4. Clin Epi: Diagnosis

Question 1

A 19 year old male is comes in to the clinic complaining of throat pain. You run a quick strep test to see if he has strep throat, and the result is negative. What is the likelihood he actually has strep? (MG)

A. PPV
B. NPV
C. 1 - PPV
D. 1 - NPV

Answer 1

D. 1 - NPV

This is the probability that a patient has the disease despite negative findings, aka the probability that your patient is a false negative. To help adjust for this, you send the specimen off to culture.

Question 2

What variable does this describe: Out of your entire tested population, how many people actually have the disease (MG)

A. Prevalence
B. PPV
C. NPV
D. True positive (TP)

Answer 2

A. Prevalence

Prevalence = (TP+FN)/(TP+FP+FN+TN)

In words:
People that actually have the disease are the true positive and false negative groups. The stem specifically refers to the ENTIRE tested population, so your denominator needs to be all the numbers in your test chart.

Question 3

A group of people are tested for rocky mountain spotted fever. 400 people have the disease. 500 people tested positive for the disease, of those 279 were true positives. 600 didn’t test positive and dont have the disease. What is the sensitivity of this test?

A.73%
B.70%
C.75%
D.56%
E. 67%

RFA

Answer 3

ANS B.

279(TP)/ (279(TP)+121(FN))= 279/400=70%

Question 4

A group of people are tested for rocky mountain spotted fever. 400 people have the disease. 500 people tested positive for the disease, of those 279 were true positives. 600 didn’t test positive and dont have the disease. What is the Specificity of this test?

A. 70%
B. 75%
C.56%
D.67%
E.73%

RFA

Answer 4

ANS E.

600(TN)/(221(FP)+600(TN))=600/821=73%

Question 5

If the criterion value is decreased (shifted to the left) it will cause ____ to increase?

A. Sensitivity 
B. Specificity
C. Positive Predictive Value
D. True Negative
E. False Negative

SZ

Answer 5

Ans. A. Sensitivity

Shifting the criterion value to the left will cause the True Positive to increase and the False Negative to decrease. Sensitivity = TP/TP+FN. Therefore sensitivity will increase.

Question 6

What is the ‘pre-test likelihood’ for a patient with a True Positive of 90, False Negative of 10, False Positive of 170, and True Negative of 30?

A. 85%
B. 15%
C. 33%
D. 2%
E. 35%

SZ

Answer 6

C. 33% “Pre-test likelihood’ is also called prevalence. Prevalence is equal to TP+FN/Total. (90+10)/300 = 33%.

Question 7

Lowering the Cut Point will _____ sensitivity and ____ NPV

MM

Answer 7

Increase, Increase

Question 8

Lowering the Cut Point will _____ specificity and ____ PPV

MM

Answer 8

Decrease, Decrease

Question 9

Raising the Cut Point will _____ sensitivity and ____ NPV

MM

Answer 9

Decrease, Decrease

Question 10

Raising the Cut Point will _____ specificity and ____ PPV

MM

Answer 10

Increase, Increase