UWorld Biostatistics and Epidemiology 2

Question 1

A screening test has 90% sensitivity and 80% specificity and is used in 1,000 people with a disease prevalence of 10%. What is the positive predictive value?

Accepted Answer

About 33%

Answer

TP = 90 and FP = 180, so PPV = 90/270 ≈ 33%.

Question 2

The cutoff for a positive fasting glucose screening test is lowered from 126 mg/dL to 110 mg/dL. Which change is expected?

Accepted Answer

Sensitivity increases and specificity decreases

Answer

A lower cutoff labels more people positive, so there are fewer false negatives and more false positives.

Question 3

A test has a sensitivity of 90% and a specificity of 80%. What is its positive likelihood ratio?

Accepted Answer

4.5

Answer

LR+ = sensitivity / (1 − specificity) = 0.9/0.2 = 4.5.

Question 4

A test with unchanged sensitivity and specificity is moved from a community clinic to a specialty referral center, where disease prevalence is much higher. What happens to its predictive values?

Accepted Answer

PPV increases and NPV decreases

Answer

Higher prevalence raises PPV and lowers NPV, while sensitivity and specificity stay the same.

Question 5

A physician wants to rule out a dangerous condition with a single test. Which test property matters most when the result is negative?

Accepted Answer

High sensitivity

Answer

A negative result on a highly sensitive test reliably rules disease out (SnNout).

UWorld Practice Test

UWorld Practice Test

UWorld Biostatistics and Epidemiology 2