Public Health
Public Health tests epidemiological measurement, levels of prevention, screening statistics and communicable disease control. The arithmetic is simple, but definitions decide the mark.
Measurement
- Incidence counts new cases in a population at risk over a period; prevalence counts all existing cases at a point or over a period. Prevalence can rise when survival improves even if incidence is unchanged.
- IMR = infant deaths under one year per 1,000 live births; MMR = maternal deaths per 100,000 live births; TFR is births per woman.
- Attack rate is used in outbreaks; case fatality rate = deaths divided by cases.
Prevention
- Primordial: reduce risk factors in the whole population. Primary: prevent disease before it occurs, as with immunisation. Secondary: early detection through screening. Tertiary: limit disability and rehabilitate. Quaternary: avoid over-medicalisation.
Screening arithmetic
Sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); predictive values depend on prevalence.
Worked example
2,000 women are screened, 100 have the disease, and the test detects 90. Sensitivity = 90/100 = 90%. Of 1,900 healthy women, 95 test positive, so specificity = 1,805/1,900 = 95%. If the same test is used in a low-prevalence group, the positive predictive value falls although sensitivity and specificity stay the same.
Traps
Sensitivity and specificity are unaffected by prevalence, but PPV and NPV are not. Prevalence is not incidence. Immunisation is primary prevention, and screening is secondary prevention even when it leads to cure.