Screening has become an essential public health strategy for early disease detection, enabling timely intervention and reducing the burden of morbidity and mortality. However, it is not a blanket process to be applied indiscriminately. The concept of screening is governed by scientific, ethical, and economic considerations, primarily laid out by Wilson and Jungner in 1968. This essay explores these criteria in depth, alongside the operational aspects of screening tests and programs.
1. Understanding Screening: Definition and Importance
Screening is a process of identifying apparently healthy individuals who may be at increased risk of a disease or condition. Unlike diagnostic testing, which confirms disease in symptomatic individuals, screening is applied to asymptomatic populations to detect disease at an early, more treatable stage.
One might ask: why not conduct comprehensive scans like MRI or PET on every individual annually? The answer lies in the structured criteria that differentiate effective screening from unnecessary or even harmful over-testing.
2. Criteria for Screening a Disease
According to Wilson and Jungner, the disease itself must meet certain prerequisites to justify screening:
- Public Health Importance: The disease should be a major health burden (e.g., hypertension, cervical cancer).
- Latent or Early Detectable Stage: There should be an asymptomatic phase that can be targeted for intervention (e.g., prediabetes).
- Well-understood Natural History: The disease’s progression must be predictable, enabling timely intervention (e.g., type 2 diabetes).
- Availability of Suitable Test: There should be a reliable, validated test (e.g., Pap smear for cervical cancer).
- Test Acceptability: The test must be culturally and socially acceptable (e.g., FIT preferred over colonoscopy).
- Clear Treatment Policy: Thresholds for treatment should be established (e.g., fasting glucose >126 mg/dL).
- Effective Treatment: The treatment must be available and impactful (e.g., antihypertensives).
- Facilities for Diagnosis and Treatment: Infrastructure should support follow-up care.
- Economic Balance: The cost of screening must be justifiable in the healthcare system (e.g., blood glucose testing vs. full-body MRI).
- Continuous Process: Screening should be ongoing, not a one-time effort.
3. Criteria for Screening Tests
A good screening test is not just about accuracy—it must also be practical and sustainable. Key characteristics include:
- Acceptability: Influenced by pain, time, privacy, cultural norms, and cost.
- Repeatability (Reliability): Consistent results across observers and time; high for automated tests like HbA1c.
- Validity: Includes both sensitivity (true positives) and specificity (true negatives). For instance, HIV ELISA is highly sensitive; Western blot is highly specific.
- Yield: The number of undiagnosed cases discovered. Yield is higher in high-prevalence groups and with high-sensitivity tests.
- Simplicity and Rapidity: The test should be easy to administer, quick, and safe.
- Cost and Feasibility: Must be affordable and scalable to the target population.
4. Acceptability and Its Determinants
Acceptability affects participation. Tests perceived as invasive, painful, time-consuming, or socially stigmatizing often face low uptake. For example:
- Blood pressure checks are highly acceptable.
- Colonoscopies or HIV tests may be avoided due to discomfort or stigma.
Improving acceptability includes using non-invasive tools, ensuring privacy, offering subsidized testing, and involving trusted community health workers.
5. Repeatability and Test Reliability
Repeatability ensures the test result is consistent when repeated under identical conditions. It includes:
- Intra-observer and inter-observer repeatability
- Instrument consistency
- Standardization of procedures
Tests like automated hemoglobin estimation score high, while manual tests with subjective interpretation (e.g., physical exams) often show poor reliability.
6. Validity and ROC Curves
A valid test accurately identifies disease presence and absence. High sensitivity is prioritized when missing a disease is risky; high specificity when false positives are problematic.
Receiver Operating Characteristic (ROC) Curve: A graphical tool plotting sensitivity against 1-specificity. The area under the curve (AUC) quantifies test accuracy—closer to 1 means better performance.
7. Managing Borderline Results
Many tests yield ambiguous results near diagnostic thresholds (e.g., fasting glucose of 110 mg/dL). Managing this “gray zone” involves:
- Repeat testing (e.g., OGTT),
- Combining clinical assessments,
- Using multiple markers (e.g., glucose + HbA1c),
- Categorizing results into indeterminate, positive, or negative.
8. Screening in Practice: Age-specific Recommendations
Antenatal Screening
Tests include hemoglobin, OGTT, VDRL, HIV, HBsAg, and thyroid function. These are scheduled at specific gestational milestones.
Infants
Heel-prick tests, physical examinations, hearing and vision screening, and developmental milestones are tracked.
Middle-aged Adults
Include checks for hypertension, diabetes, lipids, breast and cervical cancer, and oral and colon cancer.
Geriatric Population
Include osteoporosis (DEXA scan), cognitive decline (MMSE), depression (GDS), and fall risk assessments.
9. Yield of Screening
Yield is the actual number of previously undiagnosed cases detected per unit population screened. It’s influenced by:
- Disease prevalence
- Test sensitivity
- Targeting high-risk groups
- Repeated screening rounds
Corrected yield adjusts for false positives; net yield discounts cases that would have been diagnosed clinically anyway.
10. Evaluating Screening Programs
Effectiveness is judged by:
- Coverage: >80% is ideal
- Participation Rate: >70% is desirable
- Sensitivity/Specificity: Should exceed 80% and 90% respectively
- Detection Rate and Yield: Vary by condition
Study Designs for Evaluation:
- RCTs: Gold standard but costly and complex
- Uncontrolled trials: Simple, good for pilots
- Case-control and cohort studies: Used to understand impact and natural history
- Ecological studies: Useful for policy comparison
Conclusion
Screening is a cornerstone of preventive medicine, but its application demands meticulous adherence to established criteria. Not every disease is fit for screening, and not all tests are worth implementing. The balance of cost-effectiveness, test validity, acceptability, and health infrastructure determines the success of any screening program. As public health systems advance, tailoring screening strategies to demographic and cultural realities will ensure that early detection translates into tangible health outcomes.
MCQs on Principles and Criteria of Screening
1. Which of the following is NOT a criterion in Wilson and Jungner’s principles for screening a disease?
A. Disease should have a latent stage
B. Disease must be curable in all cases
C. Facilities for diagnosis should be available
D. There should be an agreed policy on whom to treat
Answer: B
2. What is the best reason for not recommending whole-body MRI as a general screening tool?
A. It is highly sensitive
B. It causes radiation exposure
C. It is expensive and has low yield
D. It detects only cancers
Answer: C
3. Which of the following is an example of a disease suitable for screening?
A. Tennis elbow
B. Pancreatic cancer
C. Hypertension
D. ALS (Amyotrophic Lateral Sclerosis)
Answer: C
4. In Wilson and Jungner’s criteria, which of the following diseases lacks a clearly defined early stage?
A. Cervical cancer
B. Type 2 diabetes
C. Pancreatic cancer
D. Hypertension
Answer: C
5. Screening should be a:
A. One-time program
B. Physician-led initiative
C. Patient-choice only
D. Continuing process
Answer: D
MCQs on Criteria for Screening Tests
6. Repeatability of a test refers to:
A. Test’s accuracy compared to gold standard
B. Ability to produce consistent results under similar conditions
C. Ease of interpretation
D. Public acceptability
Answer: B
7. Which of the following tests has the highest inter-observer repeatability?
A. Manual breast examination
B. Urine dipstick
C. HbA1c by autoanalyzer
D. Chest X-ray interpretation
Answer: C
8. Which factor decreases test repeatability the most?
A. Automated machines
B. Standard operating procedures
C. High inter-observer variation
D. Calibration
Answer: C
9. The test result is always consistent but 5 kg off the true value. This test is:
A. Valid and reliable
B. Reliable but not valid
C. Valid but not reliable
D. Neither valid nor reliable
Answer: B
10. Which one is TRUE about test validity?
A. A valid test must be simple
B. Specificity is the ability to detect true positives
C. Sensitivity is the ability to detect true positives
D. Specificity is irrelevant in screening
Answer: C
MCQs on Sensitivity, Specificity, and ROC Curve
11. Sensitivity is calculated as:
A. TP / (TP + FN)
B. TP / (TP + FP)
C. TN / (TN + FP)
D. TN / (TN + FN)
Answer: A
12. Specificity is:
A. TP / (TP + FN)
B. TN / (TN + FP)
C. TN / (TN + FN)
D. FP / (FP + TN)
Answer: B
13. In simultaneous (parallel) testing, sensitivity:
A. Decreases
B. Remains same
C. Increases
D. Becomes zero
Answer: C
14. In sequential (serial) testing, specificity:
A. Decreases
B. Increases
C. Remains same
D. Is not affected
Answer: B
15. Area under the ROC curve (AUC) represents:
A. False positive rate
B. Disease prevalence
C. Overall test accuracy
D. Cost-effectiveness
Answer: C
MCQs on Acceptability and Yield
16. Which test has low acceptability due to invasiveness and social sensitivity?
A. BP measurement
B. Hemoglobin estimation
C. Pap smear
D. Spot glucose test
Answer: C
17. Which action improves acceptability of screening in conservative communities?
A. Avoiding informed consent
B. Conducting tests publicly
C. Using male staff for female exams
D. Ensuring privacy and use of female examiners
Answer: D
18. Yield of a test refers to:
A. Cost of the test
B. Number of false positives
C. Number of previously undiagnosed cases found
D. Population not responding to test
Answer: C
19. What factor increases the yield of a screening program?
A. Low test sensitivity
B. Low disease prevalence
C. Screening low-risk groups
D. High prevalence of the condition
Answer: D
20. Corrected yield is calculated by:
A. Subtracting false positives from all positives
B. Adjusting yield for biological variability
C. Removing duplicate tests
D. Only including previously diagnosed cases
Answer: A
MCQs on Application of Screening Tests
21. What is the screening threshold for gestational diabetes by OGTT?
A. 1h >180 mg/dL
B. 2h >140 mg/dL
C. Fasting >110 mg/dL
D. Postprandial >160 mg/dL
Answer: B
22. Which test is recommended annually in elderly to prevent osteoporosis?
A. Serum calcium
B. TSH
C. DEXA scan
D. Creatinine
Answer: C
23. Newborns are screened using heel-prick at:
A. 24 hours
B. Immediately after birth
C. 48–72 hours after birth
D. 7 days after birth
Answer: C
24. Cognitive screening in geriatrics is done using:
A. GDS
B. MMSE or MoCA
C. Snellen chart
D. VDRL
Answer: B
25. A program where BP is checked every year in elderly fulfills which Wilson and Jungner criterion?
A. Test is painful
B. One-time testing is enough
C. Case-finding is a continuing process
D. No clear policy on treatment
Answer: C
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