensitivity increases, how would the NPV change?

Answer 4

Sensitivity = TP/(TP+FN)
NPV = TN/(TN+FN)

Sensitivity increases
FN decreases.
NPV INCREASES

Question 5

Linking Specificity with PPV

When Specificity increases, how would the PPV change?

Answer 5

Specificity = TN/(TN+FP)
PPV = TP/(TP+FP)

Specificity increases
FP decreases
PPV INCREASES

Question 6

How to calculate the likelihood ratio of LR+

How to calculate the likelihood ratio of LR-

Answer 6

How to calculate the likelihood ratio of LR+
(Prob result is +ve in diseased ppl) /
(Prob result is +ve in non-diseased ppl)

P (T+ | D+) /
P (T+ | D-)

(Sensitivity) /
(1 - Specificity)

How to calculate the likelihood ratio of LR-
(Prob result is -ve in diseased ppl) /
(Prob result is -ve in non-diseased ppl)

P (T- | D+) /
P (T- | D-)

(1 - Sensitivity) /
(Specificity)

Question 7

If the likelihood ratio for tests = 1

What does it mean

Answer 7

The test is useless
Cuz Sensitivity + Specificity = 1
Which is equal to flipping a coin

Question 8

Consider a R-C curve (operating characteristic curve)

The Y-axis is ____________
The X-axis is ____________

When the area under the curve increases, what does it mean?

Answer 8

Consider a R-C curve (operating characteristic curve)

The Y-axis is (SENSITIVITY)
The X-axis is (1 - SPECIFICITY)

When the area under the curve increases, what does it mean?