Preventive Medicine and Public Health: Screening, Risk, and Bias
Preventive medicine: who benefits, and how do we know?
Preventive questions test screening, counseling, immunization, outbreak investigation, and study interpretation. Know the levels of prevention: primary prevents disease, secondary detects preclinical disease, and tertiary limits complications. Screening is useful when the condition is important, detectable early, treatable, and the test is acceptable. Sensitivity is the proportion of diseased people who test positive; specificity is the proportion of nondiseased people who test negative. Positive predictive value depends on prevalence: the same sensitivity and specificity yield a higher PPV in a high-prevalence population. This is a common trap when a question asks why a test performs differently in a clinic versus the general population.
Bias and study design
Recognize lead-time bias, length-time bias, selection bias, and recall bias. Improved survival after screening may reflect earlier diagnosis rather than longer life; mortality reduction is stronger evidence. In outbreaks, calculate attack rates by exposure and use a case-control design for rare diseases. Distinguish notifiable disease reporting from clinical treatment.
Worked rule
If a test has 90% sensitivity and 90% specificity in a population with 10% prevalence, PPV is about 50%: TP = 0.09, FP = 0.09. In a 1% prevalence population, PPV falls to about 8%. Always ask what population the patient comes from before interpreting a positive test.