Master Epidemiology & Public Health
for PLAB 1

What the PLAB 1 Tests in Epidemiology & Public Health

Candidates must interpret epidemiological measures (incidence, prevalence, odds ratio, relative risk, attributable risk, number needed to treat) in clinical scenarios. They apply screening test characteristics (sensitivity, specificity, positive/negative predictive values) and understand bias, confounding, and study designs (RCT, cohort, case-control, cross-sectional, systematic review). Public health topics include UK vaccination schedules, communicable disease control (measles, TB, COVID-19, influenza, hepatitis B, meningococcal disease), health inequalities, and screening programmes (NHS cervical, breast, bowel, abdominal aortic aneurysm, diabetic retinopathy). They must know notifiable diseases, infection control (MRSA, C. difficile, norovirus), and principles of outbreak investigation.

High-Yield Concepts

• Number Needed to Treat (NNT): NNT = 1 / Absolute Risk Reduction (ARR). ARR = Control Event Rate (CER) – Experimental Event Rate (EER). For example, if CER=0.10 and EER=0.06, ARR=0.04, NNT=25. Lower NNT indicates more effective treatment.
• Screening Test Characteristics: Sensitivity = true positives / (true positives + false negatives). Specificity = true negatives / (true negatives + false positives). Positive Predictive Value (PPV) = true positives / (true positives + false positives). PPV falls as disease prevalence falls. NHS Bowel Cancer Screening uses faecal immunochemical test (FIT) at ≥120 µg Hb/g faeces.
• UK Vaccination Schedule (key ages): 8 weeks: 6-in-1 (DTaP/IPV/Hib/HepB), rotavirus, MenB. 12 weeks: 6-in-1, pneumococcal (PCV13), rotavirus. 16 weeks: 6-in-1, MenB, PCV13. 1 year: Hib/MenC, MMR, PCV13, MenB booster. 3 years 4 months: MMR, DTaP/IPV. 12-13 years: HPV (Gardasil 9, 2 doses). 14 years: Td/IPV, MenACWY.
• Notifiable Diseases (Public Health England): Acute infectious diseases that must be reported to the local health protection team under the Health Protection (Notification) Regulations 2010. Examples: measles, mumps, rubella, pertussis, diphtheria, tetanus, polio, meningococcal disease, tuberculosis, hepatitis A/B/C, COVID-19, norovirus, food poisoning (any cause), legionellosis, malaria, rabies, yellow fever.
• Odds Ratio vs Relative Risk: Odds Ratio (OR) = (a/b) / (c/d) for a 2x2 table. Relative Risk (RR) = (a/(a+b)) / (c/(c+d)). In case-control studies, use OR; in cohort and RCT, use RR. OR approximates RR only when disease is rare (<10%). Example: smoking and lung cancer OR ~15, RR ~10.
• NHS Screening Programmes: Cervical screening: ages 25-49 every 3 years, 50-64 every 5 years (HPV primary testing). Breast screening: women 50-71 every 3 years (mammography). Bowel screening: 60-74 every 2 years (FIT kit). Abdominal Aortic Aneurysm: men 65 once (ultrasound). Diabetic retinopathy: annual digital retinal photography for all diabetics ≥12 years.
• Bias and Confounding: Selection bias: systematic error in selecting participants (e.g., healthy worker effect). Information bias: misclassification (e.g., recall bias in case-control studies). Confounding: a third variable associated with both exposure and outcome (e.g., age confounding the link between grey hair and heart disease). Control by randomisation, restriction, matching, or multivariable analysis.
• Outbreak Investigation Steps: 1) Confirm diagnosis and existence of outbreak. 2) Define case (clinical, lab, time/place/person). 3) Descriptive epidemiology (epidemic curve). 4) Generate hypothesis. 5) Analytical study (cohort or case-control). 6) Implement control measures (isolation, hygiene, vaccination, treatment). 7) Communicate findings. Example: norovirus outbreak on a ward – stool samples, exclude staff/patients for 48 hours after symptom resolution.

Common Traps in Epidemiology & Public Health Questions

• Confusing incidence (new cases in a time period) with prevalence (all existing cases at a point in time).
• Thinking that a test with high sensitivity automatically has high PPV – PPV depends heavily on disease prevalence.
• Assuming odds ratio equals relative risk in common diseases (e.g., hypertension, diabetes) – it overestimates risk when event rate >10%.
• Forgetting that the NHS cervical screening now uses HPV primary testing, not cytology alone.
• Mixing up the ages for bowel screening (60-74) vs breast screening (50-71) vs AAA screening (men at 65).
• Believing that a negative screening test definitively rules out disease – false negatives occur, especially in early disease.

How to Revise Epidemiology & Public Health for the PLAB 1

Focus on applying epidemiological measures to clinical vignettes: calculate NNT, ARR, sensitivity, specificity, PPV from given 2x2 tables. Memorise key UK screening ages and vaccination schedule milestones. Understand study designs: RCT (gold standard for efficacy), cohort (for prognosis/risk factors), case-control (for rare diseases). Know the difference between bias and confounding, and how each is addressed. Questions often present a short clinical scenario (e.g., a new screening test for colorectal cancer) and ask you to interpret the results or identify the best study design. Practise interpreting odds ratios and relative risks with confidence intervals. For public health, be ready to name the correct action for a notifiable disease (e.g., immediate notification to local health protection team) and infection control measures for MRSA, C. difficile, and norovirus.

Practise it: MedLumen has 50 Epidemiology & Public Health questions for the PLAB 1, each with a full explanation and references.