Public Health Dentistry: Indices, Epidemiology and Prevention
Public Health Dentistry
This domain tests oral health indices, study design and measures of association, screening test performance, levels of prevention and community fluoride policy.
Core rules
- DMFT/dmft counts decayed, missing and filled teeth; it is an irreversible index of past and present caries experience.
- Prevalence is the proportion with disease at a point in time; incidence is the number of new cases over a period. Case-control studies yield odds ratios; cohort studies yield relative risk.
- Sensitivity = true positives / all with disease; specificity = true negatives / all without disease. Positive predictive value = true positives / all test positives and depends heavily on prevalence.
- Kappa measures agreement beyond chance; above 0.8 is excellent, 0.6–0.8 good.
- Leavell and Clark levels: primary prevention before disease (fluoride, sealants, health education), secondary prevention by early detection (screening, early restoration), tertiary prevention by limiting disability (rehabilitation, prostheses).
- Community water fluoridation at about 1 ppm is the most cost-effective population measure; Dean's index grades fluorosis.
Worked example
In a screening programme, disease prevalence is 10%, sensitivity 90% and specificity 90% in 1,000 people. True positives = 90; false positives = 90 (10% of 900 healthy people); total positives = 180; PPV = 90/180 = 50%. So half of screen positives are false - the reasoning being that when prevalence is low, false positives from the large healthy group swamp true positives. In a high-prevalence clinic the same test would look far better, which is why PPV cannot be transferred between settings.
Traps
- Interchanging prevalence and incidence.
- Assuming high sensitivity implies high predictive value.
- Classifying restoration of a carious tooth as primary prevention.
- Confusing CPITN/CPI treatment needs with a measure of disease severity alone.