Community Medicine
Core concepts
Community medicine asks you to think at the population level: epidemiology, screening principles, and social determinants of health. Understand sensitivity, specificity, positive predictive value, and how prevalence changes PPV. Know the criteria for a good screening test (important disease, acceptable test, early detection improves outcomes, cost-effective) and apply them to examples like colorectal cancer screening. Recognize social determinants-housing, food insecurity, transportation, insurance-and how they drive health outcomes. Know reportable disease concepts, outbreak investigation basics, and the difference between incidence (new cases) and prevalence (all cases).
Traps
- Confusing sensitivity (true positives among diseased) with PPV (true positives among test positives); PPV depends on prevalence.
- Recommending screening tests that do not improve outcomes (e.g., PSA without shared decision-making).
- Missing that lead-time bias can make survival look improved without true mortality benefit.
- Ignoring social barriers when a treatment plan fails-ask about cost and access.
Worked example / rule of thumb
A screening test with 90% sensitivity and 95% specificity in a low-prevalence population yields many false positives; PPV falls as prevalence falls. Rule of thumb: a positive test means something different in a high-risk versus low-risk patient.
Prevention levels
Primary (prevent disease), secondary (early detection), tertiary (limit disability). Know examples of each.