DHA Public Health & Epidemiology — Questions and Answers
Question 1: In a city of 200,000 people, 400 new cases of tuberculosis are diagnosed during one year. The total number of active TB cases at year-end is 1,200. What is the annual INCIDENCE rate per 100,000 population?
- 600 per 100,000
- 200 per 100,000 (Correct answer)
- 400 per 100,000
- 800 per 100,000
Correct answer: 200 per 100,000
Incidence = new cases / population at risk × 100,000 = 400 / 200,000 × 100,000 = 200 per 100,000. Prevalence would use total active cases (1,200), giving 600 per 100,000.
Question 2: A screening test for hepatitis C has a sensitivity of 95% and a specificity of 90%. In a population where the true prevalence is 1%, which statement is CORRECT?
- 95% of those who test positive actually have hepatitis C
- The positive predictive value will be low despite high sensitivity (Correct answer)
- Specificity is more important than sensitivity for rare diseases
- A negative result rules out infection with 90% certainty
Correct answer: The positive predictive value will be low despite high sensitivity
When prevalence is low (1%), even a highly sensitive test produces many false positives relative to true positives, resulting in a low positive predictive value (PPV). PPV depends heavily on disease prevalence, not just test characteristics.
Question 3: A national program provides vitamin A supplements to all children under 5 to prevent blindness. This is an example of which level of prevention?
- Tertiary prevention
- Secondary prevention
- Primary prevention (Correct answer)
- Quaternary prevention
Correct answer: Primary prevention
Primary prevention aims to prevent disease before it occurs. Supplementing healthy children to prevent vitamin A deficiency and its complications (blindness) acts before any disease develops. Secondary prevention involves screening for early disease; tertiary prevents disability from established disease.
Question 4: A cohort study follows 500 smokers and 500 non-smokers for 10 years. Among smokers, 50 develop lung cancer; among non-smokers, 5 develop lung cancer. What is the relative risk (RR) of lung cancer in smokers?
- 5
- 10 (Correct answer)
- 45
- 2
Correct answer: 10
RR = (risk in exposed) / (risk in unexposed) = (50/500) / (5/500) = 0.10 / 0.01 = 10. Smokers are 10 times more likely to develop lung cancer than non-smokers in this study.
Question 5: According to WHO and Dubai Health Authority communicable disease protocols, a healthcare provider who diagnoses a case of cholera must report it to public health authorities within:
- 72 hours
- 7 days
- Immediately (within 24 hours) (Correct answer)
- 28 days
Correct answer: Immediately (within 24 hours)
Cholera is a Category 1 notifiable disease under international health regulations (IHR 2005), requiring immediate notification (within 24 hours) to allow rapid outbreak response. Delayed reporting significantly hampers containment.
Question 6: Which study design is MOST appropriate for investigating the cause of a rare disease such as a specific occupational cancer?
- Randomized controlled trial
- Cross-sectional survey
- Case-control study (Correct answer)
- Prospective cohort study
Correct answer: Case-control study
Case-control studies work backward from disease outcome, comparing those with the disease (cases) to those without (controls) regarding past exposures. They are efficient for rare diseases because you start by selecting cases, avoiding the enormous sample sizes a cohort study would require.
DHA Public Health & Epidemiology
In a city of 200,000 people, 400 new cases of tuberculosis are diagnosed during one year.
The total number of active TB cases at year-end is 1,200.
What is the annual INCIDENCE rate per 100,000 population?