Biostatistics and Epidemiology Flashcards

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?

Answer: About 33%

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

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

Answer: Sensitivity increases and specificity decreases

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

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

Answer: 4.5

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

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?

Answer: PPV increases and NPV decreases

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

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

Answer: High sensitivity

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

A new screening program shows that patients diagnosed by screening survive 2 years longer after diagnosis, but the age at death does not change. Which bias best explains this?

Answer: Lead-time bias

Earlier detection starts the survival clock sooner without delaying death.

A screening program mostly detects slow-growing, indolent tumors, which makes screening look more beneficial than it really is. What is this bias?

Answer: Length-time bias

Slowly progressing disease spends longer in the detectable preclinical phase, so screening catches it more often.