Preventive Medicine

Why it matters clinically

• Most premature death in the United States traces to a small number of modifiable exposures. The classic "actual causes of death" framing (as opposed to the death-certificate diagnoses of heart disease, cancer, and stroke) places tobacco use first, followed by poor diet/physical inactivity and alcohol — which is why counseling and pharmacotherapy for smoking cessation is repeatedly the highest-yield answer choice.
• Preventive services are not uniformly beneficial. The U.S. Preventive Services Task Force (USPSTF) grades each service A through D or I; A and B services must be covered without cost sharing under the Affordable Care Act, while grade D means net harm and should be actively avoided (a favorite distractor).

Epidemiology worth recalling

• Breast cancer is the most commonly diagnosed non-skin malignancy in U.S. women; lung cancer is the leading cause of cancer death in both sexes; colorectal cancer is a leading cancer killer overall and is rising in adults under 50, the reason the USPSTF moved the starting age for average-risk colorectal screening down to 45.
• Vaccine-preventable disease burden is concentrated at the extremes of age and in the immunocompromised; the CDC/ACIP immunization schedules are the governing U.S. reference for both children and adults.
• Uptake is uneven: screening and immunization rates are consistently lower among uninsured, rural, and racial/ethnic minority populations, a disparity examiners frame as a health-systems rather than biologic problem.

Test characteristics (intrinsic, prevalence-independent)

• Sensitivity = TP/(TP+FN) — the proportion of diseased people the test catches. Specificity = TN/(TN+FP) — the proportion of healthy people correctly cleared.
• Likelihood ratios convert pre-test to post-test odds and are the prevalence-free way to express test power: LR+ = sensitivity/(1 − specificity); LR− = (1 − sensitivity)/specificity. An LR+ above roughly 10 or an LR− below roughly 0.1 meaningfully moves probability.
• Cutoff shifting: lowering the threshold for "positive" raises sensitivity and lowers specificity. The ROC curve plots sensitivity against 1 − specificity; greater area under the curve = better discrimination.

Predictive values (prevalence-dependent)

• PPV = TP/(TP+FP), NPV = TN/(TN+FN). These answer the question the patient actually asks — "I tested positive; do I have it?" — and they swing dramatically with pre-test probability.

Effect measures

• ARR = risk(control) − risk(treated); RRR = ARR/risk(control); NNT = 1/ARR, rounded up. NNH = 1/attributable risk of harm.
• Attributable risk percent and population attributable risk quantify how much disease would disappear if an exposure were removed — the population-level counterpart to NNT.

Screening-specific biases (all inflate apparent benefit)

• Lead-time bias: survival measured from an earlier diagnosis looks longer though death occurs at the same moment. Corrected by using disease-specific mortality, not 5-year survival, as the endpoint.
• Length-time bias: screening preferentially detects slow-growing, indolent disease. Overdiagnosis is its extreme — detection of disease that never would have caused symptoms (prostate cancer, some DCIS).
• Healthy-volunteer (selection) bias: people who show up for screening are healthier at baseline.

Worked case — the asymptomatic 50-year-old woman, never-smoker, no family history

• Colorectal cancer: the USPSTF recommends screening average-risk adults beginning at age 45 through 75. Colonoscopy every 10 years, annual FIT, or stool DNA-FIT are all acceptable; the best screening test is the one the patient will complete. Any positive non-colonoscopy test requires diagnostic colonoscopy, not a repeat stool test.
• Breast cancer: the USPSTF recommends biennial screening mammography starting at age 40 through 74. A first-degree relative with premenopausal breast cancer or a known BRCA pathogenic variant moves her out of the average-risk pathway toward earlier and MRI-supplemented surveillance per NCCN.
• Cervical cancer: continue screening through age 65 with cytology and/or high-risk HPV testing per USPSTF intervals; stop at 65 only if prior screening was adequate and negative.
• Cardiometabolic: blood pressure screening, lipids, and diabetes screening in adults with overweight/obesity; the USPSTF supports a statin for primary prevention in adults 40–75 with at least one risk factor and sufficiently elevated calculated 10-year ASCVD risk, mirroring the ACC/AHA primary prevention framework.
• Immunization: annual influenza, COVID-19 per current ACIP schedule, Td/Tdap booster every 10 years, and recombinant zoster vaccine at age 50.

• Prevalence changes PPV/NPV, never sensitivity/specificity. If the stem alters the population being tested, only the predictive values move. This is the most frequently tested single relationship in the topic.
• Use disease-specific mortality, not 5-year survival, to judge a screening program. A stem describing improved survival with unchanged mortality is describing lead-time bias; if the detected tumors are unusually indolent, it is length-time bias or overdiagnosis.
• The single best next step after a positive stool-based colorectal test is diagnostic colonoscopy — never a repeat FIT and never reassurance.
• Smoking cessation counseling plus pharmacotherapy outranks almost every other intervention when a stem asks which action most reduces this patient's mortality. Tobacco is the leading modifiable cause of death.
• Live vaccines (MMR, varicella, live attenuated influenza, rotavirus, yellow fever) are contraindicated in pregnancy and in severe immunosuppression; mild illness with or without low-grade fever, breastfeeding, and antibiotic use are NOT contraindications. Egg allergy is no longer a barrier to influenza vaccination.
• A one-time abdominal ultrasound for AAA in men aged 65–75 who have ever smoked is a classic USPSTF grade B answer that students miss.
• Grade D means don't do it. The USPSTF recommends against some services; "more screening" is not automatically the right answer, and screening a low-prevalence population generates mostly false positives.
• Common distractor: choosing a highly sensitive confirmatory test or a highly specific initial screen. Screening favors sensitivity (SNOUT — rule out); confirmation favors specificity (SPIN — rule in).

• Primary prevention = prevent disease occurrence (vaccination, health education); Secondary prevention = early detection (screening); Tertiary prevention = manage complications
• Screening test validity depends on sensitivity (true positive rate) and specificity (true negative rate); PPV/NPV vary with disease prevalence
• Leading causes of death in US: heart disease, cancer, chronic respiratory disease, stroke, Alzheimer's (varies by age group)
• Number Needed to Treat (NNT) = 1/ARR; lower NNT = more beneficial intervention
• Immunization schedules differ by age; catch-up vaccination requires understanding gaps and contraindications

• Question stem: "Which intervention has the greatest impact on population health?" → Calculate NNT or prevalence-adjusted metrics
• Vignette: Asymptomatic 50-year-old asking about screening → Discuss screening criteria (Wilson-Jungner): disease significance, detectable preclinical phase, effective treatment, acceptable test accuracy
• Data question: "Sensitivity 95%, specificity 85%, disease prevalence 2%" → Calculate PPV (focuses exam questions)
• Vaccine-hesitant parent → Address contraindications vs. precautions; acknowledge risks/benefits

1. 1.Confusing sensitivity/specificity with PPV/NPV — The former are TEST properties (independent of prevalence); latter are CLINICAL utilities (depend on how common disease is). Prevalence is key for calculating predictive values.
2. 2.Applying screening to low-risk populations — Even good tests generate false positives when disease rare; leads to unnecessary anxiety, further testing, and cost. Always consider pre-test probability.
3. 3.Ignoring contraindications vs. precautions in vaccination — True contraindications (e.g., anaphylaxis to component) warrant deferral; precautions (e.g., mild illness) typically do NOT prevent vaccination. Exam tests this distinction.

Prevention is primary intervention

• Primary: Health education (smoking cessation, diet, exercise), immunization (age-appropriate schedules), environmental modifications, risk-factor screening
• Secondary: Screening tests (mammography, colonoscopy, BP monitoring, lipid panel) in asymptomatic at-risk populations per evidence-based guidelines (USPSTF, CDC)
• Tertiary: Chronic disease management (medication compliance, lifestyle modification, complication surveillance) and rehabilitation