Screening and Prevention of Disease¶
Chapter 6 | Part 1: The Profession of Medicine
KEY CLINICAL POINTS¶
- Screening tests must meet rigorous criteria including important health burden, available treatment, detectable latent stage, and acceptable population test before implementation
- Lead time bias and length time bias can falsely inflate perceived survival benefits; disease incidence or mortality must be the primary endpoint for assessing screening efficacy
- The U.S. Preventive Services Task Force (USPSTF) provides evidence-based recommendations graded on benefit-to-harm ratio for average-risk populations
- Tobacco and alcohol use, diet, and exercise constitute the majority of factors influencing preventable deaths in developed countries
- Risk-stratified approaches using genomic data (BRCA1/2, polygenic risk scores), imaging, and other biomarkers enable targeted screening with improved benefit-to-harm ratios
1. DEFINITION & OVERVIEW¶
Screening and prevention represent a primary goal of health care: to prevent disease or detect it early enough that intervention will be more effective. Preventive services differ from other medical interventions because they are proactively administered to healthy individuals rather than in response to symptoms, signs, or diagnoses. This fundamental difference requires a particularly high bar of evidence that testing and intervention are both practical and effective. Screening tests encompass multiple modalities: - Biochemical: cholesterol, glucose - Physiologic: blood pressure, growth curves - Radiologic: mammogram, bone densitometry - Cytologic: Pap smear Preventive interventions include: - Counseling about risk behaviors - Vaccinations - Medications (chemoprevention) - Surgery (in select settings)
Table 6-1: WHO Principles of Screening (1968)¶
| Principle |
|---|
| Facilities for diagnosis and treatment should be available |
| There should be a latent stage of the disease |
| There should be a test or examination for the condition |
| The test should be acceptable to the population |
| The natural history of the disease should be adequately understood |
| There should be an agreed policy on whom to treat |
| The cost of finding a case should be balanced in relation to overall medical expenditure |
1.1 Importance of Risk Stratification¶
Population-based screening strategies must be extremely low risk to achieve an acceptable benefit-to-harm ratio. The ability to target individuals more likely to develop disease enables application of a wider set of potential approaches and increases efficiency. Types of predictive data for asymptomatic individuals: 1. Germline genomic data (highest attention to date) 2. Polygenic risk scores (combining variants across hundreds of genes) 3. Proteomics and metabolomics 4. Circulating tumor DNA and stool DNA 5. Imaging data (e.g., coronary CT for statin therapy decisions) Example: BRCA1/BRCA2 mutations confer 5- to 20-fold increased risk of breast and ovarian cancer, justifying more intensive interventions (prophylactic oophorectomy, breast MRI, prophylactic mastectomy) that would be inappropriate for average-risk women.
2. EPIDEMIOLOGY¶
Screening is most effective when applied to relatively common disorders that carry a large disease burden. Understanding the epidemiology of preventable conditions guides resource allocation and screening priorities.
Table 6-2: Lifetime Cumulative Risk¶
| Condition | Lifetime Risk |
|---|---|
| Breast cancer for women | 10% |
| Colon cancer | 6% |
| Cervical cancer for women (unscreened, no HPV vaccination) | 2% |
| Domestic violence for women | Up to 15% |
| Hip fracture for white women | 16% |
2.1 Leading Causes of Mortality (United States)¶
- Heart diseases
- Malignant neoplasms
- Chronic obstructive pulmonary disease
- Accidents
- Cerebrovascular diseases
- Alzheimer's disease (sixth leading cause) Global health perspective adds:
- Malaria
- Malnutrition
- AIDS
- Tuberculosis
- Violence
2.2 Lifetime Cumulative Disease Risk¶
Understanding baseline disease risk helps contextualize the potential impact of screening programs.
3. BASIC PRINCIPLES & PATHOPHYSIOLOGY OF SCREENING¶
The effectiveness of screening depends on the natural history of the disease being screened and the characteristics of the screening test.
3.1 Disease Natural History Considerations¶
Latent/Preclinical Phase: - A long latent phase where early treatment increases cure chance is ideal - Example: Polypectomy prevents progression to colon cancer - Example: Early identification of hypertension/hyperlipidemia allows therapeutic interventions reducing long-term cardiovascular/cerebrovascular events Challenges with Short Preclinical Phase: - Lung cancer: Most tumors historically not curable by time detectable on chest x-ray - Low-dose CT changed this paradigm by detecting tumors earlier - 20% reduction in lung cancer mortality in individuals with ≥ 30-pack-year smoking history Indolent Disease Concerns: - Prostate cancer: Often indolent; competing morbidities (e.g., coronary artery disease) may cause mortality before prostate cancer - Contributes to controversy about treatment and screening
3.2 Critical Biases in Screening Studies¶
Lead Time Bias: - Screening identifies a case before clinical presentation - Creates perception of longer survival simply by moving diagnosis date earlier (not death date later) - Does NOT represent true survival benefit Length Time Bias: - Screening preferentially identifies slowly progressive disease - Rapidly progressive disease less likely to be caught by screening - Screened population appears to have better survival due to case mix, not treatment effect Implication: Disease incidence or mortality (not survival) must be the primary endpoint for assessing screening effectiveness.
3.3 Available Treatment Considerations¶
Effective treatment for early disease is essential but challenging for some conditions: - Alzheimer's disease: Sixth leading cause of death but no curative treatments; limited data that early treatment improves outcomes - Developing countries: Lack of diagnosis/treatment facilities may necessitate 'see and treat' approaches (e.g., cervical cancer screening)
4. METHODS OF MEASURING HEALTH BENEFITS¶
Screening and preventive interventions are held to a high standard for demonstrating favorable risk-benefit ratio. Randomized controlled trials (RCTs) with mortality outcomes are the gold standard, though observational studies and ecologic data are sometimes used.
Table 6-3: Estimated Average Increase in Life Expectancy for a Population¶
| Screening or Preventive Intervention | Average Increase |
|---|---|
| Mammography: Women 40-50 years | 0-5 days |
| Mammography: Women 50-70 years | 1 month |
| Pap smears, age 18-65 | 2-3 months |
| Getting a 35-year-old smoker to quit | 3-5 years |
| Beginning regular exercise for a 40-year-old man (30 min, 3 times/week) | 9 months-2 years |
4.1 Absolute vs. Relative Impact¶
Absolute difference in disease incidence/mortality allows comparison of benefit size across preventive services. Mammography (Swedish meta-analysis, ages 40-70): - ~1.2 fewer women per 1000 die from breast cancer over 12 years - 14-32% relative reduction in breast cancer death Fecal Occult Blood Testing (FOBT, ages 50-75): - ~3 lives saved per 1000 over 13 years - 20-24 lives saved per 1000 over 25 years - 30% relative reduction in colon cancer death Conclusion: Colon cancer screening may save more women's lives than mammography in absolute terms, though relative impacts are similar.
4.2 Number Needed to Screen (NNS)¶
The inverse of the absolute difference in mortality represents the number of subjects who need to be screened to prevent one death or disease event. Example: 731 women aged 65-69 need DEXA screening (with appropriate treatment) to prevent one hip fracture from osteoporosis.
4.3 Increase in Average Life Expectancy¶
Important concept: Life expectancy gains are population averages, not individual benefits. Reality: - Vast majority of population derives no benefit from screening - Small subset benefits greatly - Example: Pap smears don't benefit 98% of women who never develop cervical cancer, but for the 2% who would have developed it, may add ~25 years of life Reasonable goal: A 1-month gain in life expectancy is considered reasonable for a population-based screening strategy.
5. ASSESSING THE HARMS OF SCREENING AND PREVENTION¶
Screening and preventive interventions incur potential adverse outcomes that must be weighed against benefits.
5.1 Categories of Harm¶
- Side effects from preventive medications and vaccinations
- Medication side effects: Analogous to therapeutic settings; FDA approval process addresses this
- Vaccination side effects: Primarily discomfort and minor immune reactions
- Concern about serious adverse outcomes limits vaccine acceptance despite lack of causal data
- False-positive screening tests
- Mammography/Chest CT: Abnormality identified that is not malignant; requires biopsy or short-term follow-up
- Pap smears: Wide range of potentially premalignant states identified; only small percentage progress to invasive cancer
- Overdiagnosis
- Screening identifies disease that would not have presented clinically in patient's lifetime
- Very difficult to assess; requires long-term follow-up of unscreened population
- Estimated overdiagnosis rates: - Breast cancer (mammography): 15-40% - Prostate cancer (PSA testing): 15-37%
- Anxiety
- Increased anxiety documented through screening process, especially with false-positives
- Few data suggesting long-term adverse consequences or effects on subsequent screening behavior
- Radiation exposure
- Mammography, chest CT add to cumulative radiation exposure
- Absolute amount small per test; overall impact of repeated exposure still being determined
- Discomfort
- Vaccinations, mammography may cause discomfort
- Little evidence of long-term adverse consequences
6. WEIGHING BENEFITS AND HARMS¶
Implementation of population-based screening requires comprehensive cost-benefit analysis including economic impact.
6.1 Cost-Effectiveness Analysis¶
Costs include: - Expense of intervention - Time away from work - Downstream costs from false-positives - Incidentalomas - Adverse events Cost-effectiveness metric: Cost per quality-adjusted life-year (QALY) saved Cost-effective threshold: $50,000-100,000 per QALY saved
6.2 U.S. Preventive Services Task Force (USPSTF)¶
Independent panel of experts in preventive care providing evidence-based recommendations based on benefit-to-harm ratio assessment. Areas of high consensus among advisory organizations: - Hyperlipidemia screening - Colorectal cancer screening Areas of lower consensus: - Breast cancer screening in women aged 40-49 - Prostate cancer screening Note: Guideline differences may reflect timing of data availability at guideline issuance.
6.3 Risk-Based Screening Modifications¶
When benefit-to-harm ratio is uncertain for average-risk population but favorable for higher-risk individuals: - Age: Most commonly used risk factor for screening recommendations - Other risk factors: USPSTF recommends some tests based on additional risk factors - Family history: Initiate screening earlier (e.g., colon cancer screening 10 years before youngest affected family member's diagnosis age)
6.4 Shared Decision-Making¶
Particularly important when benefit-to-harm ratio is uncertain for a specific population. Examples: 1. Prostate cancer screening: - Many expert groups (including American Cancer Society) recommend individualized discussion - Complex decision-making relying on personal issues - Some men may decline; others may accept early detection risks 2. Colon cancer screening technique selection: - Annual FOBT: 15-30% reduction in colon cancer deaths - Flexible sigmoidoscopy: 40-60% reduction - Colonoscopy: 50-70% reduction under optimal conditions - Cost per life saved similar across techniques ($10,000-25,000) - Patient preference may drive choice (ease vs. thoroughness vs. interval)
7. SCREENING TESTS: USPSTF RECOMMENDATIONS¶
The following table summarizes screening tests recommended by the USPSTF for average-risk adults.
Table 6-4: Screening Tests Recommended by USPSTF for Average-Risk Adults¶
| Disease | Test | Population | Frequency |
|---|---|---|---|
| Abdominal aortic aneurysm | Ultrasound | Men 65-75 who have ever smoked | Once |
| Alcohol misuse | Alcohol Use Disorders Identification Test | All adults | Unknown |
| Breast cancer | Mammography ± clinical breast exam | Women (40?) 50-75 | Every 2 years |
| Disease | Test | Population | Frequency |
|---|---|---|---|
| Cervical cancer | Pap smear | Women 21-65 | Every 3 years |
| Cervical cancer | Pap smear and/or HPV testing | Women 30-65 | Every 5 years if HPV negative |
| Chlamydia/gonorrhea | Nucleic acid amplification test (urine/cervical) | Sexually active women <25 | Unknown |
| Colorectal cancer | Fecal occult blood testing | 45-75 | Every year |
| Colorectal cancer | Fecal immunochemical-DNA | 45-75 | Every 1-3 years |
| Colorectal cancer | Sigmoidoscopy | 45-75 | Every 5 years |
| Colorectal cancer | Colonoscopy | 45-75 | Every 10 years |
| Depression + anxiety | Screening questions | All adults | Periodically |
| Diabetes | Fasting glucose or HgbA1c | Adults overweight, obese, or with HTN | Every 3 years |
| Hepatitis C | Anti-HCV antibody + confirmatory PCR | 18-79 | Once |
| HIV | Reactive immunoassay/rapid HIV + confirmatory | 15-65 | At least once |
| Hyperlipidemia | Cholesterol | 40-75 | Unknown |
| Hypertension | Blood pressure | All adults | Periodically |
| Intimate partner violence | Screening questions | Women of childbearing age | Unknown |
| Lung cancer | Low-dose CT | Adults 50-80 with ‡20 pack-year history, current smoker or quit within 15 years | Yearly |
| Obesity | Body mass index | All adults | Unknown |
| Osteoporosis | DEXA | Women >65 or >60 with risk factors | Unknown |
8. PREVENTIVE INTERVENTIONS: USPSTF RECOMMENDATIONS¶
Preventive interventions include immunizations and chemoprevention strategies.
Table 6-5: Preventive Interventions Recommended for Average-Risk Adults¶
| Intervention | Disease | Population | Frequency |
|---|---|---|---|
| COVID-19 vaccine | COVID-19 | >18 | - |
| Tetanus-diphtheria | - | >18 | Every 10 years |
| Varicella | - | Susceptibles only, >18 | Two doses |
| MMR | - | Women, childbearing age | One dose |
| Pneumococcal | - | >64 | 20 valent or 15/23 |
| Influenza | - | >18 | Yearly |
| HPV | - | Up to age 27 | If not done prior |
| Intervention | Disease | Population | Frequency |
|---|---|---|---|
| Zoster | - | >60 | Once |
| Aspirin | Cardiovascular disease | 40-59 with ‡10% 10-year CVD risk | - |
| Folic acid | Neural tube defects | Women planning/capable of pregnancy | - |
| Tamoxifen/Raloxifene | Breast cancer | High-risk women | - |
| Vitamin D | Fracture/falls | >64 at increased fall risk | - |
8.1 Adult Immunizations¶
Recommended vaccinations for adults: - COVID-19: Age >18 - Tetanus-diphtheria: Age >18, every 10 years - Varicella: Susceptibles only, >18, two doses - Measles-mumps-rubella (MMR): Women of childbearing age, one dose - Pneumococcal: Age >64, 20-valent option or 15/23 - Influenza: Age >18, yearly - Human papillomavirus (HPV): Up to age 27, if not done prior - Zoster: Age >60, once
8.2 Chemoprevention¶
Disease-specific chemoprevention recommendations: - Aspirin for cardiovascular disease: - Population: Aged 40-59 years with ≥ 10% 10-year CVD risk - Note: Bleeding risk may equal benefit for some groups - Folic acid for neural tube defects: - Population: Women planning or capable of pregnancy - Tamoxifen/Raloxifene for breast cancer: - Population: Women at high risk for breast cancer - Vitamin D for fractures/falls: - Population: Age >64 at increased risk for falls
9. COUNSELING ON HEALTHY BEHAVIORS¶
Tobacco and alcohol use, diet, and exercise constitute the vast majority of factors influencing preventable deaths in developed countries.
Table 6-6: Preventive Counseling Recommended by USPSTF¶
| Topic |
|---|
| Alcohol and drug use |
| Genetic counseling for BRCA1/2 testing among women at increased risk for deleterious mutations |
| Nutrition and diet |
| Sexually transmitted infections |
| Sun exposure |
| Tobacco use |
9.1 USPSTF-Recommended Counseling Topics¶
Evidence strongly supports the role of health care provider counseling in effecting behavior change: - Alcohol and drug use - Genetic counseling for BRCA1/2 testing (women at increased risk for deleterious mutations) - Nutrition and diet - Sexually transmitted infections - Sun exposure - Tobacco use Key point: Helping patients quit smoking may be the single greatest preventive health care measure.
9.2 Additional Counseling Areas¶
Although USPSTF found conclusive evidence for a limited set of counseling activities, routine primary care practice includes: - Physical activity - Injury prevention: - Seat belt use - Bicycle helmets - Motorcycle helmets - Weight management (top priority given multiple complications of obesity)
10. IMPLEMENTING DISEASE PREVENTION AND SCREENING¶
Implementation of screening and prevention strategies in practice presents challenges requiring systematic approaches.
10.1 Implementation Techniques¶
- Electronic health record (EHR) reminder systems: Track and meet guidelines more easily
- Patient portal access: Secure medical record access enhances screening adherence
- Standing orders: Effective for immunizations when given to nurses and staff
- USPSTF tools: Flow sheets and electronic tools available at uspreventiveservicestaskforce.org
- Age-specific recommendations: Help guide implementation
10.2 Integrating Prevention into Chronic Disease Care¶
Opportunities: - Patients seen for chronic illness management (hypertension, diabetes) can incorporate screening - Example: Breast cancer screening at one visit; colon cancer discussion at next visit - Some patients prefer clearly defined visits addressing all relevant screening/prevention Sunsetting Screening: - Due to age or comorbidities, may be appropriate to abandon certain screening activities - For most screening tests, benefit does not accrue until 5-10 years of follow-up - Generally few data support continuing most screening past age 75 - For patients with advanced diseases/limited life expectancy: Shift focus from screening to interventions more likely to affect quality and length of life
11. AGE-SPECIFIC PREVENTION STRATEGIES¶
Comprehensive age-stratified approach to screening and prevention based on leading causes of mortality for each age group.
Table 6-7: Age-Specific Causes of Mortality and Corresponding Preventive Options¶
| Age Group | Leading Causes of Mortality | Screening/Prevention Interventions |
|---|---|---|
| 15-24 | 1. Accident 2. Homicide 3. Suicide 4. Malignancy 5. Heart disease | (cid:127) Seat belt, helmet counseling (cid:127) Diet and exercise counseling (cid:127) Alcohol/driving dangers (cid:127) Update vaccinations (tetanus, diphtheria, hepatitis B, MMR, varicella, meningitis, HPV, COVID-19) (cid:127) Gun use/possession assessment (cid:127) Substance abuse screening (cid:127) Domestic violence screening (cid:127) Depression/suicidal/homicidal ideation screening (cid:127) Pap smear after age 21 (cid:127) Skin, breast, testicular self-exam discussion (cid:127) UV avoidance, sunscreen (cid:127) BP, height, weight, BMI measurement (cid:127) Tobacco cessation (emphasize cosmetic/economic issues) (cid:127) Chlamydia/gonorrhea screening, contraceptive counseling (sexually active females) (cid:127) Hepatitis B, syphilis testing if high-risk behavior (cid:127) Hepatitis C screening (18-79) (cid:127) HIV testing (cid:127) Annual influenza vaccination |
| 25-44 | 1. Accident 2. Malignancy 3. Heart disease 4. Suicide 5. Homicide 6. HIV | Above PLUS: (cid:127) Smoking cessation at every visit (cid:127) Detailed family history of malignancies; early screening if significant increased risk (cid:127) Assess cardiac risk factors; consider statin therapy for higher-risk patients (cid:127) Assess for chronic alcohol abuse, viral hepatitis risk factors, chronic liver disease risk (cid:127) Consider individualized breast cancer screening with mammography at age 40 |
| 45-64 | 1. Malignancy 2. Heart disease 3. Accident 4. Diabetes mellitus 5. Cerebrovascular disease 6. Chronic lower respiratory disease 7. Chronic liver disease/cirrhosis 8. Suicide | (cid:127) Consider prostate cancer screening (PSA + DRE) at age 50 (earlier in African Americans or family history) (cid:127) Colorectal cancer screening at age 45 or 50 (FOBT, stool DNA, sigmoidoscopy, or colonoscopy) (cid:127) Update vaccinations at age 50; pneumococcal vaccine for all smokers at age 50 (cid:127) Consider coronary disease screening in higher-risk patients (cid:127) Zoster vaccination at age 60 (cid:127) Begin mammography screening by age 50 (cid:127) Lung cancer screening (age 50-80): Low-dose CT yearly if ‡20 pack-year history, current smoker or quit within 15 years |
| Age Group | Leading Causes of Mortality | Screening/Prevention Interventions |
|---|---|---|
| ‡65 | 1. Heart disease 2. Malignancy 3. Cerebrovascular disease 4. Chronic lower respiratory disease 5. Alzheimer's disease 6. Influenza and pneumonia 7. Diabetes mellitus 8. Kidney disease 9. Accidents 10. Septicemia | (cid:127) Smoking cessation at every visit (cid:127) One-time AAA ultrasound (men 65-75 who have ever smoked) (cid:127) Pulmonary function testing for long-term smokers (assess for COPD) (cid:127) Osteoporosis screening (all postmenopausal women; all men with risk factors) (cid:127) Annual influenza vaccination; pneumococcal vaccine at age 65 (cid:127) Visual and hearing problem screening (cid:127) Home safety assessment (cid:127) Elder abuse screening (cid:127) Fall prevention exercise intervention if at higher risk |
12. KEY POINTS & CLINICAL PEARLS¶
12.1 Fundamental Principles¶
- Screening tests are administered to asymptomatic individuals—requires higher evidence bar than therapeutic interventions
- WHO 1968 principles remain foundational: important problem, available treatment, detectable latent stage, acceptable test
- Disease mortality (not survival) is the gold standard endpoint to avoid lead time and length time biases
12.2 Quantifying Benefits¶
- Number needed to screen (NNS) allows comparison across interventions
- Life expectancy gains are population averages; most individuals derive no benefit while a small subset benefits greatly
- 1-month population life expectancy gain is reasonable threshold for screening strategy
- Cost-effectiveness threshold: $50,000-100,000 per QALY saved
12.3 Recognizing Harms¶
- Overdiagnosis is a significant concern: 15-40% of mammography-detected breast cancers and 15-37% of PSA-detected prostate cancers may never have presented clinically
- False-positive results lead to additional testing, biopsies, and patient anxiety
- Cumulative radiation exposure from repeated screening tests is an evolving concern
12.4 Implementation Strategies¶
- EHR reminder systems and standing orders improve screening compliance
- Integrate prevention into chronic disease management visits
- Consider sunsetting screening after age 75 or when life expectancy is limited
- Shared decision-making is essential when benefit-to-harm ratio is uncertain (e.g., prostate cancer screening, colon cancer screening method selection)
12.5 Highest-Impact Interventions¶
- Smoking cessation: Single greatest preventive health care measure; may add 3-5 years of life expectancy
- Tobacco, alcohol, diet, and exercise constitute the majority of preventable mortality factors
- Risk-stratified approaches using genomic data (BRCA1/2, polygenic risk scores) and imaging enable more efficient, targeted screening