Epidemiology and Prevention
What this domain tests
Questions test whether you can choose the right measure of frequency or association, interpret a diagnostic test, and apply levels of prevention to a real population.
Core rules
- Prevalence is the proportion with disease at a point in time; incidence is new cases over a period. Prevalence rises with longer duration and better survival.
- Cohort studies give incidence and relative risk directly; case-control studies give odds ratios; cross-sectional studies give prevalence and associations only; randomised controlled trials test interventions.
- Relative risk compares incidence in exposed and unexposed groups. The odds ratio approximates relative risk only when the disease is rare.
- Sensitivity is the proportion of diseased people correctly identified; specificity is the proportion of healthy people correctly identified. Positive predictive value depends on prevalence, so a good test used in a low-prevalence population produces many false positives.
- Prevention levels: primary (fluoride, sealants, diet, smoking cessation, vaccination), secondary (screening, early lesion detection with ICDAS or bitewing radiographs), tertiary (restoration, rehabilitation, limiting disability).
- Fluoride works topically: it inhibits demineralisation, enhances remineralisation and interferes with bacterial metabolism. Dental fluorosis is a dose-related developmental effect, not a toxic emergency.
Worked example
A screening test for oral cancer is applied to 1,000 people: 90 true positives, 60 false positives, 10 false negatives, 840 true negatives. Reasoning: sensitivity = 90/100 = 90%; specificity = 840/900 = 93.3%; PPV = 90/150 = 60%. The PPV is far lower than sensitivity because the population prevalence is 10%, which is exactly why confirmation by biopsy is required.
Traps
Reporting prevalence when incidence was asked; presenting an odds ratio as if it were a risk; confusing sensitivity with predictive value; forgetting that fluoride's main action is topical, not systemic.