Community Medicine and Public Health
Community Medicine and Public Health tests screening, epidemiology, prevention, outbreak response, and health service delivery. Expect calculations and the ability to distinguish a good test from a useful screening program.
Core rules
- Incidence counts new cases; prevalence counts existing cases. Prevalence rises with longer survival and lower mortality.
- Sensitivity detects disease; specificity excludes it. Positive predictive value depends on prevalence, so a good test in a low-prevalence population can yield many false positives.
- Screening criteria: important disease, accepted treatment, detectable early stage, suitable test, and evidence that early detection improves outcomes.
- Notifiable diseases must be reported promptly; outbreak investigation starts with a case definition and line list.
- Prevention: vaccination, smoking cessation, and blood pressure control are higher-yield than opportunistic testing without follow-up.
Worked example
A test has 90% sensitivity and 95% specificity. In 10,000 people with 1% prevalence, there are 100 cases: 90 true positives and 10 false negatives. Of 9,900 without disease, 495 are false positives. PPV is 90 / (90 + 495) = 15.4%. Reasoning: most positives are false, so the test needs confirmation and should be reserved for higher-prevalence groups.
Traps
- Confusing sensitivity with PPV.
- Ignoring lead-time bias in survival statistics.
- Calling an association causal without confounding control.
- Reporting a notifiable disease only after treatment is complete.